Variability of the Tropical Ocean Surface Temperatures at Decadal-Multidecadal Timescales. Part I: The Atlantic Ocean.

NASA Astrophysics Data System (ADS)

Mehta, Vikram M.

1998-09-01

Gridded time series from the Global Ocean Surface Temperature Atlas were analyzed with a variety of techniques to identify spatial structures and oscillation periods of the tropical Atlantic sea surface temperature (SST) variations at decadal timescales, and to develop physical interpretations of statistical patterns of decadal SST variations. Each time series was 110 yr (1882-1991) long. The tropical Atlantic SST variations were compared with decadal variations in a 74-yr-long (1912-85) north Nordeste Brazil rainfall time series and a 106-yr-long (1886-1991) tropical Atlantic cyclone activity index time series. The tropical Atlantic SST variations were also compared with decadal variations in the extratropical Atlantic SST.Multiyear to multidecadal variations in the cross-equatorial dipole pattern identified as a dominant empirical pattern of the tropical Atlantic SST variations in earlier and present studies are shown to be variations in the approximately north-south gradient of SST anomalies. It is also shown that there was no dynamical-thermodynamical, dipole mode of SST variations during the analysis period. There was a distinct decadal timescale (12-13 yr) of SST variations in the tropical South Atlantic, whereas no distinct decadal timescale was found in the tropical North Atlantic SST variations. Approximately 80% of the coherent decadal variance in the cross-equatorial SST gradient was `explained' by coherent decadal oscillations in the tropical South Atlantic SSTs. There were three, possibly physical, modes of decadal variations in the tropical Atlantic SSTs during the analysis period. In the more energetic mode of the North Atlantic decadal SST variations, anomalies traveled into the tropical North Atlantic from the extratropical North Atlantic along the eastern boundary of the basin. The anomalies strengthened and resided in the tropical North Atlantic for several years, then frequently traveled northward into the mid-high-latitude North Atlantic along the western boundary of the basin, and completed a clockwise rotation around the North Atlantic basin. In the less energetic North Atlantic decadal mode, SST anomalies originated in the tropical-subtropical North Atlantic near the African coast, and traveled northwestward and southward. In the South Atlantic decadal SST mode, anomalies either developed in situ or traveled into the tropical South Atlantic from the subtropical South Atlantic along the eastern boundary of the basin. The anomalies strengthened and resided in the tropical South Atlantic for several years, then frequently traveled southward into the subtropical South Atlantic along the western boundary of the basin, and completed a counterclockwise rotation around the South Atlantic basin. These decadal modes were not a permanent feature of the tropical Atlantic SST variations. The tropical North and South Atlantic SST anomalies frequently extended across the equator. Uncorrelated alignments of decadal SST anomalies having opposite signs on two sides of the equator occasionally created the apperance of a dipole.Independent analyses of the north Nordeste Brazil rainfall showed physical consistency and high coherence with the cross-equatorial SST gradient oscillations at 12-13-yr period. The tropical Atlantic cyclone index showed physical consistency but moderate coherence with the tropical North Atlantic decadal SST variations. The quasi-regularity of the 12-13-yr oscillations in the cross-equatorial SST gradient may provide an opportunity for long lead-time, skillful predictions of climate anomalies in the tropical Atlantic sector.

Interactions Between the Thermohaline Circulation and Tropical Atlantic SST in a Coupled General Circulation Model

NASA Technical Reports Server (NTRS)

Miller, Ron; Jiang, Xing-Jian; Travis, Larry (Technical Monitor)

2001-01-01

Tropical Atlantic SST shows a (statistically well-defined) decadal time scale in a 104-year simulation of unforced variability by a coupled general circulation model (CGCM). The SST anomalies superficially resemble observed Tropical Atlantic variability (TAV), and are associated with changes in the atmospheric circulation. Brazilian rainfall is modulated with a decadal time scale, along with the strength of the Atlantic trade winds, which are associated with variations in evaporation and the net surface heat flux. However, in contrast to observed tropical Atlantic variability, the trade winds damp the associated anomalies in ocean temperature, indicating a negative feedback. Tropical SST anomalies in the CGCM, though opposed by the surface heat flux, are advected in from the Southern Hemisphere mid-latitudes. These variations modulate the strength of the thermohaline circulation (THC): warm, salty anomalies at the equator sink drawing cold, fresh mid-latitude water. Upon reaching the equator, the latter inhibit vertical overturning and advection from higher latitudes, which allows warm, salty anomalies to reform, returning the cycle to its original state. Thus, the cycle results from advection of density anomalies and the effect of these anomalies upon the rate of vertical overturning and surface advection. This decadal modulation of Tropical Atlantic SST and the thermohaline circulation is correlated with ocean heat transport to the Northern Hemisphere high latitudes and Norwegian Sea SST. Because of the central role of equatorial convection, we question whether this mechanism is present in the current climate, although we speculate that it may have operated in palaeo times, depending upon the stability of the tropical water column.

Inter-decadal variation of the Tropical Atlantic-Korea (TA-K) teleconnection pattern during boreal summer season

NASA Astrophysics Data System (ADS)

Ham, Yoo-Geun; Hwang, YeonJi; Lim, Young-Kwon; Kwon, Minho

2017-12-01

The inter-decadal variation of the positive relationship between the tropical Atlantic sea surface temperature (SST) and Korean precipitation during boreal summer season during 1900-2010 is examined. The 15-year moving correlation between the Tropical Atlantic SST (TAtlSST) index (SST anomalies from 30°S to 30°N and 60°W to 20°E) and Korean precipitation (precipitation anomalies from 35°-40°N to 120°-130°E) during June-July-August exhibits strong inter-decadal variation, which becomes positive at the 95% confidence level after the 1980s. Intensification of the linkage between the TAtlSST index and Korean precipitation after the 1980s is attributed to global warming via the increased background SST. The increase in the background SST over the Atlantic provides background conditions that enhance anomalous convective activity by anomalous Atlantic SST warming. Therefore, the overall atmospheric responses associated with the tropical Atlantic SST warming could intensify. The correlation between the TAtlSST index and Korean precipitation also exhibits strong inter-decadal variation within 1980-2010, which is over 0.8 during early 2000s, while it is relative low (i.e., around 0.6) during the early 1980s. The enhanced co-variability between the tropical and the mid-latitude Atlantic SST during the early 2000s indicates the intensification of TAtlSST-related Rossby wave source over the mid-latitude Atlantic, which excites stationary waves propagated from the Atlantic to the Korean peninsula across northern Europe and northeast Asia. This Rossby-wave train induces a cyclonic flow over the northern edge of the Korea, which intensifies southwesterly and results in precipitation over Korea. This observed decadal difference is well simulated by the stationary wave model experiments with a prescribed TAtlSST-related Rossby wave source over the mid-latitude Atlantic.

Long-Term Trends, Variability and Extremes of In Situ Sea Surface Temperature Measured Along the Eastern Adriatic Coast and its Relationship to Hemispheric Processes

NASA Astrophysics Data System (ADS)

Grbec, Branka; Matić, Frano; Beg Paklar, Gordana; Morović, Mira; Popović, Ružica; Vilibić, Ivica

2018-02-01

This paper examines long-term series of in situ sea surface temperature (SST) data measured at nine coastal and one open sea stations along the eastern Adriatic Sea for the period 1959-2015. Monthly and yearly averages were used to document SST trends and variability, while clustering and connections to hemispheric indices were achieved by applying the Principal Component Analysis (PCA) and Self-Organizing Maps (SOM) method. Both PCA and SOM revealed the dominance of temporal changes with respect to the effects of spatial differences in SST anomalies, indicating the prevalence of hemispheric processes over local dynamics, such as bora wind spatial inhomogeneity. SST extremes were connected with blocking atmospheric patterns. A substantial warming between 1979 and 2015, in total exceeding 1 °C, was preceded by a period with a negative SST trend, implying strong multidecadal variability in the Adriatic. The strongest connection was found between yearly SST and the East Atlantic (EA) pattern, while North Atlantic Oscillation (NAO) and East Atlantic/West Russia (EAWR) patterns were found to also affect February SST values. Quantification of the Adriatic SST and their connection to hemispheric indices allow for more precise projections of future SST, considered to be rather important for Adriatic thermohaline circulation, biogeochemistry and fisheries, and sensitive to ongoing climate change.

The Oceanic Contribution to Atlantic Multi-Decadal Variability

NASA Astrophysics Data System (ADS)

Wills, R. C.; Armour, K.; Battisti, D. S.; Hartmann, D. L.

2017-12-01

Atlantic multi-decadal variability (AMV) is typically associated with variability in ocean heat transport (OHT) by the Atlantic Meridional Overturning Circulation (AMOC). However, recent work has cast doubt on this connection by showing that slab-ocean climate models, in which OHT cannot vary, exhibit similar variability. Here, we apply low-frequency component analysis to isolate the variability of Atlantic sea-surface temperatures (SSTs) that occurs on decadal and longer time scales. In observations and in pre-industrial control simulations of comprehensive climate models, we find that AMV is confined to the extratropics, with the strongest temperature anomalies in the North Atlantic subpolar gyre. We show that warm subpolar temperatures are associated with a strengthened AMOC, increased poleward OHT, and local heat fluxes from the ocean into the atmosphere. In contrast, the traditional index of AMV based on the basin-averaged SST anomaly shows warm temperatures preceded by heat fluxes from the atmosphere into the ocean, consistent with the atmosphere driving this variability, and shows a weak relationship with AMOC. The autocorrelation time of the basin-averaged SST index is 1 year compared to an autocorrelation time of 5 years for the variability of subpolar temperatures. This shows that multi-decadal variability of Atlantic SSTs is sustained by OHT variability associated with AMOC, while atmosphere-driven SST variability, such as exists in slab-ocean models, contributes primarily on interannual time scales.

Interbasin Differences in the Relationship between SST and Tropical Cyclone Intensification

DOE Office of Scientific and Technical Information (OSTI.GOV)

Foltz, Gregory R.; Balaguru, Karthik; Hagos, Samson

Sea surface temperature (SST) is one of the most important parameters for tropical cyclone (TC) intensification. Here it is shown that the impact of SST on TC intensification varies considerably from basin to basin, with SST explaining less than 3% of the variance in TC intensification rates in the Atlantic, 10% in the western North Pacific, and 17% in the eastern Pacific. Two main factors are shown to be responsible for these inter-basin differences. First, variability of SST along TCs’ tracks is considerably lower in the Atlantic. This is due to smaller horizontal SST gradients in the Atlantic compared tomore » the eastern Pacific and stronger damping of pre-storm SST’s contribution to TC intensification by the storm-induced cold SST wake in the Atlantic. The damping occurs because SST tends to vary in phase with TC- induced SST cooling: in the Gulf of Mexico and northwestern basin where SSTs are highest, TCs’ translation speeds are lowest and therefore their cold wakes are strongest. In addition to this SST effect, a second factor is that SST tends to vary out of phase with vertical wind shear and outflow temperature in the western Pacific, with high SST associated with weak wind shear and a cold upper troposphere. This strengthens the relationship between SST and TC intensification more in the western Pacific than in the eastern Pacific or Atlantic. Combined, these factors explain why pre-storm SST is such a poor predictor of TC intensification in the Atlantic compared to the eastern and western North Pacific.« less

An 800-Year Tropical Atlantic Sea Surface Temperature Variability Record From the Cariaco Basin, Venezuela

NASA Astrophysics Data System (ADS)

Black, D. E.; Thunell, R. C.; Kaplan, A.; Abahazi, M. A.; Tappa, E. J.

2007-05-01

Here we present an eight century tropical Atlantic SST record based on foraminiferal Mg/Ca recovered from Cariaco Basin sediments that have been calibrated to historical instrumental SSTs. Spatial correlations indicate that the proxy record is representative of SSTs over much of the Caribbean and tropical Atlantic. The Mg/Ca-SST record also correlates well with global land and sea surface temperature anomalies, and captures decadal-scale variations in Atlantic tropical storm and hurricane frequency over the late-19th and 20th centuries. The long-term record displays a surprising amount of variability for a tropical location under essentially modern boundary conditions. The tropical North Atlantic does not appear to have experienced a pronounced Medieval Warm Period relative to the complete record. However, strong Little Ice Age cooling of as much as 3 °C occurred between A. D. 1525 and 1625. Spring SSTs gradually rose between A. D. 1650 and 1900 followed by a 2.5 °C warming over the twentieth century. Viewed in the context of the complete record, twentieth century temperatures are not the warmest in the entire record on average, but they do show the largest increase in magnitude and fastest rate of SST change over the last eight hundred years. Spectral analysis of the Mg/Ca-SST data suggests that 2-5 and ~13 year SST variability that is characteristic of tropical Atlantic instrumental records may change through time.

Low frequency North Atlantic SST variability: Weather noise forcing and coupled response

NASA Astrophysics Data System (ADS)

Fan, Meizhu

A method to diagnose the causes of low frequency SST variability is developed, tested and applied in an ideal case and real climate. In the ideal case, a free simulation of the COLA CGCM is taken as synthetic observations. For real climate, we take NCEP reanalysis atmospheric data and Reynolds SST as observations. Both the synthetic and actual observation data show that weather noise is the main component of atmospheric variability at subtropics and high-latitude. Diagnoses of results from the ideal case suggest that most of the synthetic observed SST variability can be reproduced by the weather noise surface fluxes forcing. This includes the "observed" low frequency SST patterns in the North Atlantic and their corresponding time evolution. Among all the noise surface fluxes, heat flux plays a major role. The results from simulations using actual observations also suggest that the observed SST variability is mostly atmospheric weather noise forced. The regional atmospheric noise forcing, especially the heat flux noise forcing, is the major source of the low frequency SST variability in the North Atlantic. The observed SST tripole mode has about a 12 year period and it can be reasonably reproduced by the weather noise forcing in terms of its period, spatial pattern and variance. Based on our diagnosis, it is argued that the SST tripole is mainly forced by local atmospheric heat flux noise. The gyre circulation plays a secondary role: the anomalous gyre circulation advects mean thermal features across the inter-gyre boundary, and the mean gyre advection carries SST anomalies along the inter-gyre boundary. The diagnosis is compared with a delayed oscillator theory. We find that the delayed oscillator theory is not supported and that the SST tripole mode is forced by weather noise heat flux noise. However, the result may be model dependent.

The Teleconnection Between Atlantic Sea Surface Temperature and Eastern Pacific Tropical Cyclones

NASA Astrophysics Data System (ADS)

Patricola, C. M.; Saravanan, R.; Chang, P.

2016-12-01

The El Niño-Southern Oscillation (ENSO) is a major source of seasonal tropical cyclone (TC) predictability, in both local and remote ocean basins. Unusually warm eastern tropical Pacific sea-surface temperature (SST) during El Niño tends not only to enhance local TC activity in the eastern North Pacific (ENP) but also to suppress Atlantic TCs via well-known teleconnections. Here, we demonstrate that Atlantic SST variability likewise exerts a significant influence on remote TC activity in the eastern Pacific basin using observations and 27 km resolution tropical channel model simulations. Observed and simulated accumulated cyclone energy in the ENP is substantially reduced during the positive phase of the Atlantic Meridional Mode (AMM), which is characterized by warm and cool SST anomalies in the northern and southern tropical Atlantic respectively, and vice versa during the cool AMM phase. We find that the observed anti-correlation in seasonal TC activity between the Atlantic and ENP basins is driven by interannual climate variability in both the tropical Pacific (ENSO) and Atlantic (AMM). The physical mechanisms that drive the teleconnection between Atlantic SST and ENP TC activity will also be presented. This work provides information that can be used to improve seasonal forecasts and future projections of ENP tropical cyclone activity.

Role of Tropical Atlantic SST Variability as a Modulator of El Nino Teleconnections

NASA Technical Reports Server (NTRS)

Ham, Yoo-Geun; Sung, Mi-Kyung; An, Soon-II; Schubert, Siegfried D.; Kug, Jong-Seong

2014-01-01

The present study suggests that the off-equatorial North Atlantic (NATL) SST warming plays a significant role in modulating El Niño teleconnection and its impact on the North Atlantic and European regions. The El Niño events accompanied by NATL SST warming exhibit south-north dipole pattern over the Western Europe to Atlantic, while the ENSO teleconnection pattern without NATL warming exhibits a Rossby wave-like pattern confined over the North Pacific and western Atlantic. Especially, the El Niño events with NATL warming show positive (negative) geopotential-height anomalies over the North Atlantic (Western Europe) which resemble the negative phase of the NAO. Consistently, it is shown using a simple statistical model that NATL SSTA in addition to the tropical Pacific SSTA leads to better prediction on regional climate variation over the North Atlantic and European regions. This role of NATL SST on ENSO teleconnection is also validated and discussed in a long term simulation of coupled global circulation model (CGCM).

The summer North Atlantic Oscillation (SNAO) variability on decadal to paleoclimate time scales

NASA Astrophysics Data System (ADS)

Linderholm, H. W.; Folland, C. K.; Zhang, P.; Gunnarson, B. E.; Jeong, J. H.; Ren, H.

2017-12-01

The summer North Atlantic Oscillation (SNAO), strongly related to the latitude of the North Atlantic and European summer storm tracks, exerts a considerable influence on European summer climate variability and extremes. Here we extend the period covered by the SNAO from July and August to June, July and August (JJA). As well as marked interannual variability, the JJA SNAO has shown a large inter-decadal change since the 1970s. Decadally averaged, there has been a change from a very positive to a rather negative SNAO phase. This change in SNAO phase is opposite in sign from that expected by a number of climate models under enhanced greenhouse forcing by the late twenty first century. It has led to noticeably wetter summers in North West Europe in the last decade. On interannual to multidecadal timescales, SNAO variability is linked to variations in North Atlantic sea surface temperature (SST): observations and models indicate an association between the Atlantic Multi-decadal Oscillation (AMO) where the cold (warm) phase of the AMO corresponds a positive (negative) phase of the SNAO. Observations also indicate a link with SST in the Gulf Stream region of the North Atlantic where, particularly on decadal time scales, SST warming may favour a more positive phase of the SNAO. Influences of Arctic climate change on North Atlantic and European atmospheric circulation may also exist, particularly reduced sea ice coverage, perhaps favouring the negative phase of the SNAO. A new tree-ring data based JJA SNAO reconstruction extending over the last millennium, as well as climate model output for the same period, enables us to examine the influence of North Atlantic SST and Arctic sea-ice coverage, as well as SNAO impacts on European summer climate, in a long-term, pre-industrial context.

How predictable are equatorial Atlantic surface winds?

NASA Astrophysics Data System (ADS)

Richter, Ingo; Doi, Takeshi; Behera, Swadhin

2017-04-01

Sensitivity tests with the SINTEX-F general circulation model (GCM) as well as experiments from the Coupled Model Intercomparison Project phase 5 (CMIP5) are used to examine the extent to which sea-surface temperature (SST) anomalies contribute to the variability and predictability of monthly mean surface winds in the equatorial Atlantic. In the SINTEX-F experiments, a control experiment with prescribed observed SST for the period 1982-2014 is modified by inserting climatological values in certain regions, thereby eliminating SST anomalies. When SSTs are set to climatology in the tropical Atlantic only (30S to 30N), surface wind variability over the equatorial Atlantic (5S-5N) decreases by about 40% in April-May-June (AMJ). This suggests that about 60% of surface wind variability is due to either internal atmospheric variability or SSTs anomalies outside the tropical Atlantic. A further experiment with climatological SSTs in the equatorial Pacific indicates that another 10% of variability in AMJ may be due to remote influences from that basin. Experiments from the CMIP5 archive, in which climatological SSTs are prescribed globally, tend to confirm the results from SINTEX-F but show a wide spread. In some models, the equatorial Atlantic surface wind variability decreases by more than 90%, while in others it even increases. Overall, the results suggest that about 50-60% of surface wind variance in AMJ is predictable, while the rest is due to internal atmospheric variability. Other months show significantly lower predictability. The relatively strong internal variability as well as the influence of remote SSTs suggest a limited role for coupled ocean-atmosphere feedbacks in equatorial Atlantic variability.

A volcanic wind-stress origin of the Atlantic Multidecadal Oscillation

NASA Astrophysics Data System (ADS)

Birkel, S. D.; Mayewski, P. A.; Maasch, K. A.; Auger, J.; Lyon, B.

2016-12-01

The Atlantic Multidecadal Oscillation (AMO) is a mode of sea-surface temperature (SST) variability in the North Atlantic that has significant impact on global climate. Most previous studies ascribe the origin of the AMO to oceanic mechanisms, and suggest only a limited role for the atmosphere. Here, we suggest that the AMO is manifested from basin-wide changes in surface wind stress that arise in response to episodic volcanic activity. Our interpretation is based on historical SST, reanalysis, and stratospheric aerosol optical thickness data, wherein it is evident that cool (warm) intervals of the AMO coincide with emergence of strong (weak) winds and high (low) volcanic activity. We find that SST excursions ultimately develop from atmospheric forcing as volcanic events project onto the North Atlantic Oscillation (NAO). A volcanic signature is particularly evident beneath the westerlies in the subpolar region south of Greenland, where several large SST excursions occur coincident with identifiable major eruptions. High latitude surface waters cool when NAO+ circulation, which includes a deepened Icelandic Low, draws cold flow out of the Labrador Sea and into the subpolar region. Important feedbacks that cause SST anomalies to spread across the basin include cloud cover, wind-driven upwelling, and entrainment of Saharan dust into the tropical easterlies. Finally, we speculate that cooling in the North Atlantic observed since 2011 could be linked to renewed volcanic activity over Iceland, namely from the eruptions of Grímsvötn (2011) and Bárðarbunga (2014). An important question remains how North Atlantic SST variability will evolve as atmospheric circulation becomes increasingly modified by human activity.

Enhanced biennial variability in the Pacific due to Atlantic capacitor effect.

PubMed

Wang, Lei; Yu, Jin-Yi; Paek, Houk

2017-03-20

The El Niño-Southern Oscillation (ENSO) and the variability in the Pacific subtropical highs (PSHs) have major impacts on social and ecological systems. Here we present an Atlantic capacitor effect mechanism to suggest that the Atlantic is a key pacemaker of the biennial variability in the Pacific including that in ENSO and the PSHs during recent decades. The 'charging' (that is, ENSO imprinting the North Tropical Atlantic (NTA) sea surface temperature (SST) via an atmospheric bridge mechanism) and 'discharging' (that is, the NTA SST triggering the following ENSO via a subtropical teleconnection mechanism) processes alternate, generating the biennial rhythmic changes in the Pacific. Since the early 1990s, a warmer Atlantic due to the positive phase of Atlantic multidecadal oscillation and global warming trend has provided more favourable background state for the Atlantic capacitor effect, giving rise to enhanced biennial variability in the Pacific that may increase the occurrence frequency of severe natural hazard events.

Enhanced biennial variability in the Pacific due to Atlantic capacitor effect

PubMed Central

Wang, Lei; Yu, Jin-Yi; Paek, Houk

2017-01-01

The El Niño-Southern Oscillation (ENSO) and the variability in the Pacific subtropical highs (PSHs) have major impacts on social and ecological systems. Here we present an Atlantic capacitor effect mechanism to suggest that the Atlantic is a key pacemaker of the biennial variability in the Pacific including that in ENSO and the PSHs during recent decades. The ‘charging' (that is, ENSO imprinting the North Tropical Atlantic (NTA) sea surface temperature (SST) via an atmospheric bridge mechanism) and ‘discharging' (that is, the NTA SST triggering the following ENSO via a subtropical teleconnection mechanism) processes alternate, generating the biennial rhythmic changes in the Pacific. Since the early 1990s, a warmer Atlantic due to the positive phase of Atlantic multidecadal oscillation and global warming trend has provided more favourable background state for the Atlantic capacitor effect, giving rise to enhanced biennial variability in the Pacific that may increase the occurrence frequency of severe natural hazard events. PMID:28317857

Enhanced biennial variability in the Pacific due to Atlantic capacitor effect

NASA Astrophysics Data System (ADS)

Wang, Lei; Yu, Jin-Yi; Paek, Houk

2017-03-01

The El Niño-Southern Oscillation (ENSO) and the variability in the Pacific subtropical highs (PSHs) have major impacts on social and ecological systems. Here we present an Atlantic capacitor effect mechanism to suggest that the Atlantic is a key pacemaker of the biennial variability in the Pacific including that in ENSO and the PSHs during recent decades. The `charging' (that is, ENSO imprinting the North Tropical Atlantic (NTA) sea surface temperature (SST) via an atmospheric bridge mechanism) and `discharging' (that is, the NTA SST triggering the following ENSO via a subtropical teleconnection mechanism) processes alternate, generating the biennial rhythmic changes in the Pacific. Since the early 1990s, a warmer Atlantic due to the positive phase of Atlantic multidecadal oscillation and global warming trend has provided more favourable background state for the Atlantic capacitor effect, giving rise to enhanced biennial variability in the Pacific that may increase the occurrence frequency of severe natural hazard events.

Model under-representation of decadal Pacific trade wind trends and its link to tropical Atlantic bias

NASA Astrophysics Data System (ADS)

Kajtar, Jules B.; Santoso, Agus; McGregor, Shayne; England, Matthew H.; Baillie, Zak

2018-02-01

The strengthening of the Pacific trade winds in recent decades has been unmatched in the observational record stretching back to the early twentieth century. This wind strengthening has been connected with numerous climate-related phenomena, including accelerated sea-level rise in the western Pacific, alterations to Indo-Pacific ocean currents, increased ocean heat uptake, and a slow-down in the rate of global-mean surface warming. Here we show that models in the Coupled Model Intercomparison Project phase 5 underestimate the observed range of decadal trends in the Pacific trade winds, despite capturing the range in decadal sea surface temperature (SST) variability. Analysis of observational data suggests that tropical Atlantic SST contributes considerably to the Pacific trade wind trends, whereas the Atlantic feedback in coupled models is muted. Atmosphere-only simulations forced by observed SST are capable of recovering the time-variation and the magnitude of the trade wind trends. Hence, we explore whether it is the biases in the mean or in the anomalous SST patterns that are responsible for the under-representation in fully coupled models. Over interannual time-scales, we find that model biases in the patterns of Atlantic SST anomalies are the strongest source of error in the precipitation and atmospheric circulation response. In contrast, on decadal time-scales, the magnitude of the model biases in Atlantic mean SST are directly linked with the trade wind variability response.

Simulation of Tropical Rainfall Variability

NASA Astrophysics Data System (ADS)

Bader, J.; Latif, M.

2002-12-01

The impact of sea surface temperature (SST) - especially the role of the tropical Atlantic meridional SST gradient and the El Nino-Southern Oscillation - on precipitation is investigated with the atmospheric general circulation model ECHAM4/T42. Ensemble experiments - driven with observed SST - show that Atlantic SST has a significant influence on precipitation over West Africa and northeast Brazil. SST sensitivity experiments were performed in which the climatological SST was enhanced or decreased by one Kelvin in certain ocean areas. Changing SST in the eastern tropical Atlantic caused only significant changes along the Guinea Coast, with a positive anomaly (SSTA) increasing rainfall and a negative SSTA reducing it. The response was nearly linear. Changing SST in other ocean areas caused significant changes over West Africa, especially in the Sahel area. The response is found to be non linear, with only negative SSTA leading to significant reduction in Sahel rainfall. Also, the impact of the SSTAs from the different ocean regions was not additive with respect to the rainfall. The influence of SST on precipitation over northeast Brazil (Nordeste) was also investigated. Three experiments were performed in which the climatological SST was enhanced/decreased or decreased/enhanced by one Kelvin in the North/South Atlantic and increased by two Kelvin in the Nino3 ocean area. All experiments caused significant changes over Nordeste, with an enhanced/reduced SST gradient in the Atlantic increasing/reducing rainfall. The response was nearly linear. The main effect of the Atlantic SST gradient was a shift of the ITCZ, caused by trade wind changes. The ''El Nino'' event generates a significant reduction in Nordeste rainfall. A significant positive SLP anomaly occurs in northeast Brazil which may be associated with the descending branch of the Walker circulation. Also a significant positive SLP over the Atlantic from 30S to 10N north occurs. This results in a reduced SLP gradient from the subtropical highs to the equator and a weakening of the trade winds.

North Atlantic climate model bias influence on multiyear predictability

NASA Astrophysics Data System (ADS)

Wu, Y.; Park, T.; Park, W.; Latif, M.

2018-01-01

The influences of North Atlantic biases on multiyear predictability of unforced surface air temperature (SAT) variability are examined in the Kiel Climate Model (KCM). By employing a freshwater flux correction over the North Atlantic to the model, which strongly alleviates both North Atlantic sea surface salinity (SSS) and sea surface temperature (SST) biases, the freshwater flux-corrected integration depicts significantly enhanced multiyear SAT predictability in the North Atlantic sector in comparison to the uncorrected one. The enhanced SAT predictability in the corrected integration is due to a stronger and more variable Atlantic Meridional Overturning Circulation (AMOC) and its enhanced influence on North Atlantic SST. Results obtained from preindustrial control integrations of models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) support the findings obtained from the KCM: models with large North Atlantic biases tend to have a weak AMOC influence on SAT and exhibit a smaller SAT predictability over the North Atlantic sector.

A note on Bjerkne's hypothesis for North Atlantic variability

NASA Astrophysics Data System (ADS)

Bryan, Kirk; Stouffer, Ron

1991-01-01

On decadal time-scales the historical surface temperature record over land in the Northern Hemisphere is dominated by polar amplified variations. These variations are coherent with SST anomalies concentrated in the Northwest Atlantic, but extending with lesser amplitude into the North Pacific as well. Bierknes suggested that multi-year SST anomalies in the subpolar North Atlantic were due to irregular changes in the intensity of the thermohaline circulation. In support of the Bjerknes hypothesis there is evidence that winter overturning in the Labrador Sea was suppressed for a brief period from 1967-1969 by a cap of relative fresh water at the surface. Cause and effect are unclear, but this event was associated with a marked cooling of the entire Northern Hemisphere. The difference in SST averaged over the Northern Hemisphere oceans and SST averaged over the Southern Hemisphere oceans from the equator to 40°S is coherent with Sahel summer rainfall on decadal time scales. Empirical evidence is supported by numerical experiments with the British Meteorological Office atmospheric climate model which simulate augmented monsoonal rainfall in the Sahel region of Africa in response to realistic warm SST anomalies in the Northwest Atlantic. A coupled ocean-atmosphere global model exhibits two equilibrium climate states. One has an active thermohaline circulation in the North Atlantic and the other does not. The two climate states provide an extreme example which illustrates the type of large scale air sea interaction Bjerknes visualized as a mechanism for North Atlantic climate variability on decadal time-scales.

Reconstructing the history of the Atlantic Multidecadal Oscillation using high-resolution Mg/Ca paleothermometry from a Cariaco Basin core

NASA Astrophysics Data System (ADS)

Wurtzel, J. B.; Black, D. E.; Rahman, S.; Thunell, R.; Peterson, L. C.; Tappa, E.

2010-12-01

Instrumental and proxy-reconstructions show the existence of an approximately 70-year periodicity in Atlantic sea surface temperature (SST), known as the Atlantic Multidecadal Oscillation (AMO). The AMO is correlated with circum-tropical Atlantic climate phenomena such as Sahel and Nordeste rainfall, and Atlantic hurricane patterns. Though it has been suggested that the AMO is controlled by thermohaline circulation, much debate exists as to whether the SST fluctuations are a result of anthropogenic forcing or a natural climate mode, or even if the AMO is a true oscillation at all. Our ability to address this issue has been limited by instrumental SST records that rarely extend back more than 50-100 years and proxy reconstructions that are mostly terrestrial-based. Additionally, the modern instrumental variability likely contains an anthropogenic component that is not easily distinguished from the natural background of the system. From a marine sediment core taken in the Cariaco Basin, we have developed a high-resolution SST reconstruction for the past ca. 1500 years using Mg/Ca paleothermometry on seasonally-representative foraminifera, with the most recent data calibrated to the instrumental record. Previous studies have shown Cariaco Basin Mg/Ca-SSTs to be well-correlated to the Caribbean Sea and much of the western tropical Atlantic, which allows us to create a record that can be used to determine pre-anthropogenic rates and ranges of SST variability and observe how they change over time. Averaging the seasonal temperatures derived from the two foraminiferal species over the instrumental period yields a strong correlation to the AMO index from A. D. 1880 through 1970 (r = 0.44, p<0.0001). Wavelet analysis of the proxy average annual SST data indicates that modern AMO variability is not a consistent feature through time, and may be a function of warm-period climate.

Vertical Motion Changes Related to North-East Brazil Rainfall Variability: a GCM Simulation

NASA Astrophysics Data System (ADS)

Roucou, Pascal; Oribe Rocha de Aragão, José; Harzallah, Ali; Fontaine, Bernard; Janicot, Serge

1996-08-01

The atmospheric structure over north-east Brazil during anomalous rainfall years is studied in the 11 levels of the outputs of the Laboratoire de Météorologie Dynamique atmospheric general circulation model (LMD AGCM). Seven 19-year simulations were performed using observed sea-surface temperature (SST) corresponding to the period 1970- 1988. The ensemble mean is calculated for each month of the period, leading to an ensemble-averaged simulation. The simulated March-April rainfall is in good agreement with observations. Correlations of simulated rainfall and three SST indices relative to the equatorial Pacific and northern and southern parts of the Atlantic Ocean exhibit stronger relationships in the simulation than in the observations. This is particularly true with the SST gradient in the Atlantic (Atlantic dipole). Analyses on 200 ;hPa velocity potential, vertical velocity, and vertical integral of the zonal component of mass flux are performed for years of abnormal rainfall and positive/negative SST anomalies in the Pacific and Atlantic oceans in March-April during the rainy season over the Nordeste region. The results at 200 hPa show a convergence anomaly over Nordeste and a divergence anomaly over the Pacific concomitant with dry seasons associated with warm SST anomalies in the Pacific and warm (cold) waters in the North (South) Atlantic. During drought years convection inside the ITCZ indicated by the vertical velocity exhibits a displacement of the convection zone corresponding to a northward migration of the ITCZ. The east-west circulation depicted by the zonal divergent mass flux shows subsiding motion over Nordeste and ascending motion over the Pacific in drought years, accompanied by warm waters in the eastern Pacific and warm/cold waters in northern/southern Atlantic. Rainfall variability of the Nordeste rainfall is linked mainly to vertical motion and SST variability through the migration of the ITCZ and the east-west circulation.

Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability

NASA Astrophysics Data System (ADS)

Tokinaga, Hiroki; Xie, Shang-Ping; Mukougawa, Hitoshi

2017-06-01

With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean-atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region.

Understanding the predictability of seasonal precipitation over northeast Brazil

NASA Astrophysics Data System (ADS)

Misra, Vasubandhu

2006-05-01

Using multiple long-term simulations of the Center for Ocean-Land-Atmosphere Studies (COLA) atmospheric general circulation model (AGCM) forced with observed sea surface temperature (SST), it is shown that the model has high skill in simulating the February-March-April (FMA) rainy season over northeast Brazil (Nordeste). Separate sensitivity experiments conducted with the same model that entails suppression of all variability except for the climatological annual cycle in SST over the Pacific and Atlantic Oceans reveal that this skill over Nordeste is sensitive to SST anomalies in the tropical Atlantic Ocean. However, the spatial pattern of SST anomalies in the tropical Atlantic Ocean that correlate with FMA Nordeste rainfall are in fact a manifestation of El Niño Southern Oscillation (ENSO) phenomenon in the Pacific Ocean. This study also analyzes the failure of the COLA AGCM in capturing the correct FMA precipitation anomalies over Nordeste in several years of the simulation. It is found that this failure occurs when the SST anomalies over the northern tropical Atlantic Ocean are large and not significantly correlated with contemporaneous SST anomalies over the eastern Pacific Ocean. In two of the relatively large ENSO years when the model failed to capture the correct signal of the interannual variability of precipitation over Nordeste, it was found that the meridional gradient of SST anomalies over the tropical Atlantic Ocean was inconsistent with the canonical development of ENSO. The analysis of the probabilistic skill of the model revealed that it has more skill in predicting flood years than drought. Furthermore, the model has no skill in predicting normal seasons. These model features are consistent with the model systematic errors.

Atlantic Induced Pan-tropical Climate Variability in the Upper-ocean and Atmosphere

NASA Astrophysics Data System (ADS)

Li, X.; Xie, S. P.; Gille, S. T.; Yoo, C.

2016-02-01

During the last three decades, tropical sea surface temperature (SST) exhibited dipole-like trends, with warming over the tropical Atlantic and Indo-Western Pacific but cooling over the Eastern Pacific. The Eastern Pacific cooling has recently been identified as a driver of the global warming hiatus. Previous studies revealed atmospheric bridges between the tropical Pacific, Atlantic, and Indian Ocean, which could potentially contribute to this zonally asymmetric SST pattern. However, the mechanisms and the interactions between these teleconnections remain unclear. To investigate these questions, we performed a `pacemaker' simulation by restoring the tropical Atlantic SST changes in a state-of-the-art climate model - the CESM1. Results show that the Atlantic plays a key role in initiating the tropical-wide teleconnections, and the Atlantic-induced anomalies contribute 55%-75% of the total tropical SST and circulation changes during the satellite era. A hierarchy of oceanic and atmospheric models are then used to investigate the physical mechanisms of these teleconnections: the Atlantic warming enhances atmospheric deep convection, drives easterly wind anomalies over the Indo-Western Pacific through the Kelvin wave, and westerly anomalies over the eastern Pacific as Rossby waves, in line with Gill's solution (Fig1a). These wind changes induce an Indo-Western Pacific warming via the wind-evaporation-SST effect, and this warming intensifies the La Niña-type response in the upper Pacific Ocean by enhancing the easterly trade winds and through the Bjerknes ocean-dynamical processes (Fig1b). The teleconnection finally develops into a tropical-wide SST dipole pattern with an enhanced trade wind and Walker circulation, similar as the observed changes during the satellite era. This mechanism reveals that the tropical ocean basins are more tightly connected than previously thought, and the Atlantic plays a key role in the tropical climate pattern formation and further the global warming hiatus. The tropical Atlantic warming is likely due to radiative forcing and Atlantic meridional overturning circulation (AMOC). Our study suggests that the AMOC may force the decadal variability of the tropical ocean and atmosphere, and thus contributes to the decadal predictability of the global climate.

Role of tropical Indian and Atlantic Oceans variability on ENSO

NASA Astrophysics Data System (ADS)

Prodhomme, Chloé; Terray, Pascal; Masson, Sebastien; Boschat, Ghyslaine

2014-05-01

There are strong evidences of an interaction between tropical Indian, Atlantic and Pacific Oceans. Nevertheless, these interactions remain deeply controversial. While some authors claim the tropical Indian and Atlantic oceans only play a passive role with respect to ENSO, others suggest a driving role for these two basins on ENSO. The mecanisms underlying these relations are not fully understood and, in the Indian Ocean, the possible role of both modes of tropical variability (the Indian Ocean Dipole (IOD) and the Indian Ocean Basin mode (IOB)) remain unclear. To better quantify and understand how the variability of the tropical Indian and Atlantic Oceans impact ENSO variability, we performed two sensitivity experiments using the SINTEX-F2 coupled model. For each experiment, we suppressed the variability of SST and the air-sea coupling in either the tropical Indian Ocean or tropical Atlantic Ocean by applying a strong nudging of the SST to the observed SST climatology. In both experiments, the ENSO periodicity increases. In the Atlantic experiment, our understanding of this increased periodicity is drastically limited by the strongly biased mean state in this region. Conversely, in the Indian Ocean experiment, the increase of ENSO periodicity is related to the absence of the IOB following the El Niño peak, which leads to a decrease of westerly winds in the western Pacific during late winter and spring after the peak. These weaker westerlies hinders the transition to a La Niña phase and thus increase the duration and periodicity of the event.

Intensified impact of tropical Atlantic SST on the western North Pacific summer climate under a weakened Atlantic thermohaline circulation

NASA Astrophysics Data System (ADS)

Chen, Wei; Lee, June-Yi; Lu, Riyu; Dong, Buwen; Ha, Kyung-Ja

2015-10-01

The tropical North Atlantic (TNA) sea surface temperature (SST) has been identified as one of regulators on the boreal summer climate over the western North Pacific (WNP), in addition to SSTs in the tropical Pacific and Indian Oceans. The major physical process proposed is that the TNA warming induces a pair of cyclonic circulation anomaly over the eastern Pacific and negative precipitation anomalies over the eastern to central tropical Pacific, which in turn lead to an anticyclonic circulation anomaly over the western to central North Pacific. This study further demonstrates that the modulation of the TNA warming to the WNP summer climate anomaly tends to be intensified under background of the weakened Atlantic thermohaline circulation (THC) by using a water-hosing experiment. The results suggest that the weakened THC induces a decrease in thermocline depth over the TNA region, resulting in the enhanced sensitivity of SST variability to wind anomalies and thus intensification of the interannual variation of TNA SST. Under the weakened THC, the atmospheric responses to the TNA warming are westward shifted, enhancing the anticyclonic circulation and negative precipitation anomaly over the WNP. This study supports the recent finding that the negative phase of the Atlantic multidecadal oscillation after the late 1960s has been favourable for the strengthening of the connection between TNA SST variability and WNP summer climate and has important implications for seasonal prediction and future projection of the WNP summer climate.

Early 20th-century Arctic warming intensified by Pacific and Atlantic multidecadal variability

PubMed Central

Tokinaga, Hiroki; Xie, Shang-Ping; Mukougawa, Hitoshi

2017-01-01

With amplified warming and record sea ice loss, the Arctic is the canary of global warming. The historical Arctic warming is poorly understood, limiting our confidence in model projections. Specifically, Arctic surface air temperature increased rapidly over the early 20th century, at rates comparable to those of recent decades despite much weaker greenhouse gas forcing. Here, we show that the concurrent phase shift of Pacific and Atlantic interdecadal variability modes is the major driver for the rapid early 20th-century Arctic warming. Atmospheric model simulations successfully reproduce the early Arctic warming when the interdecadal variability of sea surface temperature (SST) is properly prescribed. The early 20th-century Arctic warming is associated with positive SST anomalies over the tropical and North Atlantic and a Pacific SST pattern reminiscent of the positive phase of the Pacific decadal oscillation. Atmospheric circulation changes are important for the early 20th-century Arctic warming. The equatorial Pacific warming deepens the Aleutian low, advecting warm air into the North American Arctic. The extratropical North Atlantic and North Pacific SST warming strengthens surface westerly winds over northern Eurasia, intensifying the warming there. Coupled ocean–atmosphere simulations support the constructive intensification of Arctic warming by a concurrent, negative-to-positive phase shift of the Pacific and Atlantic interdecadal modes. Our results aid attributing the historical Arctic warming and thereby constrain the amplified warming projected for this important region. PMID:28559341

Contrasting spatial structures of Atlantic Multidecadal Oscillation between observations and slab ocean model simulations

NASA Astrophysics Data System (ADS)

Sun, Cheng; Li, Jianping; Kucharski, Fred; Xue, Jiaqing; Li, Xiang

2018-04-01

The spatial structure of Atlantic multidecadal oscillation (AMO) is analyzed and compared between the observations and simulations from slab ocean models (SOMs) and fully coupled models. The observed sea surface temperature (SST) pattern of AMO is characterized by a basin-wide monopole structure, and there is a significantly high degree of spatial coherence of decadal SST variations across the entire North Atlantic basin. The observed SST anomalies share a common decadal-scale signal, corresponding to the basin-wide average (i. e., the AMO). In contrast, the simulated AMO in SOMs (AMOs) exhibits a tripole-like structure, with the mid-latitude North Atlantic SST showing an inverse relationship with other parts of the basin, and the SOMs fail to reproduce the observed strong spatial coherence of decadal SST variations associated with the AMO. The observed spatial coherence of AMO SST anomalies is identified as a key feature that can be used to distinguish the AMO mechanism. The tripole-like SST pattern of AMOs in SOMs can be largely explained by the atmosphere-forced thermodynamics mechanism due to the surface heat flux changes associated with the North Atlantic Oscillation (NAO). The thermodynamic forcing of AMOs by the NAO gives rise to a simultaneous inverse NAO-AMOs relationship at both interannual and decadal timescales and a seasonal phase locking of the AMOs variability to the cold season. However, the NAO-forced thermodynamics mechanism cannot explain the observed NAO-AMO relationship and the seasonal phase locking of observed AMO variability to the warm season. At decadal timescales, a strong lagged relationship between NAO and AMO is observed, with the NAO leading by up to two decades, while the simultaneous correlation of NAO with AMO is weak. This lagged relationship and the spatial coherence of AMO can be well understood from the view point of ocean dynamics. A time-integrated NAO index, which reflects the variations in Atlantic meridional overturning circulation (AMOC) and northward ocean heat transport caused by the accumulated effect of NAO forcing, reasonably well captures the observed multidecadal fluctuations in the AMO. Further analysis using the fully coupled model simulations provides direct modeling evidence that the observed spatial coherence of decadal SST variations across North Atlantic basin can be reproduced only by including the AMOC-related ocean dynamics, and the AMOC acts as a common forcing signal that results in a spatially coherent variation of North Atlantic SST.

Variations of Sea Surface Temperature, Wind Stress, and Rainfall over the Tropical Atlantic and South America.

NASA Astrophysics Data System (ADS)

Nobre, Paulo; Srukla, J.

1996-10-01

Empirical orthogonal functions (E0Fs) and composite analyses are used to investigate the development of sea surface temperature (SST) anomaly patterns over the tropical Atlantic. The evolution of large-scale rainfall anomaly patterns over the equatorial Atlantic and South America are also investigated. 71e EOF analyses revealed that a pattern of anomalous SST and wind stress asymmetric relative to the equator is the dominant mode of interannual and longer variability over the tropical Atlantic. The most important findings of this study are as follows.Atmospheric circulation anomalies precede the development of basinwide anomalous SST patterns over the tropical Atlantic. Anomalous SST originate off the African coast simultaneously with atmospheric circulation anomalies and expand westward afterward. The time lag between wind stress relaxation (strengthening) and maximum SST warming (cooling) is about two months.Anomalous atmospheric circulation patterns over northern tropical Atlantic are phase locked to the seasonal cycle. Composite fields of SLP and wind stress over northern tropical Atlantic can be distinguished from random only within a few months preceding the March-May (MAM) season. Observational evidence is presented to show that the El Niño-Southern Oscillation phenomenon in the Pacific influences atmospheric circulation and SST anomalies over northern tropical Atlantic through atmospheric teleconnection patterns into higher latitudes of the Northern Hemisphere.The well-known droughts over northeastern Brazil (Nordeste) are a local manifestation of a much larger-scale rainfall anomaly pattern encompassing the whole equatorial Atlantic and Amazon region. Negative rainfall anomalies to the south of the equator during MAM, which is the rainy season for the Nordeste region, are related to an early withdrawal of the intertropical convergence zone toward the warm SST anomalies over the northern tropical Atlantic. Also, it is shown that precipitation anomalies over southern and northern parts of the Nordeste are out of phase: drought years over the northern Nordeste are commonly preceded by wetter years over the southern Nordeste, and vice versa.

Multi-species coral Sr/Ca-based sea-surface temperature reconstruction using Orbicella faveolata and Siderastrea siderea from the Florida Straits

USGS Publications Warehouse

Flannery, Jennifer A.; Richey, Julie N.; Thirumalai, Kaustubh; Poore, Richard Z.; DeLong, Kristine L.

2017-01-01

We present new, monthly-resolved Sr/Ca-based sea-surface temperature (SST) records from two species of massive coral, Orbicella faveolata and Siderastrea siderea, from the Dry Tortugas National Park, FL, USA (DTNP). We combine these new records with published data from three additional S. siderea coral colonies to generate a 278-year long multi-species stacked Sr/Ca-SST record from DTNP. The composite record of mean annual Sr/Ca-SST at DTNP shows pronounced decadal-scale variability with a range of 1 to 2°C. Notable cool intervals in the Sr/Ca-derived SST lasting about a decade centered at ~1845, ~1935, and ~1965 are associated with reduced summer Sr/Ca-SST (monthly maxima < 29°C), and imply a reduction in the spatial extent of the Atlantic Warm Pool (AWP). There is significant coherence between the composite DTNP Sr/Ca-SST record and the Atlantic Multidecadal Oscillation (AMO) index, with the AMO lagging Sr/Ca-SST at DTNP by 9 years. Low frequency variability in the Gulf Stream surface transport, which originates near DTNP, may provide a link for the lagged relationship between multidecadal variability at DTNP and the AMO.

North Atlantic sub-decadal variability in climate models

NASA Astrophysics Data System (ADS)

Reintges, Annika; Martin, Thomas; Latif, Mojib; Park, Wonsun

2017-04-01

The North Atlantic Oscillation (NAO) is the dominant variability mode for the winter climate of the North Atlantic sector. During a positive (negative) NAO phase, the sea level pressure (SLP) difference between the subtropical Azores high and the subpolar Icelandic low is anomalously strong (weak). This affects, for example, temperature, precipitation, wind, and surface heat flux over the North Atlantic, and over large parts of Europe. In observations we find enhanced sub-decadal variability of the NAO index that goes along with a dipolar sea surface temperature (SST) pattern. The corresponding SLP and SST patterns are reproduced in a control experiment of the Kiel Climate Model (KCM). Large-scale air-sea interaction is suggested to be essential for the North Atlantic sub-decadal variability in the KCM. The Atlantic Meridional Overturning Circulation (AMOC) plays a key role, setting the timescale of the variability by providing a delayed negative feedback to the NAO. The interplay of the NAO and the AMOC on the sub-decadal timescale is further investigated in the CMIP5 model ensemble. For example, the average CMIP5 model AMOC pattern associated with sub-decadal variability is characterized by a deep-reaching dipolar structure, similar to the KCM's sub-decadal AMOC variability pattern. The results suggest that dynamical air-sea interactions are crucial to generate enhanced sub-decadal variability in the North Atlantic climate.

Long-term variations of SST and heat content in the Atlantic Ocean

NASA Astrophysics Data System (ADS)

Huonsou-gbo, Aubains; Servain, Jacques; Caniaux, Guy; Araujo, Moacyr; Bourlès, Bernard; Veleda, Doris

2015-04-01

Recent studies (eg. Wen et al. 2010; Servain et al. 2014) suggest that subsurface processes influence the interannual variability of sea surface temperature (SST) in the tropical Atlantic through the Meridional Overturning Circulation (MOC) with time lags of several months. In this study, we used observed SST and Ocean heat content to test such hypothesis during the period 1964-2013. First results indicate great similarities in the positive linear trends of monthly standardized anomalies of SST, upper ocean heat content (0-500m) and deeper ocean heat content (500-2000m) averaged over the whole Atlantic Ocean. Strong positive trends of SST and deeper heat content occurred in the equatorial Atlantic, while a strong positive trend of the upper heat content was observed in the northeast Atlantic. These positive trends were the highest during the last two decades. The lagged positive correlation patterns between upper heat content anomalies over the whole gridded Atlantic Ocean and SST anomalies averaged over the equatorial region (60°W-15°E; 10°N-10°S) show a slow temporal evolution, which is roughly in agreement with the upper MOC. More detailed works about the mechanism, as well as about the origin of the highest positive trend of the deeper heat content in the equatorial region, are presently under investigation. References Servain J., G. Caniaux, Y. K. Kouadio, M. J. McPhaden, M. Araujo (2014). Recent climatic trends in the tropical Atlantic. Climate Dynamics, Vol. 43, 3071-3089, DOI 10.1007/s00382-014-2168-7.

WES feedback and the Atlantic Meridional Mode: observations and CMIP5 comparisons

NASA Astrophysics Data System (ADS)

Amaya, Dillon J.; DeFlorio, Michael J.; Miller, Arthur J.; Xie, Shang-Ping

2017-09-01

The Atlantic Meridional Mode (AMM) is the dominant mode of tropical SST/wind coupled variability. Modeling studies have implicated wind-evaporation-SST (WES) feedback as the primary driver of the AMM's evolution across the Atlantic basin; however, a robust coupling of the SST and winds has not been shown in observations. This study examines observed AMM growth, propagation, and decay as a result of WES interactions. Investigation of an extended maximum covariance analysis shows that boreal wintertime atmospheric forcing generates positive SST anomalies (SSTA) through a reduction of surface evaporative cooling. When the AMM peaks in magnitude during spring and summer, upward latent heat flux anomalies occur over the warmest SSTs and act to dampen the initial forcing. In contrast, on the southwestern edge of the SSTA, SST-forced cross-equatorial flow reduces the strength of the climatological trade winds and provides an anomalous latent heat flux into the ocean, which causes southwestward propagation of the initial atmosphere-forced SSTA through WES dynamics. Additionally, the lead-lag relationship of the ocean and atmosphere indicates a transition from an atmosphere-forcing-ocean regime in the northern subtropics to a highly coupled regime in the northern tropics that is not observed in the southern hemisphere. CMIP5 models poorly simulate the latitudinal transition from a one-way interaction to a two-way feedback, which may explain why they also struggle to reproduce spatially coherent interactions between tropical Atlantic SST and winds. This analysis provides valuable insight on how meridional modes act as links between extratropical and tropical variability and focuses future research aimed at improving climate model simulations.

The complexity of millennial-scale variability in southwestern Europe during MIS 11

NASA Astrophysics Data System (ADS)

Oliveira, Dulce; Desprat, Stéphanie; Rodrigues, Teresa; Naughton, Filipa; Hodell, David; Trigo, Ricardo; Rufino, Marta; Lopes, Cristina; Abrantes, Fátima; Sánchez Goñi, Maria Fernanda

2016-11-01

Climatic variability of Marine Isotope Stage (MIS) 11 is examined using a new high-resolution direct land-sea comparison from the SW Iberian margin Site U1385. This study, based on pollen and biomarker analyses, documents regional vegetation, terrestrial climate and sea surface temperature (SST) variability. Suborbital climate variability is revealed by a series of forest decline events suggesting repeated cooling and drying episodes in SW Iberia throughout MIS 11. Only the most severe events on land are coeval with SST decreases, under larger ice volume conditions. Our study shows that the diverse expression (magnitude, character and duration) of the millennial-scale cooling events in SW Europe relies on atmospheric and oceanic processes whose predominant role likely depends on baseline climate states. Repeated atmospheric shifts recalling the positive North Atlantic Oscillation mode, inducing dryness in SW Iberia without systematical SST changes, would prevail during low ice volume conditions. In contrast, disruption of the Atlantic meridional overturning circulation (AMOC), related to iceberg discharges, colder SST and increased hydrological regime, would be responsible for the coldest and driest episodes of prolonged duration in SW Europe.

Linkages of Remote Sea Surface Temperatures and Atlantic Tropical Cyclone Activity Mediated by the African Monsoon

DOE Office of Scientific and Technical Information (OSTI.GOV)

Taraphdar, Sourav; Leung, Lai-Yung R.; Hagos, Samson M.

2015-01-28

Warm sea surface temperatures (SSTs) in North Atlantic and Mediterranean (NAMED) can influence tropical cyclone (TC) activity in the tropical East Atlantic by modulating summer convection over western Africa. Analysis of 30 years of observations show that the NAMED SST is linked to a strengthening of the Saharan heat low and enhancement of moisture and moist static energy in the lower atmosphere over West Africa, which favors a northward displacement of the monsoonal front. These processes also lead to a northward shift of the African easterly jet that introduces an anomalous positive vorticity from western Africa to the main developmentmore » region (50W–20E; 10N–20N) of Atlantic TC. By modulating multiple processes associated with the African monsoon, this study demonstrates that warm NAMED SST explains 8% of interannual variability of Atlantic TC frequency. Thus NAME SST may provide useful predictability for Atlantic TC activity on seasonal-to-interannual time scale.« less

Hurricane intensification along United States coast suppressed during active hurricane periods

NASA Astrophysics Data System (ADS)

Kossin, James P.

2017-01-01

The North Atlantic ocean/atmosphere environment exhibits pronounced interdecadal variability that is known to strongly modulate Atlantic hurricane activity. Variability in sea surface temperature (SST) is correlated with hurricane variability through its relationship with the genesis and thermodynamic potential intensity of hurricanes. Another key factor that governs the genesis and intensity of hurricanes is ambient environmental vertical wind shear (VWS). Warmer SSTs generally correlate with more frequent genesis and greater potential intensity, while VWS inhibits genesis and prevents any hurricanes that do form from reaching their potential intensity. When averaged over the main hurricane-development region in the Atlantic, SST and VWS co-vary inversely, so that the two factors act in concert to either enhance or inhibit basin-wide hurricane activity. Here I show, however, that conditions conducive to greater basin-wide Atlantic hurricane activity occur together with conditions for more probable weakening of hurricanes near the United States coast. Thus, the VWS and SST form a protective barrier along the United States coast during periods of heightened basin-wide hurricane activity. Conversely, during the most-recent period of basin-wide quiescence, hurricanes (and particularly major hurricanes) near the United States coast, although substantially less frequent, exhibited much greater variability in their rate of intensification, and were much more likely to intensify rapidly. Such heightened variability poses greater challenges to operational forecasting and, consequently, greater coastal risk during hurricane events.

Early summer southern China rainfall variability and its oceanic drivers

NASA Astrophysics Data System (ADS)

Li, Weijing; Ren, Hong-Chang; Zuo, Jinqing; Ren, Hong-Li

2018-06-01

Rainfall in southern China reaches its annual peak in early summer (May-June) with strong interannual variability. Using a combination of observational analysis and numerical modeling, the present study investigates the leading modes of this variability and its dynamic drivers. A zonal dipole pattern termed the southern China Dipole (SCD) is found to be the dominant feature in early summer during 1979-2014, and is closely related to a low-level anomalous anticyclone over the Philippine Sea (PSAC) and a Eurasian wave-train pattern over the mid-high latitudes. Linear regressions based on observations and numerical experiments using the CAM5 model suggest that the associated atmospheric circulation anomalies in early summer are linked to decaying El Niño-Southern Oscillation-like sea surface temperature (SST) anomalies in the tropical Pacific, basin-scale SST anomalies in the tropical Indian Ocean, and meridional tripole-like SST anomalies in the North Atlantic in the previous winter to early summer. The tropical Pacific and Indian Ocean SST anomalies primarily exert an impact on the SCD through changing the polarity of the PSAC, while the North Atlantic tripole-like SST anomalies mainly exert a downstream impact on the SCD by inducing a Eurasian wave-train pattern. The North Atlantic tripole-like SST anomalies also make a relatively weak contribution to the variations of the PSAC and SCD through a subtropical teleconnection. Modeling results indicate that the three-basin combined forcing has a greater impact on the SCD and associated circulation anomalies than the individual influence from any single oceanic basin.

North Atlantic SST Patterns and NAO Flavors

NASA Astrophysics Data System (ADS)

Rousi, E.; Rahmstorf, S.; Coumou, D.

2017-12-01

North Atlantic SST variability results from the interaction of atmospheric and oceanic processes. The North Atlantic Oscillation (NAO) drives changes in SST patterns but is also driven by them on certain time-scales. These interactions are not very well understood and might be affected by anthropogenic climate change. Paleo reconstructions indicate a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in recent decades leading to a pronounced cold anomaly ("cold blob") in the North Atlantic (Rahmstorf et al., 2015). The latter may favor NAO to be in its negative mode. In this work, sea surface temperature (SST) patterns are studied in relation to NAO variations, with the aim of discovering preferred states and understanding their interactions. SST patterns are analyzed with Self-Organizing Maps (SOM), a clustering technique that helps identify different spatial patterns and their temporal evolution. NAO flavors refer to different longitudinal positions and tilts of the NAO action centers, also defined with SOMs. This way the limitations of the basic, index-based, NAO-definition are overcome, and the method handles different spatially shapes associated with NAO. Preliminary results show the existence of preferred combinations of SSTs and NAO flavors, which in turn affect weather and climate of Europe and North America. The possible influence of the cold blob on European weather is discussed.

Alleviating tropical Atlantic sector biases in the Kiel climate model by enhancing horizontal and vertical atmosphere model resolution: climatology and interannual variability

NASA Astrophysics Data System (ADS)

Harlaß, Jan; Latif, Mojib; Park, Wonsun

2018-04-01

We investigate the quality of simulating tropical Atlantic (TA) sector climatology and interannual variability in integrations of the Kiel climate model (KCM) with varying atmosphere model resolution. The ocean model resolution is kept fixed. A reasonable simulation of TA sector annual-mean climate, seasonal cycle and interannual variability can only be achieved at sufficiently high horizontal and vertical atmospheric resolution. Two major reasons for the improvements are identified. First, the western equatorial Atlantic westerly surface wind bias in spring can be largely eliminated, which is explained by a better representation of meridional and especially vertical zonal momentum transport. The enhanced atmospheric circulation along the equator in turn greatly improves the thermal structure of the upper equatorial Atlantic with much reduced warm sea surface temperature (SST) biases. Second, the coastline in the southeastern TA and steep orography are better resolved at high resolution, which improves wind structure and in turn reduces warm SST biases in the Benguela upwelling region. The strongly diminished wind and SST biases at high atmosphere model resolution allow for a more realistic latitudinal position of the intertropical convergence zone. Resulting stronger cross-equatorial winds, in conjunction with a shallower thermocline, enable a rapid cold tongue development in the eastern TA in boreal spring. This enables simulation of realistic interannual SST variability and its seasonal phase locking in the KCM, which primarily is the result of a stronger thermocline feedback. Our findings suggest that enhanced atmospheric resolution, both vertical and horizontal, could be a key to achieving more realistic simulation of TA climatology and interannual variability in climate models.

Last interglacial temperature seasonality reconstructed from tropical Atlantic corals

NASA Astrophysics Data System (ADS)

Brocas, William M.; Felis, Thomas; Obert, J. Christina; Gierz, Paul; Lohmann, Gerrit; Scholz, Denis; Kölling, Martin; Scheffers, Sander R.

2016-09-01

Reconstructions of last interglacial (LIG, MIS 5e, ∼127-117 ka) climate offer insights into the natural response and variability of the climate system during a period partially analogous to future climate change scenarios. We present well preserved fossil corals (Diploria strigosa) recovered from the southern Caribbean island of Bonaire (Caribbean Netherlands). These have been precisely dated by the 230Th/U-method to between 130 and 120 ka ago. Annual banding of the coral skeleton enabled construction of time windows of monthly resolved strontium/calcium (Sr/Ca) temperature proxy records. In conjunction with a previously published 118 ka coral record, our eight records of up to 37 years in length, cover a total of 105 years within the LIG period. From these, sea surface temperature (SST) seasonality and variability in the tropical North Atlantic Ocean is reconstructed. We detect similar to modern SST seasonality of ∼2.9 °C during the early (130 ka) and the late LIG (120-118 ka). However, within the mid-LIG, a significantly higher than modern SST seasonality of 4.9 °C (at 126 ka) and 4.1 °C (at 124 ka) is observed. These findings are supported by climate model simulations and are consistent with the evolving amplitude of orbitally induced changes in seasonality of insolation throughout the LIG, irrespective of wider climatic instabilities that characterised this period. The climate model simulations suggest that the SST seasonality changes documented in our LIG coral Sr/Ca records are representative of larger regions within the tropical North Atlantic. These simulations also suggest that the reconstructed SST seasonality increase during the mid-LIG is caused primarily by summer warming. A 124 ka old coral documents, for the first time, evidence of decadal SST variability in the tropical North Atlantic during the LIG, akin to that observed in modern instrumental records.

Secular spring rainfall variability at local scale over Ethiopia: trend and associated dynamics

NASA Astrophysics Data System (ADS)

Tsidu, Gizaw Mengistu

2017-10-01

Spring rainfall secular variability is studied using observations, reanalysis, and model simulations. The joint coherent spatio-temporal secular variability of gridded monthly gauge rainfall over Ethiopia, ERA-Interim atmospheric variables and sea surface temperature (SST) from Hadley Centre Sea Ice and SST (HadISST) data set is extracted using multi-taper method singular value decomposition (MTM-SVD). The contemporaneous associations are further examined using partial Granger causality to determine presence of causal linkage between any of the climate variables. This analysis reveals that only the northwestern Indian Ocean secular SST anomaly has direct causal links with spring rainfall over Ethiopia and mean sea level pressure (MSLP) over Africa inspite of the strong secular covariance of spring rainfall, SST in parts of subtropical Pacific, Atlantic, Indian Ocean and MSLP. High secular rainfall variance and statistically significant linear trend show consistently that there is a massive decline in spring rain over southern Ethiopia. This happened concurrently with significant buildup of MSLP over East Africa, northeastern Africa including parts of the Arabian Peninsula, some parts of central Africa and SST warming over all ocean basins with the exception of the ENSO regions. The east-west pressure gradient in response to the Indian Ocean warming led to secular southeasterly winds over the Arabian Sea, easterly over central Africa and equatorial Atlantic. These flows weakened climatological northeasterly flow over the Arabian Sea and southwesterly flow over equatorial Atlantic and Congo basins which supply moisture into the eastern Africa regions in spring. The secular divergent flow at low level is concurrent with upper level convergence due to the easterly secular anomalous flow. The mechanisms through which the northwestern Indian Ocean secular SST anomaly modulates rainfall are further explored in the context of East Africa using a simplified atmospheric general circulation model (AGCM) coupled to mixed-layer oceanic model. The rainfall anomaly (with respect to control simulation), forced by the northwestern Indian Ocean secular SST anomaly and averaged over the 30-year period, exhibits prevalence of dry conditions over East and equatorial Africa in agreement with observation. The atmospheric response to secular SST warming anomaly led to divergent flow at low levels and subsidence at the upper troposphere over regions north of 5° S on the continent and vice versa over the Indian Ocean. This surface difluence over East Africa, in addition to its role in suppressing convective activity, deprives the region of moisture supply from the Indian Ocean as well as the Atlantic and Congo basins.

An out of phase coupling between the atmosphere and the ocean over the North Atlantic Ocean

NASA Astrophysics Data System (ADS)

Ribera, Pedro; Ordoñez, Paulina; Gallego, David; Peña-Ortiz, Cristina

2017-04-01

An oscillation band, with a period ranging between 40 and 60 years, has been identified as the most intense signal over the North Atlantic Ocean using several oceanic and atmospheric reanalyses between 1856 and the present. This signal represents the Atlantic Multidecadal Oscillation, an oscillation between warmer and colder than normal conditions in SST. Simultaneously, those changes in SST are accompanied by changes in atmospheric conditions represented by surface pressure, temperature and circulation. In fact, the evolution of the surface pressure pattern along this oscillation shows a North Atlantic Oscillation-like pattern, suggesting the existence of an out of phase coupling between atmospheric and oceanic conditions. Further analysis shows that the evolution of the oceanic SST distribution modifies atmospheric baroclinic conditions in the mid to high latitudes of the North Atlantic and leads the atmospheric variability by 6-7 years. If AMO represents the oceanic conditons and NAO represents the atmospheric variability then it could be said that AMO of one sign leads NAO of the opposite sign with a lag of 6-7 years. On the other hand, the evolution of atmospheric conditions, represented by pressure distribution patterns, favors atmospheric circulation anomalies and induces a heat advection which tends to change the sign of the existing SST distribution and oceanic conditions with a lag of 16-17 years. In this case, NAO of one sign leads AMO of the same sign with a lag of 16-17 years.

Tropical Atlantic-Korea teleconnection pattern during boreal summer season

NASA Astrophysics Data System (ADS)

Ham, Yoo-Geun; Chikamoto, Yoshimitsu; Kug, Jong-Seong; Kimoto, Masahide; Mochizuki, Takashi

2017-10-01

The remote impact of tropical Atlantic sea surface temperature (SST) variability on Korean summer precipitation is examined based on observational data analysis along with the idealized and hindcast model experiments. Observations show a significant correlation (i.e. 0.64) between Korean precipitation anomalies (averaged over 120-130°E, 35-40°N) and the tropical Atlantic SST index (averaged over 60°W-20°E, 30°S-30°N) during the June-July-August (JJA) season for the 1979-2010 period. Our observational analysis and partial-data assimilation experiments using the coupled general circulation model demonstrate that tropical Atlantic SST warming induces the equatorial low-level easterly over the western Pacific through a reorganization of the global Walker Circulation, causing a decreased precipitation over the off-equatorial western Pacific. As a Gill-type response to this diabatic forcing, an anomalous low-level anticyclonic circulation appears over the Philippine Sea, which transports wet air from the tropics to East Asia through low-level southerly, resulting an enhanced precipitation in the Korean peninsula. Multi-model hindcast experiments also show that predictive skills of Korean summer precipitation are improved by utilizing predictions of tropical Atlantic SST anomalies as a predictor for Korean precipitation anomalies.

Centennial-scale links between Atlantic Ocean dynamics and hydroclimate over the last 4400 years: Insights from the northern Gulf of Mexico

NASA Astrophysics Data System (ADS)

Thirumalai, K.; Quinn, T. M.; Okumura, Y.; Richey, J. N.; Partin, J. W.; Poore, R. Z.

2015-12-01

Surface circulation in the Atlantic Ocean is an important mediator of global climate and yet its variability is poorly constrained on centennial timescales. Changes in the Atlantic meridional overturning circulation (AMOC) have been implicated in late Holocene climate variability in the Western Hemisphere, although the relationship between AMOC variability and hydroclimate is uncertain due to the lack of sufficiently highly resolved proxy records. Here we present a replicated reconstruction of sea-surface temperature (SST) and salinity (SSS) from the Garrison Basin in the northern Gulf of Mexico (NGOM) spanning the last 4,400 years to better constrain past sea-surface conditions. We generated time series of paired Mg/Ca (SST proxy) and δ18O (SST and SSS proxy) variations in planktic foraminifer Globigerinoides ruber (white variety) from three multi-cores collected in 2010. Using a Monte Carlo-based technique we produce a stacked record from the three multi-cores and constrain analytical, calibration, chronological, and sampling uncertainties. We apply this technique to existing paired Mg/Ca- δ18O studies in the Gulf of Mexico and Atlantic Ocean to facilitate comparison between time-uncertain proxy reconstructions. The Garrison Basin stack exhibits large centennial-scale variability (σSST~0.6°C; δ18Osw~0.17‰) and indicates a substantially cool (0.9±0.5°C) and fresh (0.26±0.1‰) Little Ice Age (LIA; 1450-1850 A.D.), corroborating extant records from the Gulf of Mexico. Focusing on the last millennium, we analyze a suite of oceanic and terrestrial proxy records to demonstrate a centennial-scale link between salt advection in the Atlantic Ocean, a diagnostic parameter of ocean circulation, and hydroclimate in the adjacent continents. The ensuing multiproxy relationships seem to be consistent with spatial field correlations of limited salinity and rainfall instrumental/reanalysis data, which suggest that NGOM salinity varies with large-scale Atlantic Ocean circulation and continental precipitation. Our results imply significant centennial-scale variability over the late Holocene and are consistent with limited observational analysis indicating a slowdown of AMOC during the LIA.

The Relationships between Tropical Pacific and Atlantic SST and Northeast Brazil Monthly Precipitation.

NASA Astrophysics Data System (ADS)

Bertacchi Uvo, Cintia; Repelli, Carlos A.; Zebiak, Stephen E.; Kushnir, Yochanan

1998-04-01

The monthly patterns of northeast Brazil (NEB) precipitation are analyzed in relation to sea surface temperature (SST) in the tropical Pacific and Atlantic Oceans, using singular value decomposition. It is found that the relationships between precipitation and SST in both basins vary considerably throughout the rainy season (February-May). In January, equatorial Pacific SST is weakly correlated with precipitation in small areas of southern NEB, but Atlantic SST shows no significant correlation with regional precipitation. In February, Pacific SST is not well related to precipitation, but south equatorial Atlantic SST is positively correlated with precipitation over the northern Nordeste, the latter most likely reflecting an anomalously early (or late) southward migration of the ITCZ precipitation zone. During March, equatorial Pacific SST is negatively correlated with Nordeste precipitation, but no consistent relationship between precipitation and Atlantic SST is found. Atlantic SST-precipitation correlations for April and May are the strongest found among all months or either ocean. Precipitation in the Nordeste is positively correlated with SST in the south tropical Atlantic and negatively correlated with SST in the north tropical Atlantic. These relationships are strong enough to determine the structure of the seasonal mean SST-precipitation correlations, even though the corresponding patterns for the earlier months of the season are quite different. Pacific SST-precipitation correlations for April and May are similar to those for March. Extreme wet (dry) years for the Nordeste occur when both Pacific and Atlantic SST patterns for April and May occur simultaneously. A separate analysis reinforces previous findings in showing that SST in the tropical Pacific and the northern tropical Atlantic are positively correlated and that tropical Pacific-south Atlantic correlations are negligible.Time-lagged analyses show the potential for forecasting either seasonal mean or monthly precipitation patterns with some degree of skill. In some instances, individual monthly mean SST versus seasonal mean (February-May) precipitation relationships differ considerably from the corresponding monthly SST versus monthly precipitation relationships. It is argued that the seasonal mean relationships result from the relatively strong monthly relationships toward the end of the season, combined with the considerable persistence of SST in both oceans.

Subpolar Atlantic cooling and North American east coast warming linked to AMOC slowdown

NASA Astrophysics Data System (ADS)

Rahmstorf, Stefan; Caesar, Levke; Feulner, Georg; Saba, Vincent

2017-04-01

Reconstructing the history of the Atlantic Meridional Overturning Circulation (AMOC) is difficult due to the limited availability of data. One approach has been to use instrumental and proxy data for sea surface temperature (SST), taking multi-decadal and longer SST variations in the subpolar gyre region as indicator for AMOC changes [Rahmstorf et al., 2015]. Recent high-resolution global climate model results [Saba et al., 2016] as well as dynamical theory and conceptual modelling [Zhang and Vallis, 2007] suggest that an AMOC weakening will not only cool the subpolar Atlantic but simultaneously warm the Northwest Atlantic between Cape Hatteras and Nova Scotia, thus providing a characteristic SST pattern associated with AMOC variations. We analyse sea surface temperature (SST) observations from this region together with high-resolution climate model simulations to better understand the linkages of SST variations to AMOC variability and to provide further evidence for an ongoing AMOC slowdown. References Rahmstorf, S., J. E. Box, G. Feulner, M. E. Mann, A. Robinson, S. Rutherford, and E. J. Schaffernicht (2015), Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Nature Climate Change, 5(5), 475-480, doi: 10.1038/nclimate2554. Saba, V. S., et al. (2016), Enhanced warming of the Northwest Atlantic Ocean under climate change, Journal of Geophysical Research-Oceans, 121(1), 118-132, doi: 10.1002/2015JC011346. Zhang, R., and G. K. Vallis (2007), The Role of Bottom Vortex Stretching on the Path of the North Atlantic Western Boundary Current and on the Northern Recirculation Gyre, Journal of Physical Oceanography, 37(8), 2053-2080, doi: 10.1175/jpo3102.1.

Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region

NASA Astrophysics Data System (ADS)

Fontaine, B.; Janicot, Serge; Roucou, P.

This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes of variability in the tropical Atlantic and some climate anomalies over the tropical 120°W-60°W region using selected historical files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System on the period 1968-1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress data over the tropical Atlantic (30°N-20°S) expresses a modulation in the thermal transequatorial gradient of SST anomalies conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall. It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal well observed in the last 20-30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific. This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the north-south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations: anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level pressure over West Africa.

North Tropical Atlantic Climate Variability and Model Biases

NASA Astrophysics Data System (ADS)

Yang, Y.

2017-12-01

Remote forcing from El Niño-Southern Oscillation (ENSO) and local ocean-atmosphere feedback are important for climate variability over the North Tropical Atlantic. These two factors are extracted by the ensemble mean and inter-member difference of a 10-member Pacific Ocean-Global Atmosphere (POGA) experiment, in which sea surface temperatures (SSTs) are restored to the observed anomalies over the tropical Pacific but fully coupled to the atmosphere elsewhere. POGA reasonably captures main features of observed North Tropical Atlantic variability. ENSO forced and local North Tropical Atlantic modes (NTAMs) develop with wind-evaporation-SST feedback, explaining one third and two thirds of total variance respectively. Notable biases, however, exist. The seasonality of the simulated NTAM is delayed by one month, due to the late development of the North Atlantic Oscillation (NAO) in the model. A spurious band of enhanced sea surface temperature (SST) variance (SBEV) is identified over the northern equatorial Atlantic in POGA and 14 out of 23 CMIP5 models. The SBEV is especially pronounced in boreal spring and due to the combined effect of both anomalous atmospheric thermal forcing and oceanic vertical upwelling. While the tropical North Atlantic variability is only weakly correlated with the Atlantic Zonal Mode (AZM) in observations, the SBEV in CMIP5 produces conditions that drive and intensify the AZM variability via triggering the Bjerknes feedback. This partially explains why AZM is strong in some CMIP5 models even though the equatorial cold tongue and easterly trades are biased low.

The effect of atmospheric variability at intra-seasonal time scale on the SST of the Southwestern Atlantic Continental Shelf

NASA Astrophysics Data System (ADS)

Simionato, Claudia; Clara, Moira Luz; Jaureguizar, Andrés

2017-04-01

The Southwestern Atlantic Continental Shelf is characterized by large SST variability which origin remains unknown. In this work, we use blended SST data provided by NOAA CoastWatch Program, which combine the information coming from infrared and microwave sensors to provide daily images of an intermediate spatial resolution (11 km) with a noise floor of less than 0.2 °C. The data base starts at the middle of 2002, when an increase in signal variance is observed due to the fact that the Advanced Microwave Scanning Radiometer became available and as a consequence to its near all-weather coverage. Several years of observations are thus available, and even though the temporal and spatial resolution of these data is intermediate, they are reasonable for observing and characterizing the most significant patterns of SST variability in the (atmospheric) synoptic to intra-seasonal time scales, so as to help on understanding the physical processes which occur in the area and their forcing mechanisms. As we hypothesize that most of the variability in those time scales is wind forced, the study is complemented with the use of atmospheric observations -coming from remote sensing and reanalysis-. To perform the analysis, the long-term trend, inter-annual and seasonal variability are subtracted to the SST data to obtain the signal on intra-seasonal time scales. Then, Principal Components (EOF) analysis is applied to the data and composites of SST and several meteorological variables (wind, sea level pressure, air temperature, OLR, etc.) are computed for the days when the leading modes are active. It is found that the first three modes account for more than 70% of the variance. Modes 1 and 2 seem to be related to atmospheric waves generated in the tropical Pacific. Those waves, through atmospheric teleconnections, affect the SST on the southwestern South Atlantic Continental Shelf very rapidly. The oceanic anomalies exceed 0.7°C and are quite persistent. Mode 2 seems to be forced by an atmospheric 3-4 mode and might be related to SAM. Besides showing the impact of intra-seasonal atmospheric variability on the ocean at mid latitudes, the knowledge of the connections between the ocean and the atmosphere could aid on improving the ocean predictability on those time scales.

A 80-Year Long Coral-Based Temperature Reconstruction for the Last Interglacial from Northern Hispaniola

NASA Astrophysics Data System (ADS)

DeLong, K. L.; Ouellette, G., Jr.; Goodkin, N.; Martin, E. R.; Rosendahl, D. H.; Taylor, F. W.; WU, C. C.; Shen, C. C.

2016-12-01

The Last Interglacial (LIG; 117-128 ka), when sea level was 6 m higher than today, can serve as an analog for future climate scenarios, yet minimal paleoclimatic information exists with seasonal to decadal resolution. The island of Hispaniola is a particularly desirable site for producing sea surface temperature (SST) reconstructions, as it displays significant correlations with SST and precipitation anomalies for much of the tropical and North Atlantic Ocean, and Hispaniola is located in the northern sector of the Atlantic Warm Pool (AWP), a primary moisture source for precipitation in the Americas. Here we present an early LIG (128,626 ±438 (2σ) years) monthly-resolved coral Sr/Ca-SST reconstruction from a well-preserved Siderastrea siderea subfossil coral spanning 80 years from the northern coast of Hispaniola (19.913ºN, 70.925ºW). We compare our LIG SST reconstruction with coral Sr/Ca-SST from three modern coral microatolls of the same species, the longest spanning 80 years, recovered near Port-au Prince, Haiti (18.479070°N, 72.668659°W) after the 2010 Haiti earthquake, as well as a 125 ka LIG model simulation spanning 300 years. We find similar mean SST for the LIG (27.4ºC) and modern corals (27.9ºC) that is consistent with MIS 5e reconstructions in the tropical oceans (27.3-29.6ºC); however, these reconstructions are warmer than the LIG model mean SST for our study site (25.6ºC). Seasonal variability is similar (1.5ºC LIG, 1.0-1.7ºC modern) consistent with the findings of LIG coral reconstructions using the tropical Atlantic coral Diploria strigosa and with climate model simulations suggesting orbital insolation changes driving LIG seasonality. However, our LIG coral contains decadal variability (1.7-3.1ºC) not evident in the shorter LIG coral reconstructions or modern SST records and coral reconstructions yet are present in the LIG model simulation for our study site. This decadal variability may reflect variations in the northern extent of the AWP on decadal time scales, which may vary trade wind strength, westward moisture transport to the Americas, and precipitation in the Atlantic.

Joint spatiotemporal variability of global sea surface temperatures and global Palmer drought severity index values

USGS Publications Warehouse

Apipattanavis, S.; McCabe, G.J.; Rajagopalan, B.; Gangopadhyay, S.

2009-01-01

Dominant modes of individual and joint variability in global sea surface temperatures (SST) and global Palmer drought severity index (PDSI) values for the twentieth century are identified through a multivariate frequency domain singular value decomposition. This analysis indicates that a secular trend and variability related to the El Niño–Southern Oscillation (ENSO) are the dominant modes of variance shared among the global datasets. For the SST data the secular trend corresponds to a positive trend in Indian Ocean and South Atlantic SSTs, and a negative trend in North Pacific and North Atlantic SSTs. The ENSO reconstruction shows a strong signal in the tropical Pacific, North Pacific, and Indian Ocean regions. For the PDSI data, the secular trend reconstruction shows high amplitudes over central Africa including the Sahel, whereas the regions with strong ENSO amplitudes in PDSI are the southwestern and northwestern United States, South Africa, northeastern Brazil, central Africa, the Indian subcontinent, and Australia. An additional significant frequency, multidecadal variability, is identified for the Northern Hemisphere. This multidecadal frequency appears to be related to the Atlantic multidecadal oscillation (AMO). The multidecadal frequency is statistically significant in the Northern Hemisphere SST data, but is statistically nonsignificant in the PDSI data.

Interannual and Decadal Variability of Summer Rainfall over South America

NASA Technical Reports Server (NTRS)

Zhou, Jiayu; Lau, K.-M.

1999-01-01

Using the CPC (Climate Prediction Center) Merged Analysis of Precipitation product along with the Goddard Earth Observing System reanalysis and the Climate Analysis Center sea surface temperature (SST) data, we conduct a diagnostic study of the interannual and decadal scale variability of summer rainfall over South America. Results show three leading modes of rainfall variation identified with interannual, decadal, and long-term trend variability. Together, these modes explain more than half the total variance. The first mode is highly correlated with El Nino/southern oscillation (ENSO), showing severe drought over Northeast Brazil and copious rainfall over the Ecuador coast and the area of Uruguay-Southern Brazil in El Nino years. This pattern is attributed to the large scale zonal shift of the Walker circulation and local Hadley cell anomaly induced by positive (negative) SST anomaly over the eastern (western) equatorial Pacific. In El Nino years, two convective belts indicated by upper tropospheric velocity potential trough and mid-tropospheric rising motion, which are somewhat symmetric about the equator, extend toward the northeast and the southeast into the tropical North and South Atlantic respectively. Sandwiched between the ascent is a region of descending motion over Northeast Brazil. The southern branch of the anomalous Hadley cell is dynamically linked to the increase of rainfall over Uruguay-Southern Brazil. The regional response of anomalous circulation shows a stronger South American summer monsoon and an enhanced (weakened) subtropical high over the South Atlantic (South Pacific) Ocean. The decadal variation displays a meridional shift of the Intertropical Convergence Zone (ITCZ), which is tie to the anomalous cross-equatorial SST gradient over the Atlantic and the eastern Pacific. In conjunction with this mode is a large scale mass swing between the polar regions and midlatitudes in both hemispheres. Over the South Atlantic and the South Pacific, the changes of the strength of the subtropical high and the associated surface wind are dynamically consistent with the distribution of local SST anomalies, suggesting the importance of the atmospheric forcing in the decadal time scale. The decadal mode also presents a weak summer monsoon in its positive phase, which reduces the moisture supply from the equatorial Atlantic and the Amazon Basin and results in negative rainfall anomalies over the central Andes and Gran Chaco. The long-term trend shows decrease of rainfall from the northwest coast to the southeast subtropical region and a southward shift of Atlantic ITCZ that leads to increased rainfall over northern and eastern Brazil. Our result shows a close link of this mode to the observed SST warming trend over the subtropical South Atlantic and a remote connection to the interdecadal SST variation over the extratropical North Atlantic found in previous studies.

Enhanced Biennial Variability in the Pacific due to Atlantic Capacitor Effect after the Early 1990s

NASA Astrophysics Data System (ADS)

WANG, L.; Yu, J. Y.; Paek, H.

2016-12-01

The El Niño-Southern Oscillation (ENSO) and Pacific subtropical highs (PSHs) have major impacts on social and ecological systems through their influences on severe natural hazards including tropical storms, coastal erosions, droughts and floods. The ability to forecast ENSO and PSHs requires an understanding of the underlying physical mechanisms that drive their variability. Here we present an Atlantic capacitor effect mechanism to suggest the Atlantic as a key pacemaker of the biennial variability in the Pacific including ENSO and PSHs in recent decades, while the pacemaker was previously considered to be mainly lied within the Pacific or Indian Oceans. The "charging" (i.e., ENSO imprinting the North Tropical Atlantic (NTA) sea surface temperature (SST) via an atmospheric bridge mechanism) and "discharging" (i.e., the NTA SST triggering the following ENSO via a subtropical teleconnection mechanism) process works alternately, generating the biennial rhythmic changes in the Pacific. After the early-1990s, the positive phase of the Atlantic Multidecadal Oscillation and global warming provides more favorable background states over the NTA that enable the Atlantic capacitor effect to operate more efficiently, giving rise to enhanced biennial variability in the Pacific which may increase the occurrence frequency of severe natural hazard events. The results highlight the increasing important role of the Atlantic-Pacific coupling as an important pacemaker of the ENSO cycle in recent decades.

Mg/Ca Ratios in Coralline Red Algae as Temperature Proxies for Reconstructing Labrador Current Variability

NASA Astrophysics Data System (ADS)

Gamboa, G.; Hetzinger, S.; Halfar, J.; Zack, T.; Kunz, B.; Adey, W.

2009-05-01

Marine ecosystems and fishery productivity in the Northwestern Atlantic have been considerably affected by regional climate and oceanographic changes. Fluctuations of North Atlantic marine climate have been linked in part to a dominant pattern of atmospheric circulation known as the North Atlantic Oscillation, which has a strong influence on transport variability of the Labrador Current (LC). The cold LC originates in the Labrador Sea and flows southbound along the Eastern Canadian coastline causing an important cooling effect on marine waters off the Canadian Atlantic provinces. Although interdecadal and interannual variability of sea surface temperatures (SST) in the LC system have been documented, a long-term pattern has not been identified. In order to better understand the observed ecosystem changes and their relationship with climate variability in the Northwestern Atlantic, a century-scale reconstruction of spatial and temporal variations of the LC is needed. This, however, requires reliable long-term and high-resolution SST records, which are not available from short instrumental observations. Here we present the first century-scale SST reconstructions from the Northwest Atlantic using long-lived coralline red algae. Coralline red algae have a high-Mg calcite skeleton, live in shallow water worldwide and develop annual growth bands. It has previously been demonstrated that subannual resolution SSTs can be obtained from coralline red algal Mg/Ca ratios, a commonly used paleotemperature proxy. Specimens of the long-lived coralline red algae Clathromorphum compactum were collected alive in August 2008 along a latitudinal transect spanning the southern extent of LC flow in Nova Scotia and Newfoundland. This collection is supplemented with specimens from the same region collected in the 1960's. In order to reconstruct spatial and temporal patterns of the LC, selected samples of C. compactum were analyzed for Mg/Ca using Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS). Mg/Ca ratios range from 0.048 to 0.138 (measured in weight %) and relate to water temperatures of -1 to 16°C. Age models were established by comparing annual growth increments (average increment width 350 microns/year) with Mg/Ca cycles. This yielded subannually-resolved Mg/Ca-based SST reconstructions spanning the past century.

Development of the Wintertime Sr/Ca-SST Record from Red Sea Corals as a Proxy for the North Atlantic Oscillation

NASA Astrophysics Data System (ADS)

Bernstein, W. N.; Hughen, K. A.

2009-12-01

The North Atlantic Oscillation (NAO) is one of the most pronounced and influential patterns in winter atmospheric circulation variability. This meridional redistribution of atmospheric mass across the Atlantic Ocean produces large changes in the intensity, number and direction of storms generated within the basin, and the regional climate of surrounding continents. The NAO exerts a significant impact on society, through influences on agriculture, fisheries, water management, energy generation and coastal development. NAO effects on climate extend from eastern North America across Europe to the eastern Mediterranean and Middle East. Changes in NAO behavior during the late 20th century have been linked to global warming; yet despite its importance, the causes and long-term patterns of NAO variability in the past remain poorly understood. In order to better predict the influence of the NAO on climate in the future, it is critical to examine multi-century NAO variability. The Red Sea is an excellent location from which to generate long NAO records for two reasons. First, patterns of wintertime sea surface temperature (SST) and salinity (SSS) in the Red Sea are highly correlated with NAO variability (Visbeck et al. 2001; Hurrell et al. 2003). Second, the tropical/subtropical Red Sea region contains fast growing long-lived massive Porites spp. corals with annually banded skeletons. These corals are ideal for generating well-dated high-resolution paleoclimatic records that extend well beyond the instrumental period. Here we present a study of winter SST and NAO variability in the Red sea region based on coral Sr/Ca data. In 2008, we collected multiple drill cores ranging in length from 1 to 4.1 meters from Porites corals at six sites spanning a large SST gradient. Sr/Ca measurements from multiple corals will be regressed against 23 years of satellite SST data, expanding the SST range over which we calibrate. A sampling resolution of 0.5mm will yield greater than bi-weekly temporal resolution for downcore SST reconstructions over the past 140 years, which will be used to evaluate the ability of the coral proxies to capture instrumental NAO variability. We expect that this winter Sr/Ca record will exhibit coherence with the NAO similar to that evident between Red Sea instrumental SST and the NAO index. Future work will involve construction of an NAO record back ~400 years, using the multi-core Sr/Ca-SST calibration applied to a combination of new records from modern and fossil coral material. This record will be examined to identify changes in NAO behavior as a function of frequency, and to compare frequency-dependent NAO variability between periods of relatively warm and cold hemispheric climate. This analysis will allow us to test the hypothesized link between NAO behavior and mean climate conditions, and if confirmed, improve predictions regarding the role of the NAO in impending climate change. References Hurrell, J. et al., 2003, in The North Atlantic Oscillation: Climatic Significance and Environmental Impact, 1-36 (A.G.U., Washington, D.C.). Visbeck, M. et al., 2001, Proc. Nat. Acad. Sci. 98, 12876-12877.

Observations of Local Positive Low Cloud Feedback Patterns and Their Role in Internal Variability and Climate Sensitivity

NASA Astrophysics Data System (ADS)

Yuan, Tianle; Oreopoulos, Lazaros; Platnick, Steven E.; Meyer, Kerry

2018-05-01

Modeling studies have shown that cloud feedbacks are sensitive to the spatial pattern of sea surface temperature (SST) anomalies, while cloud feedbacks themselves strongly influence the magnitude of SST anomalies. Observational counterparts to such patterned interactions are still needed. Here we show that distinct large-scale patterns of SST and low-cloud cover (LCC) emerge naturally from objective analyses of observations and demonstrate their close coupling in a positive local SST-LCC feedback loop that may be important for both internal variability and climate change. The two patterns that explain the maximum amount of covariance between SST and LCC correspond to the Interdecadal Pacific Oscillation and the Atlantic Multidecadal Oscillation, leading modes of multidecadal internal variability. Spatial patterns and time series of SST and LCC anomalies associated with both modes point to a strong positive local SST-LCC feedback. In many current climate models, our analyses suggest that SST-LCC feedback strength is too weak compared to observations. Modeled local SST-LCC feedback strength affects simulated internal variability so that stronger feedback produces more intense and more realistic patterns of internal variability. To the extent that the physics of the local positive SST-LCC feedback inferred from observed climate variability applies to future greenhouse warming, we anticipate significant amount of delayed warming because of SST-LCC feedback when anthropogenic SST warming eventually overwhelm the effects of internal variability that may mute anthropogenic warming over parts of the ocean. We postulate that many climate models may be underestimating both future warming and the magnitude of modeled internal variability because of their weak SST-LCC feedback.

Variability in pigment concentration in warm-core rings as determined by coastal zone color scanner satellite imagery from the Mid-Atlantic Bight

NASA Technical Reports Server (NTRS)

Garcia-Moliner, Graciela; Yoder, James A.

1994-01-01

A time series of coastal zone color scanner (CZCS) derived chlorophyll (CZCS-chl) and sea surface temperature (SST) satellite imagery was developed for the Mid-Atlantic Bight (MAB). Warm-core rings (WCR) were identified by both the warmer SST signal as well as the low pigment concentrations of their cores. The variation in pigment concentrations and SST observed in satellite imagery over the geographic range and life span of four WCRs is investigated. The hypotheses are that pigment concentration increase during the lifetime of the WCR is a response to processes such as convective overturn, upwelling, edge enhancement due to increased vertical mixing, active convergence, or lateral exchange. Empirical orthogonal function analysis (EOF) is used to investigate the relationship between SST and pigment patterns observed in the presence of a WCR. The first two EOF modes explain more than 80% of the variability observed in all four WCRs and in both (SST and pigment) data sets. The results of this study show that, at the synoptic scales of staellite data, the variability observed in the WCRs is greater at the periphery of the rings. These results show that advective entrainment, rather than processes at ring center (e.g., shoaling of the pycnocline/nutricline in response to frictional decay) or at the periphery due to other processes such as vertical mixing, is the mechanism responsible for the observed variability.

Role of the North Atlantic Ocean in Low Frequency Climate Variability

NASA Astrophysics Data System (ADS)

Danabasoglu, G.; Yeager, S. G.; Kim, W. M.; Castruccio, F. S.

2017-12-01

The Atlantic Ocean is a unique basin with its extensive, North - South overturning circulation, referred to as the Atlantic meridional overturning circulation (AMOC). AMOC is thought to represent the dynamical memory of the climate system, playing an important role in decadal and longer time scale climate variability as well as prediction of the earth's future climate on these time scales via its large heat and salt transports. This oceanic memory is communicated to the atmosphere primarily through the influence of persistent sea surface temperature (SST) variations. Indeed, many modeling studies suggest that ocean circulation, i.e., AMOC, is largely responsible for the creation of coherent SST variability in the North Atlantic, referred to as Atlantic Multidecadal Variability (AMV). AMV has been linked to many (multi)decadal climate variations in, e.g., Sahel and Brazilian rainfall, Atlantic hurricane activity, and Arctic sea-ice extent. In the absence of long, continuous observations, much of the evidence for the ocean's role in (multi)decadal variability comes from model simulations. Although models tend to agree on the role of the North Atlantic Oscillation in creating the density anomalies that proceed the changes in ocean circulation, model fidelity in representing variability characteristics, mechanisms, and air-sea interactions remains a serious concern. In particular, there is increasing evidence that models significantly underestimate low frequency variability in the North Atlantic compared to available observations. Such model deficiencies can amplify the relative influence of external or stochastic atmospheric forcing in generating (multi)decadal variability, i.e., AMV, at the expense of ocean dynamics. Here, a succinct overview of the current understanding of the (North) Atlantic Ocean's role on the regional and global climate, including some outstanding questions, will be presented. In addition, a few examples of the climate impacts of the AMV via atmospheric teleconnections from a set of coupled simulations, also considering the relative roles of its tropical and extratropical components, will be highlighted.

Last interglacial temperature seasonality reconstructed from tropical Atlantic corals

NASA Astrophysics Data System (ADS)

Felis, T.; Brocas, W.; Obert, J. C.; Gierz, P.; Lohmann, G.; Scholz, D.; Kölling, M.; Pfeiffer, M.; Scheffers, S. R.

2016-12-01

Reconstructions of last interglacial ( 127-117 ka) climate offer insights into the natural response and variability of the climate system during a period partially analogous to future climate change scenarios. However, the seasonal temperature changes of the tropical ocean are not well known for the last interglacial period. Here we present well preserved fossil corals (Diploria strigosa) recovered from the southern Caribbean island of Bonaire. These corals have been precisely dated by the 230Th/U-method to between 130 and 118 ka ago. Annual banding of the coral skeleton enabled construction of time windows of monthly resolved Sr/Ca temperature proxy records. Our eight coral records of up to 37 years in length cover a total of 105 years within the last interglacial period. From these coral records, sea surface temperature (SST) seasonality in the tropical North Atlantic Ocean is reconstructed. We detect similar to modern SST seasonality of 2.9 °C during the early (130 ka) and the late last interglacial (120 - 118 ka). However, within the mid-last interglacial, a significantly higher than modern SST seasonality of 4.9 °C (at 126 ka) and 4.1 °C (at 124 ka) is observed. These findings are supported by climate model simulations (COSMOS) and are consistent with the evolving amplitude of orbitally induced changes in seasonality of insolation throughout the last interglacial, irrespective of wider climatic instabilities that characterised this period, e.g. at 118 ka ago. The climate model simulations suggest that the SST seasonality changes documented in our last interglacial coral Sr/Ca records are representative of larger regions within the tropical North Atlantic. These simulations also suggest that the reconstructed SST seasonality increase during the mid-last interglacial is caused primarily by summer warming. Furthermore, a 124 ka old coral documents evidence of decadal SST variability in the tropical North Atlantic during the last interglacial, akin to that observed in modern instrumental records. Our results indicate that the dense theca walls of brain coral skeletons (e.g., Diploria strigosa) can provide robust seasonally resolved proxy records of tropical SST and reliable 230Th/U-ages for the last interglacial period.

Seasonal predictions of equatorial Atlantic SST in a low-resolution CGCM with surface heat flux correction

NASA Astrophysics Data System (ADS)

Dippe, Tina; Greatbatch, Richard; Ding, Hui

2016-04-01

The dominant mode of interannual variability in tropical Atlantic sea surface temperatures (SSTs) is the Atlantic Niño or Zonal Mode. Akin to the El Niño-Southern Oscillation in the Pacific sector, it is able to impact the climate both of the adjacent equatorial African continent and remote regions. Due to heavy biases in the mean state climate of the equatorial-to-subtropical Atlantic, however, most state-of-the-art coupled global climate models (CGCMs) are unable to realistically simulate equatorial Atlantic variability. In this study, the Kiel Climate Model (KCM) is used to investigate the impact of a simple bias alleviation technique on the predictability of equatorial Atlantic SSTs. Two sets of seasonal forecasting experiments are performed: An experiment using the standard KCM (STD), and an experiment with additional surface heat flux correction (FLX) that efficiently removes the SST bias from simulations. Initial conditions for both experiments are generated by the KCM run in partially coupled mode, a simple assimilation technique that forces the KCM with observed wind stress anomalies and preserves SST as a fully prognostic variable. Seasonal predictions for both sets of experiments are run four times yearly for 1981-2012. Results: Heat flux correction substantially improves the simulated variability in the initialization runs for boreal summer and fall (June-October). In boreal spring (March-May), however, neither the initialization runs of the STD or FLX-experiments are able to capture the observed variability. FLX-predictions show no consistent enhancement of skill relative to the predictions of the STD experiment over the course of the year. The skill of persistence forecasts is hardly beat by either of the two experiments in any season, limiting the usefulness of the few forecasts that show significant skill. However, FLX-forecasts initialized in May recover skill in July and August, the peak season of the Atlantic Niño (anomaly correlation coefficients of about 0.3). Further study is necessary to determine the mechanism that drives this potentially useful recovery.

Interannual rainfall variability in the Amazon basin and sea-surface temperatures in the equatorial Pacific and the tropical Atlantic Oceans

NASA Astrophysics Data System (ADS)

Ronchail, Josyane; Cochonneau, Gérard; Molinier, Michel; Guyot, Jean-Loup; Chaves, Adriana Goretti De Miranda; Guimarães, Valdemar; de Oliveira, Eurides

2002-11-01

Rainfall variability in the Amazon basin is studied in relation to sea-surface temperatures (SSTs) in the equatorial Pacific and the northern and southern tropical Atlantic during the 1977-99 period, using the HiBAm original rainfall data set and complementary cluster and composite analyses.The northeastern part of the basin, north of 5 °S and east of 60 °W, is significantly related with tropical SSTs: a rainier wet season is observed when the equatorial Pacific and the northern (southern) tropical Atlantic are anomalously cold (warm). A shorter and drier wet season is observed during El Niño events and negative rainfall anomalies are also significantly associated with a warm northern Atlantic in the austral autumn and a cold southern Atlantic in the spring. The northeastern Amazon rainfall anomalies are closely related with El Niño-southern oscillation during the whole year, whereas the relationships with the tropical Atlantic SST anomalies are mainly observed during the autumn. A time-space continuity is observed between El Niño-related rainfall anomalies in the northeastern Amazon, those in the northern Amazon and south-eastern Amazon, and those in northern South America and in the Nordeste of Brazil.A reinforcement of certain rainfall anomalies is observed when specific oceanic events combine. For instance, when El Niño and cold SSTs in the southern Atlantic are associated, very strong negative anomalies are observed in the whole northern Amazon basin. Nonetheless, the comparison of the cluster and the composite analyses results shows that the rainfall anomalies in the northeastern Amazon are not always associated with tropical SST anomalies.In the southern and western Amazon, significant tropical SST-related rainfall anomalies are very few and spatially variable. The precipitation origins differ from those of the northeastern Amazon: land temperature variability, extratropical perturbations and moisture advection are important rainfall factors, as well as SSTs. This could partially explain why: (a) the above-mentioned signals weaken or disappear, with the exception of the relative dryness that is observed at the peak of an El Niño event and during the dry season when northern Atlantic SSTs are warmer than usual; (b) rainfall anomalies tend to resemble those of southeastern South America, noticeably at the beginning and the end of El Niño and La Niña events; (c) some strong excesses of rain are not associated with any SST anomalies and merit further investigation.

Effects of Atlantic warm pool variability over climate of South America tropical transition zone

NASA Astrophysics Data System (ADS)

Ricaurte Villota, Constanza; Romero-Rodríguez, Deisy; Andrés Ordoñez-Zuñiga, Silvio; Murcia-Riaño, Magnolia; Coca-Domínguez, Oswaldo

2016-04-01

Colombia is located in the northwestern corner of South America in a climatically complex region due to the influence processes modulators of climate both the Pacific and Atlantic region, becoming in a transition zone between phenomena of northern and southern hemisphere. Variations in the climatic conditions of this region, especially rainfall, have been attributed to the influence of the El Nino Southern Oscillation (ENSO), but little is known about the interaction within Atlantic Ocean and specifically Caribbean Sea with the environmental conditions of this region. In this work We studied the influence of the Atlantic Warm Pool (AWP) on the Colombian Caribbean (CC) climate using data of Sea Surface Temperature (SST) between 1900 - 2014 from ERSST V4, compared with in situ data SIMAC (National System for Coral Reef Monitoring in Colombia - INVEMAR), rainfall between 1953-2013 of meteorological stations located at main airports in the Colombian Caribbean zone, administered by IDEAM, and winds data between 2003 - 2014 from WindSat sensor. The parameters analyzed showed spatial differences throughout the study area. SST anomalies, representing the variability of the AWP, showed to be associated with Multidecadal Atlantic Oscillation (AMO) and with the index of sea surface temperature of the North-tropical Atlantic (NTA), the variations was on 3 to 5 years on the ENSO scale and of approximately 11 years possibly related to solar cycles. Rainfall anomalies in the central and northern CC respond to changes in SST, while in the south zone these are not fully engage and show a high relationship with the ENSO. Finally, the winds also respond to changes in SST and showed a signal approximately 90 days possibly related to the Madden-Julian Oscillation, whose intensity depends on the CC region being analyzed. The results confirm that region is a transition zone in which operate several forcing, the variability of climate conditions is difficult to attribute only one, as ENSO, since the role of the AWP in the climate of this region and especially in the central part proves to be decisive, probably due to changes in moisture and heat flows transferred to the atmosphere.

Positive Low Cloud and Dust Feedbacks Amplify Tropical North Atlantic Multidecadal Variability

NASA Technical Reports Server (NTRS)

Yuan, Tianle; Oraiopoulos, Lazaros; Zelinka, Mark; Yu, Hongbin; Norris, Joel R.; Chin, Mian; Platnick, Steven; Meyer, Kerry

2016-01-01

The Atlantic Multidecadal Oscillation (AMO) is characterized by a horseshoe pattern of sea surface temperature (SST) anomalies and has a wide range of climatic impacts. While the tropical arm of AMO is responsible for many of these impacts, it is either too weak or completely absent in many climate model simulations. Here we show, using both observational and model evidence, that the radiative effect of positive low cloud and dust feedbacks is strong enough to generate the tropical arm of AMO, with the low cloud feedback more dominant. The feedbacks can be understood in a consistent dynamical framework: weakened tropical trade wind speed in response to a warm middle latitude SST anomaly reduces dust loading and low cloud fraction over the tropical Atlantic, which warms the tropical North Atlantic SST. Together they contribute to appearance of the tropical arm of AMO. Most current climate models miss both the critical wind speed response and two positive feedbacks though realistic simulations of them may be essential for many climatic studies related to the AMO.

The Global Warming Hiatus Tied to the North Atlantic Oscillation and Its Prediction

NASA Astrophysics Data System (ADS)

Li, J.; Sun, C.

2015-12-01

The twentieth century Northern Hemisphere mean surface temperature (NHT) is characterized by a multidecadal warming-cooling-warming pattern followed by a flat trend since about 2000 (recent warming hiatus). Here we demonstrate that the multidcadal variability in NHT including the recent warming hiatus is tied to the North Atlantic Oscillation (NAO) and the NAO is implicated as a useful predictor of NHT multidecadal variability. Observational analysis shows that the NAO leads both the detrended NHT and oceanic Atlantic Multidecadal Oscillation (AMO) by 15-20 years. Theoretical analysis illuminates that the NAO precedes NHT multidecadal variability through its delayed effect on the AMO due to the large thermal inertia associated with slow oceanic processes. The CCSM4 model is employed to investigate possible physical mechanisms. The positive NAO forces the strengthening of the Atlantic meridional overturning circulation (AMOC) and induces a basin-wide uniform sea surface temperature (SST) warming that corresponds to the AMO. The SST field exhibits a delayed response to the preceding enhanced AMOC, and shows a pattern similar to the North Atlantic tripole (NAT), with SST warming in the northern North Atlantic and cooling in the southern part. This SST pattern (negative NAT phase) may lead to an atmospheric response that resembles the negative NAO phase, and subsequently the oscillation proceeds, but in the opposite sense. Based on these mechanisms, a simple delayed oscillator model is established to explain the quasi-periodic multidecadal variability of the NAO. The magnitude of the NAO forcing of the AMOC/AMO and the time delay of the AMOC/AMO feedback are two key parameters of the delayed oscillator. For a given set of parameters, the quasi 60-year cycle of the NAO can be well predicted. This delayed oscillator model is useful for understanding of the oscillatory mechanism of the NAO, which has potential for decadal predictions as well as the interpretation of proxy data records. An NAO-based linear model is therefore established to predict the NHT, which gives an excellent hindcast for NHT in 1971-2011 with the recent flat trend well predicted. NHT in 2012-2027 is predicted to fall slightly over the next decades, due to the recent NAO decadal weakening that temporarily offsets the anthropogenically induced warming.

Forced vs unforced drivers of Atlantic SST variability - linking forced role to magnitude of aerosol forcing

NASA Astrophysics Data System (ADS)

Booth, B.; Dunstone, N.; Halloran, P. R.; Andrews, T.; Bellouin, N.; Martin, E. R.

2014-12-01

Historical variations in North Atlantic SSTs have been a key driver of regional climate change - linked to drought frequency in the Sahel, Amazon and American Mid-West, rainfall and heat waves in Europe and frequency of Atlantic tropical storms. Traditionally these SST variations were deemed to arise from internally generated ocean variability. We present results from recent studies (Booth et al, 2012, Dunstone, 2013) that identify a mechanism via which volcanic and industrial aerosols could explain a large fraction of observed Atlantic variability, and its associated climate impacts. This work has prompted a lot of subsequent discussion about the relative contribution of ocean generated and external forced variability in the Atlantic. Here we present new results, that extend this earlier work, by looking at forced variability in the CMIP5 modelling context. This provides new insights into the potential externally forced role aerosols may play in the real world. CMIP5 models that represent aerosol-cloud interactions tend to have stronger correlations to observed variations in SSTs, but disagree on the magnitude of forced variability that they explain. We can link this contribution to the magnitude of aerosol forcing in each of these models - a factor that is both dependent on the aerosol parameterisation and the representation of boundary layer cloud in this region. This suggests that whether aerosols have played a larger or smaller role in historical Atlantic variability is tied to whether aerosols have a larger or smaller aerosol forcing (particularly indirect) in the real world. This in turn suggests that benefits of reducing current aerosol uncertainty are likely to extend beyond better estimates of global forcing, to providing a clearer picture of the past aerosol driven role in historical regional climate change.

Forced and Internal Multi-Decadal Variability in the North Atlantic and their Climate Impacts

NASA Astrophysics Data System (ADS)

Ting, M.

2017-12-01

Atlantic Multidecadal Variability (AMV), a basin-wide North Atlantic sea surface temperature warming or cooling pattern varying on decadal and longer time scales, is one of the most important climate variations in the Atlantic basin. The AMV has shown to be associated with significant climate impacts regionally and globally, from Atlantic hurricane activities, frequency and severity of droughts across North America, as well as rainfall anomalies across the African Sahel and northeast Brazil. Despite the important impacts of the AMV, its mechanisms are not completely understood. In particular, it is not clear how much of the historical Atlantic SST fluctuations were forced by anthropogenic sources such as greenhouse warming and aerosol cooling, versus driven internally by changes in the coupled ocean-atmosphere processes in the Atlantic. Using climate models such as the NCAR large ensemble simulations, we were able to successfully separate the forced and internally generated North Atlantic sea surface temperature anomalies through a signal-to-noise maximizing Empirical Orthogonal Function (S/N EOF) analysis method. Two forced modes were identified with one representing a hemispherical symmetric mode and one asymmetric mode. The symmetric mode largely represents the greenhouse forced component while the asymmetric mode resembles the anthropogenic aerosol forcing. When statistically removing both of the forced modes, the residual multidecadal Atlantic SST variability shows a very similar structure as the AMV in the preindustrial simulation. The distinct climate impacts of each of these modes are also identified and the implications and challenges for decadal climate prediction will be discussed.

Mechanisms of northeastern Brazil rainfall anomalies due to Southern Tropical Atlantic variability

NASA Astrophysics Data System (ADS)

Neelin, J.; Su, H.

2004-05-01

Observational studies have shown that the rainfall anomalies in eastern equatorial South America, including Nordeste Brazil, have a positive correlation with tropical southern Atlantic sea surface temperature (SST) anomalies. Such relationships are reproduced in model simulations with the quasi-equilibrium tropical circulation model (QTCM), which includes a simple land model. A suite of model ensemble experiments is analysed using observed SST over the tropical oceans, the tropical Atlantic and the tropical southern Atlantic (30S-0), respectively (with climatological SST in the remainder of the oceans). Warm tropical south Atlantic SST anomalies yield positive precipitation anomalies over the Nordeste and the southern edge of the Atlantic marine intertropical convergence zone (ITCZ). Mechanisms associated with moisture variations are responsible for the land precipitation changes. Increases in moisture over the Atlantic cause positive anomalies in moisture advection, spreading increased moisture downwind. Where the basic state is far from the convective stability threshold, moisture changes have little effect, but the margins of the climatological convection zone are affected. The increased moisture supply due to advection is enhanced by increases in low-level convergence required by moist static energy balances. The moisture convergence term is several times larger, but experiments altering the moisture advection confirm that the feedback is initiated by wind acting on moisture gradient. This mechanism has several features in common with the recently published "upped-ante" mechanism for El Nino impacts on this region. In that case, the moisture gradient is initiated by warm free tropospheric temperature anomalies increasing the typical value of low-level moisture required to sustain convection in the convection zones. Both mechanisms suggest the usefulness of coordinating ocean and land in situ observations of boundary layer moisture.

Late Quaternary surface circulation in the east equatorial South Atlantic: Evidence from Alkenone sea surface temperatures

NASA Astrophysics Data System (ADS)

Schneider, Ralph R.; Müller, Peter J.; Ruhland, GöTz

1995-04-01

Angola Basin and Walvis Ridge records of past sea surface temperatures (SST) derived from the alkenone Uk37 index are used to reconstruct the surface circulation in the east equatorial South Atlantic for the last 200,000 years. Comparison of SST estimates from surface sediments between 5° and 20°S with modern SST data suggests that the alkenone temperatures represent annual mean values of the surface mixed layer. Alkenone-derived temperatures for the warm climatic maxima of the Holocene and the penultimate interglacial are 1 to 4°C higher than latest Holocene values. All records show glacial to interglacial differences of about 3.5°C in annual mean SST, which is about 1.5°C greater than the difference estimated by CLIMAP (1981) for the eastern Angola Basin. At the Walvis Ridge, significant SST variance is observed at all of the Earth's orbital periodicities. SST records from the Angola Basin vary predominantly at 23- and 100-kyr periodicities. For the precessional cycle, SST changes at the Walvis Ridge correspond to variations of boreal summer insolation over Africa and lead ice volume changes, suggesting that the east equatorial South Atlantic is sensitive to African monsoon intensity via trade-wind zonality. Angola Basin SST records lag those from the Walvis Ridge and the equatorial Atlantic by about 3 kyr. The comparison of Angola Basin and Walvis Ridge SST records implies that the Angola-Benguela Front (ABF) (currently at about 14-16°S) has remained fairly stationary between 12° and 20°S (the limits of our cores) during the last two glacial-interglacial cycles. The temperature contrast associated with the ABF exhibits a periodic 23-kyr variability which is coherent with changes in boreal summer insolation over Africa. These observations suggest that surface waters north of the present ABF have not directly responded to monsoon-modulated changes in the trade-wind vector, that the central field of zonally directed trades in the southern hemisphere was not shifted or extended northward by several degrees of latitude during glacials, and that a cyclonic gyre circulation has existed in the east equatorial South Atlantic over the last 200,000 years. This scenario contradicts former assumptions of glacial intensification of the Benguela Current into the eastern Angola Basin and increased coastal upwelling off Angola.

Atlantic Meridional Overturning Circulation Influence on North Atlantic Sector Surface Air Temperature and its Predictability in the Kiel Climate Model

NASA Astrophysics Data System (ADS)

Latif, M.

2017-12-01

We investigate the influence of the Atlantic Meridional Overturning Circulation (AMOC) on the North Atlantic sector surface air temperature (SAT) in two multi-millennial control integrations of the Kiel Climate Model (KCM). One model version employs a freshwater flux correction over the North Atlantic, while the other does not. A clear influence of the AMOC on North Atlantic sector SAT only is simulated in the corrected model that depicts much reduced upper ocean salinity and temperature biases in comparison to the uncorrected model. Further, the model with much reduced biases depicts significantly enhanced multiyear SAT predictability in the North Atlantic sector relative to the uncorrected model. The enhanced SAT predictability in the corrected model is due to a stronger and more variable AMOC and its enhanced influence on North Atlantic sea surface temperature (SST). Results obtained from preindustrial control integrations of models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) support the findings obtained from the KCM: models with large North Atlantic biases tend to have a weak AMOC influence on SST and exhibit a smaller SAT predictability over the North Atlantic sector.

Late Holocene sea level variability and Atlantic Meridional Overturning Circulation

USGS Publications Warehouse

Cronin, Thomas M.; Farmer, Jesse R.; Marzen, R. E.; Thomas, E.; Varekamp, J.C.

2014-01-01

Pre-twentieth century sea level (SL) variability remains poorly understood due to limits of tide gauge records, low temporal resolution of tidal marsh records, and regional anomalies caused by dynamic ocean processes, notably multidecadal changes in Atlantic Meridional Overturning Circulation (AMOC). We examined SL and AMOC variability along the eastern United States over the last 2000 years, using a SL curve constructed from proxy sea surface temperature (SST) records from Chesapeake Bay, and twentieth century SL-sea surface temperature (SST) relations derived from tide gauges and instrumental SST. The SL curve shows multidecadal-scale variability (20–30 years) during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), as well as the twentieth century. During these SL oscillations, short-term rates ranged from 2 to 4 mm yr−1, roughly similar to those of the last few decades. These oscillations likely represent internal modes of climate variability related to AMOC variability and originating at high latitudes, although the exact mechanisms remain unclear. Results imply that dynamic ocean changes, in addition to thermosteric, glacio-eustatic, or glacio-isostatic processes are an inherent part of SL variability in coastal regions, even during millennial-scale climate oscillations such as the MCA and LIA and should be factored into efforts that use tide gauges and tidal marsh sediments to understand global sea level rise.

An Eight-Century High-Resolution Paleoclimate Record From the Cariaco Basin: Baseline Variability and the 20th Century

NASA Astrophysics Data System (ADS)

Black, D. E.; Thunell, R. C.; Kaplan, A.; Tappa, E. J.; Peterson, L. C.

2007-12-01

The Cariaco Basin, Venezuela is well-positioned to record a detailed history of surface ocean changes along the southern margin of the Caribbean and the tropical Atlantic. Varved, high deposition rate sediments deposited under anoxic conditions and an abundance of well-preserved microfossils result in one of the few marine records capable of preserving evidence of interannual- to decadal-scale climate variability in the tropical Atlantic. Here we present Mg/Ca and stable oxygen isotope data with sub-decadal resolution derived from sediments deposited over the last 800 years. Mg/Ca measured on the planktic foraminifer Globigerina bulloides from a Cariaco Basin sediment core strongly correlates with spring (March-May) instrumental SSTs between AD 1870 and 1990. The long-term record displays a surprising amount of variability for a tropical location. The temperature swings are not necessarily related to local upwelling variability, but instead represent wider conditions in the Caribbean and western tropical Atlantic. The Mg/Ca-SST record also captures the decadal and multidecadal variability observed in global land and sea surface temperature anomalies, and correlates with Atlantic tropical storm and hurricane frequency over the late-19th and 20th centuries. On average, 20th century temperatures are not the warmest in the entire record, but they do show the largest increase in magnitude and fastest rate of SST change over the last eight hundred years. Stable oxygen isotope data also correlate well with instrumental SSTs, but not over the full instrumental record. Poor correlations with early instrumental SST data suggest a salinity overprint. However, reconstructing δ- water variability using combined Mg/Ca and δ18O data is not straightforward as the δ- water/salinity relationship varies seasonally in the Cariaco Basin. Comparisons with percent titanium data suggest intervals of both local and regional surface salinity changes over the length of the record.

Impact of tropical Atlantic sea-surface temperature biases on the simulated atmospheric circulation and precipitation over the Atlantic region: An ECHAM6 model study

NASA Astrophysics Data System (ADS)

Eichhorn, Astrid; Bader, Jürgen

2017-09-01

As many coupled atmosphere-ocean general circulation models, the coupled Earth System Model developed at the Max Planck Institute for Meteorology suffers from severe sea-surface temperature (SST) biases in the tropical Atlantic. We performed a set of SST sensitivity experiments with its atmospheric model component ECHAM6 to understand the impact of tropical Atlantic SST biases on atmospheric circulation and precipitation. The model was forced by a climatology of observed global SSTs to focus on simulated seasonal and annual mean state climate. Through the superposition of varying tropical Atlantic bias patterns extracted from the MPI-ESM on top of the control field, this study investigates the relevance of the seasonal variation and spatial structure of tropical Atlantic biases for the simulated response. Results show that the position and structure of the Intertropical Convergence Zone (ITCZ) across the Atlantic is significantly affected, exhibiting a dynamically forced shift of annual mean precipitation maximum to the east of the Atlantic basin as well as a southward shift of the oceanic rain belt. The SST-induced changes in the ITCZ in turn affect seasonal rainfall over adjacent continents. However not only the ITCZ position but also other effects arising from biases in tropical Atlantic SSTs, e.g. variations in the wind field, change the simulation of precipitation over land. The seasonal variation and spatial pattern of tropical Atlantic SST biases turns out to be crucial for the simulated atmospheric response and is essential for analyzing the contribution of SST biases to coupled model mean state biases. Our experiments show that MPI-ESM mean-state biases in the Atlantic sector are mainly driven by SST biases in the tropical Atlantic while teleconnections from other basins seem to play a minor role.

SSTs from Fossil Corals using Sr-U Thermometry

NASA Astrophysics Data System (ADS)

Cohen, A. L.; Alpert, A.; Soucy, A.; DeCarlo, T. M.; Vasquez-Bedoya, L. F.; Blanchon, P.; Oppo, D.; Gaetani, G. A.

2017-12-01

Earth's climate varies naturally on decadal through millennial timescales. Resolving and attributing the anthropogenic influence on climate therefore, requires accurate, continuous records that exceed the duration of the short observational dataset. Sea surface temperatures (SSTs) of warm tropical regions are especially important because the tropics are regions of deep atmospheric convection that redistribute heat and moisture. The skeletons of long-lived corals are valuable archives of tropical ocean temperature, yet the pre-instrumental SST evolution of the global tropical oceans remains poorly constrained. One reason is the limited lifespan of individual coral colonies, which seldom exceeds 150-200 years. Thus, extending SST records well beyond the observational period requires use of well-dated sub-fossil material but the current coral-based temperature proxy, Sr/Ca, is not well-suited for application to non-living material. The sensitivity of the Sr/Ca-SST relationship can vary from coral to coral, limiting the accuracy with which absolute temperature and trends can be interpreted from non-living corals. To overcome this constraint, we developed a new thermometer, Sr-U, based on a robust understanding of the processes responsible for colony-to-colony variability. Our Sr-U SST calibration is derived from three coral species representing two Atlantic and one Pacific site, validated against the instrumental record of SST and spanning a temperature range of 24.5 through 28.5 °C. We applied Sr-U to U-series dated fossil corals that grew on tropical Atlantic reefs during the Little Ice Age (1450-1650 AD) and Last Interglacial (122 000 yr BP). Our results show that SSTs in the region fluctuated within 1°C of modern values, with much of the late LIA slightly cooler and the LIG slightly warmer than late 20th century SSTs. Each continuous coral-based record spans multiple decades, enabling us to identify multi-decadal AMO-like variability as a persistent characteristic of tropical Atlantic variability.

Tropical Atlantic Impacts on the Decadal Climate Variability of the Tropical Ocean and Atmosphere.

NASA Astrophysics Data System (ADS)

Li, X.; Xie, S. P.; Gille, S. T.; Yoo, C.

2015-12-01

Previous studies revealed atmospheric bridges between the tropical Pacific, Atlantic, and Indian Ocean. In particular, several recent works indicate that the Atlantic sea surface temperature (SST) may contribute to the climate variability over the equatorial Pacific. Inspired by these studies, our work aims at investigating the impact of the tropical Atlantic on the entire tropical climate system, and uncovering the physical dynamics under these tropical teleconnections. We first performed a 'pacemaker' simulation by restoring the satellite era tropical Atlantic SST changes in a fully coupled model - the CESM1. Results reveal that the Atlantic warming heats the Indo-Western Pacific and cools the Eastern Pacific, enhances the Walker circulation and drives the subsurface Pacific to a La Niña mode, contributing to 60-70% of the above tropical changes in the past 30 years. The same pan-tropical teleconnections have been validated by the statistics of observations and 106 CMIP5 control simulations. We then used a hierarchy of atmospheric and oceanic models with different complexities, to single out the roles of atmospheric dynamics, atmosphere-ocean fluxes, and oceanic dynamics in these teleconnections. With these simulations we established a two-step mechanism as shown in the schematic figure: 1) Atlantic warming generates an atmospheric deep convection and induces easterly wind anomalies over the Indo-Western Pacific in the form of Kelvin waves, and westerly wind anomalies over the eastern equatorial Pacific as Rossby waves, in line with Gill's solution. This circulation changes warms the Indo-Western Pacific and cools the Eastern Pacific with the wind-evaporation-SST effect, forming a temperature gradient over the Indo-Pacific basins. 2) The temperature gradient further generates a secondary atmospheric deep convection, which reinforces the easterly wind anomalies over the equatorial Pacific and enhances the Walker circulation, triggering the Pacific to a La Niña mode with Bjerknes ocean dynamical feedback. This mechanism contributes to the understanding of the global decadal climate variability and predictability. In particular, Atlantic contributes to the Eastern Pacific cooling, which is considered as an important source of the recent global warming hiatus.

Tropical North Atlantic Coral-Based Sea Surface Temperature and Salinity Reconstructions From the Little Ice Age and Early Holocene

NASA Astrophysics Data System (ADS)

Saenger, C.; Cohen, A.; Oppo, D.; Hubbard, D.

2006-12-01

Understanding the magnitude and spatial extent of tropical sea surface temperature (SST) cooling during the Little Ice Age (~1400-1850 A.D.; LIA) is important for elucidating low-latitude paleoclimate, but present estimates are poorly constrained. We used Sr/Ca and δ18O variability within the aragonitic skeleton of the coral genus Montastrea to reconstruct SST and sea surface salinity (SSS) during the LIA and early Holocene (EH) in the tropical Atlantic. Four seasonally-resolved coral Sr/Ca records from St. Croix, U.S. Virgin Islands, and Bermuda indicate SST is highly correlated (r2 = 0.94) with modern Montastrea Sr/Ca and mean annual coral extension. A Sr/Ca -SST calibration that combines temperature and growth rate effects on coral Sr/Ca was applied to fossil St. Croix corals to reconstruct Caribbean climate during 5-10 year intervals of the LIA (440 ± 30 yBP) and EH (7200 ± 30; EH). Contrary to previous coral-based LIA proxy reconstructions, we find mean SST during the LIA was similar to today, but approximately 1.2°C cooler during the EH. Both periods exhibited higher amplitude seasonal variability indicating other SST estimates may be seasonally biased. Based on residual coral δ18O, we find the LIA and EH were saltier, which suggests previous cooling estimates of 1-3°C relative to today may be exaggerated by changes in seawater δ18O. Our results are consistent with a southerly migration of the Intertropical Convergence Zone (ITCZ) during the LIA, but their corroboration requires longer, high-resolution proxy reconstructions that place our two brief multi-annual coral records from the LIA and EH, respectively, within the context of multi-decadal variability.

The relative importance of ENSO and tropical Atlantic sea surface temperature anomalies for seasonal precipitation over South America: a numerical study

NASA Astrophysics Data System (ADS)

Pezzi, L. P.; Cavalcanti, I. F. A.

The role of tropical Atlantic sea surface temperature (SST) anomalies during ENSO episodes over northeast Brazil (Nordeste) is investigated using the CPTEC/COLA Atmospheric General Circulation Model (AGCM). Four sets of integrations are performed using SST in El Niño and La Niña (ENSO) episodes, changing the SST of the Atlantic Ocean. A positive dipole (SST higher than normal in the tropical North Atlantic and below normal in the tropical South Atlantic) and a negative dipole (opposite conditions), are set as the boundary conditions of SST in the Atlantic Ocean. The four experiments are performed using El Niño or La Niña SST in all oceans, except in the tropical Atlantic where the two phases of the SST dipole are applied. Five initial conditions were integrated in each case in order to obtain four ensemble results. The positive SST dipole over the tropical Atlantic Ocean and El Niño conditions over the Pacific Ocean resulted in dry conditions over the Nordeste. When the negative dipole and El Niño conditions over the Pacific Ocean were applied, the results showed precipitation above normal over the north of Nordeste. When La Niña conditions over Pacific Ocean were tested together with a negative dipole, positive precipitation anomalies occurred over the whole Nordeste. Using the positive dipole over the tropical Atlantic, the precipitation over Nordeste was below average. During La Niña episodes, the Atlantic Ocean conditions have a larger effect on the precipitation of Nordeste than the Pacific Ocean. In El Niño conditions, only the north region of Nordeste is affected by the Atlantic SST. Other tropical areas of South America show a change only in the intensity of anomalies. Central and southeast regions of South America are affected by the Atlantic conditions only during La Niña conditions, whereas during El Niño these regions are influenced only by conditions in the Pacific Ocean.

Relation between climatic factors, diet and reproductive parameters of Little Terns over a decade

NASA Astrophysics Data System (ADS)

Ramos, Jaime A.; Pedro, Patrícia; Matos, Antonio; Paiva, Vitor H.

2013-11-01

We used 10 years of data on clutch size, egg size and diet, and 8 years of data on timing of laying on Little Terns (Sternula albifrons) breeding in Ria Formosa lagoon system, Algarve, Portugal to assess whether diet acts as an important intermediary between climatic conditions and breeding parameters. We used Generalized Linear Models to relate (1) the relative occurrence and size of the main prey species, sand smelts (Atherina spp.), with environmental variables, a large-scale climate variable, the North Atlantic Oscillation (NAO) index, and a local scale variable, the sea-surface temperature (SST), and (2) the respective effects of sand smelts relative occurrence, NAO index and SST on Little Tern breeding parameters. The diet of Little Terns was dominated by sand smelts, with a frequency occurrence of over 60% in all years. The winter SST (February) was negatively associated with the relative occurrence of sand smelts in the diet of Little Terns during the breeding season which, in turn, was positively associated with Little Tern clutch size. Our results suggest that negative NAO conditions in the Atlantic Ocean, often associated with rougher sea conditions (greater vertical mixing, stronger winds and lower SST) were related with earlier breeding, and lower SST in the surroundings of the colony during winter-spring favour the abundance of prey fish for Little Terns as well as their reproductive parameters. Climate patterns at both large and local scales are likely to change in the future, which may have important implications for estuarine seabirds in Southern Europe.

The Low-Frequency Variability of the Tropical Atlantic Ocean

NASA Technical Reports Server (NTRS)

Haekkinen, Sirpa; Mo, Kingtse C.; Koblinsky, Chester J. (Technical Monitor)

2001-01-01

Upper ocean temperature variability in the tropical Atlantic is examined from the Comprehensive Ocean Atmosphere Data Set (COADS) as well as from an ocean model simulation forced by COADS anomalies appended to a monthly climatology. Our findings are as follows: Only the sea surface temperatures (SST) in the northern tropics are driven by heat fluxes, while the southern tropical variability arises from wind driven ocean circulation changes. The subsurface temperatures in the northern and southern tropics are found to have a strong linkage to buoyancy forcing changes in the northern North Atlantic. Evidence for Kelvin-like boundary wave propagation from the high latitudes is presented from the model simulation. This extratropical influence is associated with wintertime North Atlantic Oscillation (NAO) forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depth of 100-200 meters as result of a Rossby wave propagation away from the eastern boundary in the wake of the boundary wave passage. The most apparent association of the southern tropical sea surface temperature anomalies (STA) arises with the anomalous cross-equatorial winds which can be related to both NAO and the remote influence from the Pacific equatorial region. These teleconnections are seasonal so that the NAO impact on the tropical SST is the largest it mid-winter but in spring and early summer the Pacific remote influence competes with NAO. However, NAO appears to have a more substantial role than the Pacific influence at low frequencies during the last 50 years. The dynamic origin of STA is indirectly confirmed from the SST-heat flux relationship using ocean model experiments which remove either anomalous wind stress forcing or atmospheric forcing anomalies contributing to heat exchange.

Patterns of interannual climate variability in large marine ecosystems

NASA Astrophysics Data System (ADS)

Soares, Helena Cachanhuk; Gherardi, Douglas Francisco Marcolino; Pezzi, Luciano Ponzi; Kayano, Mary Toshie; Paes, Eduardo Tavares

2014-06-01

The purpose of this study is to investigate the vulnerability of the Brazilian and western African Large Marine Ecosystems (LMEs) to local and remote forcing, including the Pacific Decadal Oscillation (PDO) regime shift. The analyses are based on the total and partial correlation between climate indices (Niño3, tropical South Atlantic (TSA), tropical North Atlantic (TNA) and Antarctic oscillation (AAO) and oceanic and atmospheric variables (sea surface temperature (SST), wind stress, Ekman transport, sea level pressure and outgoing longwave radiation). Differences in the correlation fields between the cold and warm PDO indicate that this mode exerts a significant impact on the thermodynamic balance of the ocean-atmosphere system on the South Atlantic ocean, mainly in the South Brazil and Benguela LMEs. The PDO regime shift also resulted in an increase in the spatial variability of SST and wind stress anomalies, mainly along the western African LMEs. Another important finding is the strong AAO influence on the SST anomalies (SSTA) in the South Brazil LME. It is also striking that TSA modulates the relation between El Niño-Southern Oscillation (ENSO) and SSTA, by reducing the influence of ENSO on SSTA during the warm PDO period in the North and East Brazil LMEs and in the Guinea Current LME. The relation between AAO and SSTA on the tropical area is also influenced by the TSA. The results shown here give a clear indication that future ecosystem-based management actions aimed at the conservation of marine resources under climate change need to consider the high complexity of basin-scale interactions between local and remote climate forcings, including their effects on the ocean-atmosphere system of the South Atlantic ocean.

A teleconnection study of interannual sea surface temperature fluctuations in the northern North Atlantic and precipitation and runoff over Western Siberia

DOE Office of Scientific and Technical Information (OSTI.GOV)

Peng, S.; Mysak, L.A.

The spatial distributions of northern North Atlantic sea surface temperature and the high-latitude Northern Hemisphere sea level pressure anomalies averaged over six consecutive warm SST winters (1951-1956) and six consecutive cold SST winters (1971-1976) are examined. Three SLP anomaly difference (i.e., warm - cold winters) centers, significant at the 5% level, are observed over the northern North Atlantic, Europe, and western Siberia. This anomaly pattern is consistent in principle with what was identified in a related analyses by Palmer and Sun, who used composite data from selected winter months. The SLP difference centers over the northern North Atlantic and westernmore » Siberia are in phase. The impact of the latter center upon the runoff from the underlying Ob and Yenisey rivers and especially the teleconnection between SST anomalies in the northern North Atlantic and runoff of those two rivers via the atmosphere are investigated. The temporal cross-correlation analyses of 50 years (1930-1979) of records of SST, precipitation, and runoff anomalies indicate that the winter SST anomalies in the northern North Atlantic are significantly correlated with the winter and following summer runoff fluctuations of the Ob and Yenisey rivers. Positive (negative) northern North Atlantic SST anomalies are related to less (more) precipitation, and hence, less (more) runoff, over western Siberia. Discussions of possible physical mechanisms and processes that lead to the above relationships are attempted. The analyses of spatial distributions of precipitation in the warm and cold SST winters suggest that precipitation fluctuations over Europe and western Siberia may be affected by shifts of cyclone tracks associated with the SST variations in the northern North Atlantic. 27 refs., 9 figs.« less

Forced and Unforced Variability of Twentieth Century North American Droughts and Pluvials

NASA Technical Reports Server (NTRS)

Cook, Benjamin I.; Cook, Edward R.; Anchukaitis, Kevin J.; Seager, Richard; Miller, Ron L.

2010-01-01

Research on the forcing of drought and pluvial events over North America is dominated by general circulation model experiments that often have operational limitations (e.g., computational expense, ability to simulate relevant processes, etc). We use a statistically based modeling approach to investigate sea surface temperature (SST) forcing of the twentieth century pluvial (1905-1917) and drought (1932-1939, 1948-1957, 1998-2002) events. A principal component (PC) analysis of Palmer Drought Severity Index (PDSI) from the North American Drought Atlas separates the drought variability into five leading modes accounting for 62% of the underlying variance. Over the full period spanning these events (1900-2005), the first three PCs significantly correlate with SSTs in the equatorial Pacific (PC 1), North Pacific (PC 2), and North Atlantic (PC 3), with spatial patterns (as defined by the empirical orthogonal functions) consistent with our understanding of North American drought responses to SST forcing. We use a large ensemble statistical modeling approach to determine how successfully we can reproduce these drought/pluvial events using these three modes of variability. Using Pacific forcing only (PCs 1-2), we are able to reproduce the 1948-1957 drought and 1905-1917 pluvial above a 95% random noise threshold in over 90% of the ensemble members; the addition of Atlantic forcing (PCs 1-2-3) provides only marginal improvement. For the 1998-2002 drought, Pacific forcing reproduces the drought above noise in over 65% of the ensemble members, with the addition of Atlantic forcing increasing the number passing to over 80%. The severity of the drought, however, is underestimated in the ensemble median, suggesting this drought intensity can only be achieved through internal variability or other processes. Pacific only forcing does a poor job of reproducing the 1932-1939 drought pattern in the ensemble median, and less than one third of ensemble members exceed the noise threshold (28%). Inclusion of Atlantic forcing improves the ensemble median drought pattern and nearly doubles the number of ensemble members passing the noise threshold (52%). Even with the inclusion of Atlantic forcing, the intensity of the simulated 1932-1939 drought is muted, and the drought itself extends too far into the southwest and southern Great Plains. To an even greater extent than the 1998-2002 drought, these results suggest much of the variance in the 1932-1939 drought is dependent on processes other than SST forcing. This study highlights the importance of internal noise and non SST processes for hydroclimatic variability over North America, complementing existing research using general circulation models.

North American Tropical Cyclone Landfall and SST: A Statistical Model Study

NASA Technical Reports Server (NTRS)

Hall, Timothy; Yonekura, Emmi

2013-01-01

A statistical-stochastic model of the complete life cycle of North Atlantic (NA) tropical cyclones (TCs) is used to examine the relationship between climate and landfall rates along the North American Atlantic and Gulf Coasts. The model draws on archived data of TCs throughout the North Atlantic to estimate landfall rates at high geographic resolution as a function of the ENSO state and one of two different measures of sea surface temperature (SST): 1) SST averaged over the NA subtropics and the hurricane season and 2) this SST relative to the seasonal global subtropical mean SST (termed relSST). Here, the authors focus on SST by holding ENSO to a neutral state. Jackknife uncertainty tests are employed to test the significance of SST and relSST landfall relationships. There are more TC and major hurricane landfalls overall in warm years than cold, using either SST or relSST, primarily due to a basinwide increase in the number of storms. The signal along the coast, however, is complex. Some regions have large and significant sensitivity (e.g., an approximate doubling of annual major hurricane landfall probability on Texas from -2 to +2 standard deviations in relSST), while other regions have no significant sensitivity (e.g., the U.S. mid-Atlantic and Northeast coasts). This geographic structure is due to both shifts in the regions of primary TC genesis and shifts in TC propagation.

Variability of North Atlantic Hurricane Frequency in a Large Ensemble of High-Resolution Climate Simulations

NASA Astrophysics Data System (ADS)

Mei, W.; Kamae, Y.; Xie, S. P.

2017-12-01

Forced and internal variability of North Atlantic hurricane frequency during 1951-2010 is studied using a large ensemble of climate simulations by a 60-km atmospheric general circulation model that is forced by observed sea surface temperatures (SSTs). The simulations well capture the interannual-to-decadal variability of hurricane frequency in best track data, and further suggest a possible underestimate of hurricane counts in the current best track data prior to 1966 when satellite measurements were unavailable. A genesis potential index (GPI) averaged over the Main Development Region (MDR) accounts for more than 80% of the forced variations in hurricane frequency, with potential intensity and vertical wind shear being the dominant factors. In line with previous studies, the difference between MDR SST and tropical mean SST is a simple but useful predictor; a one-degree increase in this SST difference produces 7.1±1.4 more hurricanes. The hurricane frequency also exhibits internal variability that is comparable in magnitude to the interannual variability. The 100-member ensemble allows us to address the following important questions: (1) Are the observations equivalent to one realization of such a large ensemble? (2) How many ensemble members are needed to reproduce the variability in observations and in the forced component of the simulations? The sources of the internal variability in hurricane frequency will be identified and discussed. The results provide an explanation for the relatively week correlation ( 0.6) between MDR GPI and hurricane frequency on interannual timescales in observations.

European Climate and Pinot Noir Grape-Harvest Dates in Burgundy, since the 17th Century

NASA Astrophysics Data System (ADS)

Tourre, Y. M.

2011-12-01

Time-series of growing season air temperature anomalies in the Parisian region and of 'Pinot Noir' grape-harvest dates (GHD) in Burgundy (1676-2004) are analyzed in the frequency-domain. Variability of both time-series display three significant frequency-bands (peaks significant at the 5% level) i.e., a low-frequency band (multi-decadal) with a 25-year peak period; a 3-to-8 year band period (inter-annual) with a 3.1-year peak period; and a 2-to-3 year band period (quasi-biennial) with a 2.4-year peak period. Joint sea surface temperature/sea level pressure (SST/SLP) empirical orthogonal functions (EOF) analyses during the 20th century, along with spatio-temporal patterns for the above frequency-bands are presented. It is found that SST anomalies display early significant spatial SST patterns in the North Atlantic Ocean (air temperature lagging by 6 months) similar to those obtained from EOF analyses. It is thus proposed that the robust power spectra for the above frequency-bands could be linked with Atlantic climate variability metrics modulating Western European climate i.e., 1) the global Multi-decadal Oscillation (MDO) with its Atlantic Multi-decadal Oscillation (AMO) footprint; 2) the Atlantic Inter-Annual (IA) fluctuations; and 3) the Atlantic Quasi-Biennial (QB) fluctuations, respectively. Moreover these specific Western European climate signals have effects on ecosystem health and can be perceived as contributors to the length of the growing season and the timing of GHD in Burgundy. Thus advance knowledge on the evolution and phasing of the above climate fluctuations become important elements for viticulture and wine industry management. It is recognized that anthropogenic effects could have modified time-series patterns presented here, particularly since the mid 1980s.

Remote SST Forcing and Local Land-Atmosphere Moisture Coupling as Drivers of Amazon Temperature and Carbon Cycle Variability

NASA Astrophysics Data System (ADS)

Levine, P. A.; Xu, M.; Chen, Y.; Randerson, J. T.; Hoffman, F. M.

2017-12-01

Interannual variability of climatic conditions in the Amazon rainforest is associated with El Niño-Southern Oscillation (ENSO) and ocean-atmosphere interactions in the North Atlantic. Sea surface temperature (SST) anomalies in these remote ocean regions drive teleconnections with Amazonian surface air temperature (T), precipitation (P), and net ecosystem production (NEP). While SST-driven NEP anomalies have been primarily linked to T anomalies, it is unclear how much the T anomalies result directly from SST forcing of atmospheric circulation, and how much result indirectly from decreases in precipitation that, in turn, influence surface energy fluxes. Interannual variability of P associated with SST anomalies lead to variability in soil moisture (SM), which would indirectly affect T via partitioning of turbulent heat fluxes between the land surface and the atmosphere. To separate the direct and indirect influence of the SST signal on T and NEP, we performed a mechanism-denial experiment to decouple SST and SM anomalies. We used the Accelerated Climate Modeling for Energy (ACMEv0.3), with version 5 of the Community Atmosphere Model and version 4.5 of the Community Land Model. We forced the model with observed SSTs from 1982-2016. We found that SST and SM variability both contribute to T and NEP anomalies in the Amazon, with relative contributions depending on lag time and location within the Amazon basin. SST anomalies associated with ENSO drive most of the T variability at shorter lag times, while the ENSO-driven SM anomalies contribute more to T variability at longer lag times. SM variability and the resulting influence on T anomalies are much stronger in the eastern Amazon than in the west. Comparing modeled T with observations demonstrate that SST alone is sufficient for simulating the correct timing of T variability, but SM anomalies are necessary for simulating the correct magnitude of the T variability. Modeled NEP indicated that variability in carbon fluxes results from both SST and SM anomalies. As with T, SM anomalies affect NEP at a much longer lag time than SST anomalies. These results highlight the role of land-atmosphere coupling in driving climate variability within the Amazon, and suggest that land atmospheric coupling may amplify and delay carbon cycle responses to ocean-atmosphere teleconnections.

Multidecadal Atlantic climate variability and its impact on marine pelagic communities

NASA Astrophysics Data System (ADS)

Harris, Victoria; Edwards, Martin; Olhede, Sofia C.

2014-05-01

A large scale analysis of sea surface temperature (SST) and climate variability over the North Atlantic and its interactions with plankton over the North East Atlantic was carried out to better understand what drives both temperature and species abundance. The spatio-temporal pattern of SST was found to correspond to known climate indices, namely the Atlantic Multidecadal Oscillation (AMO), the East Atlantic Pattern (EAP) and the North Atlantic Oscillation (NAO). The spatial influence of these indices is heterogeneous. Although the AMO is present across all regions, it is most strongly represented in the SST signal in the subpolar gyre region. The NAO instead is strongly weighted in the North Sea and the pattern of its influence is oscillatory in space with a wavelength of approximately 6000 km. Natural oscillations might obscure the influence of climate change effects, making it difficult to determine how much of the variation is attributable to longer term trends. In order to separate the influences of different climate signals the SST signals were decomposed in to spatial and temporal components using principal component analysis (PCA). A similar analysis is carried out on various indicator species of plankton: Calanus finmarchicus, Phytoplankton Colour Index and total copepod abundance, as well as phytoplankton and zooplankton communities. By comparing the two outputs it is apparent that the dominant driver is the recent warming trend, which has a negative influence on C. finmarchicus and total copepods, but has a positive one on phytoplankton colour. However natural oscillations also influence the abundance of plankton, in particular the AMO is a driver of diatom abundance. Fourier principal component analysis, an approach which is novel in terms of the ecological data, was used to analyse the behaviour of various communities averaged over space. The zooplankton community is found to be primarily influenced by climate warming trends. The analysis provides compelling evidence for the hypothesis that cold water species are gradually being replaced by more temperate species in the North Atlantic. This may have detrimental effects for the entire marine ecosystem, by affecting on organisms such as fish larva for example. The second group, a phytoplankton subset consisting primarily of diatom species, is primarily influenced by the AMO rather than the average temperature trend. This result highlights the importance of natural oscillations to certain functional groups, in particular those subgroups which are less directly metabolically affected by changes in temperature.

US regional tornado outbreaks and their links to spring ENSO phases and North Atlantic SST variability

NASA Astrophysics Data System (ADS)

Lee, Sang-Ki; Wittenberg, Andrew T.; Enfield, David B.; Weaver, Scott J.; Wang, Chunzai; Atlas, Robert

2016-04-01

Recent violent and widespread tornado outbreaks in the US, such as occurred in the spring of 2011, have caused devastating societal impact with significant loss of life and property. At present, our capacity to predict US tornado and other severe weather risk does not extend beyond seven days. In an effort to advance our capability for developing a skillful long-range outlook for US tornado outbreaks, here we investigate the spring probability patterns of US regional tornado outbreaks during 1950-2014. We show that the four dominant springtime El Niño-Southern Oscillation (ENSO) phases (persistent versus early-terminating El Niño and resurgent versus transitioning La Niña) and the North Atlantic sea surface temperature tripole variability are linked to distinct and significant US regional patterns of outbreak probability. These changes in the probability of outbreaks are shown to be largely consistent with remotely forced regional changes in the large-scale atmospheric processes conducive to tornado outbreaks. An implication of these findings is that the springtime ENSO phases and the North Atlantic SST tripole variability may provide seasonal predictability of US regional tornado outbreaks.

Decadal Air-Sea Interaction in the North Atlantic Based on Observations and Modeling Results

NASA Technical Reports Server (NTRS)

Hakkinen, Sirpa

1998-01-01

The decadal, 12-14 year, cycle observed in the North Atlantic SST and tide gauge data was examined using the NCEP/NCAR reanalyses, COADS data and an ocean model simulation. Besides this decadal mode, a shorter, subdecadal period of about 8 years exists in tide gauge data north of 40N, in the subpolar SST and in the winter North Atlantic Oscillation (NAO) index and in subpolar winter heat flux values. The decadal cycle is a well separated mode in a singular spectrum analysis (SSA) for a time series of SST EOF mode 1 with a center over the Gulf Stream extension. Tide gauge and SST data are consistent in that both show a significant subdecadal periodicity exclusively in the subpolar gyre, but in subtropics the 12-14 year period is the prominent, but nonstationary, decadal signal. The main finding of this study is that this 12-14 year cycle can be constructed based on the leading mode of the surface heat flux. This connection to the surface heat flux implicates the participation of the thermohaline circulation in the decadal cycle. During the cycle starting from the positive index phase of NAO, SST and oceanic heat content anomalies are created in subtropics due to local heat flux and intensification of the thermohaline circulation. The anomalies advect to the subpolar gyre where they are amplified by local heat flux and are part of the negative feedback of thermohaline circulation on itself. Consequently the oceanic thermohaline circulation slows down and the opposite cycle starts. The oscillatory nature would not be possible without the active atmospheric participation in the cycle, because it provides the unstable interaction through heat flux, without it, the oceanic mode would be damped. This analysis suggests that the two principal modes of heat flux variability, corresponding to patterns similar to North Atlantic Oscillation (NAO) and Western Atlantic (WA), are part of the same decadal cycle and an indirect measure of the north-south movement of the storm tracks.

A USCLVAR Multi-Model Assessment of the Impact of SST Anomalies and Land-Atmosphere Feedbacks on Drought

NASA Technical Reports Server (NTRS)

Schubert, Siegfried

2009-01-01

The USCLIVAR working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land-atmosphere feedbacks on regional drought. Specific questions that the runs are designed to address include, What are the mechanisms that maintain drought across the seasonal cycle and from one year to the next? What is the role of the leading patterns of SST variability, and what are the physical mechanisms linking the remote SST forcing to regional drought, including the role of land-atmosphere coupling? The runs were carried out with five different atmospheric general circulation models (AGCMs), and one coupled atmosphere-ocean model in which the model was continuously nudged to the imposed SST forcing. This talk provides an overview of the experiments and some initial results focusing on the responses to the leading patterns of annual mean SST variability consisting of a Pacific El Nino/Southern Oscillation (ENSO)-like pattern, a pattern that resembles the Atlantic Multi-decadal Oscillation (AMO), and a global trend pattern. One of the key findings is that all the AGCMs produce broadly similar (though different in detail) precipitation responses to the Pacific forcing pattern, with a cold Pacific leading to reduced precipitation and a warm Pacific leading to enhanced precipitation over most of the United States. While the response to the Atlantic pattern is less robust, there is general agreement among the models that the largest precipitation response over the U.S. tends to occur when the two oceans have anomalies of opposite sign. That is, a cold Pacific and warm Atlantic tend to produce the largest precipitation reductions, whereas a warm Pacific and cold Atlantic tend to produce the greatest precipitation enhancements. Further analysis of the response over the U.S. to the Pacific forcing highlights a number of noteworthy and to some extent unexpected results. These include a seasonal dependence of the precipitation response that is characterized by signal-to-noise ratios that peak in spring, and surface temperature signal-to-noise ratios that are both lower and show less agreement among the models than those found for the precipitation response. Another interesting result concerns what appears to be a substantially different character in the surface temperature response over the U.S. to the Pacific forcing by the only model examined here that was developed for use in numerical weather prediction. The response to the positive SST trend forcing pattern is an overall surface warming over the world's land areas with substantial regional variations that are in part reproduced in runs forced with a globally uniform SST trend forcing. The precipitation response to the trend forcing is weak in all the models. It is hoped that these early results will serve to stimulate further analysis of these simulations, as well as suggest new research on the physical mechanisms contributing to hydroclimatic variability and change throughout the world.

Main processes of the Atlantic cold tongue interannual variability

NASA Astrophysics Data System (ADS)

Planton, Yann; Voldoire, Aurore; Giordani, Hervé; Caniaux, Guy

2018-03-01

The interannual variability of the Atlantic cold tongue (ACT) is studied by means of a mixed-layer heat budget analysis. A method to classify extreme cold and warm ACT events is proposed and applied to ten various analysis and reanalysis products. This classification allows 5 cold and 5 warm ACT events to be selected over the period 1982-2007. Cold (warm) ACT events are defined by the presence of negative (positive) sea surface temperature (SST) anomalies at the center of the equatorial Atlantic in late boreal spring, preceded by negative (positive) zonal wind stress anomalies in the western equatorial Atlantic. An ocean general circulation model capable of reconstructing the interannual variability of the ACT correctly is used to demonstrate that cold ACT events develop rapidly from May to June mainly due to intense cooling by vertical mixing and horizontal advection. The simulated cooling at the center of the basin is the result of the combined effects of non-local and local processes. The non-local process is an upwelling associated with an eastward-propagating Kelvin wave, which makes the mixed-layer more shallow and preconditions the upper layers to be cooled by an intense heat loss at the base of the mixed-layer, which is amplified by a stronger local injection of energy from the atmosphere. The early cooling by vertical mixing in March is also shown to be a good predictor of June cooling. In July, horizontal advection starts to warm the mixed-layer abnormally and damps SST anomalies. The advection anomalies, which result from changes in the horizontal temperature gradient, are associated in some cases with the propagation of Rossby waves along the equator. During warm ACT events, processes are reversed, generating positive SST anomalies: a downwelling Kelvin wave triggers stratification anomalies and mixed-layer depth anomalies, amplified by a weaker injection of energy from the atmosphere in May-June. In July, warm ACT events are abnormally cooled due to negative horizontal advection anomalies resulting from processes similar to those that occur during cold ACT events. This additional cooling process extends the period of cooling of the ACT, reducing SST anomalies.

Sea Surface Temperature Records Using Sr/Ca Ratios in a Siderastrea siderea Coral from SE Cuba

NASA Astrophysics Data System (ADS)

Fargher, H. A.; Hughen, K. A.; Ossolinski, J. E.; Bretos, F.; Siciliano, D.; Gonzalez, P.

2015-12-01

Sea surface temperature (SST) variability from Cuba remains relatively unknown compared to the rest of the Caribbean. Cuba sits near an inflection point in the spatial pattern of SST from the North Atlantic Oscillation (NAO), and long SST records from the region could reveal changes in the influence of this climate system through time. A Siderastrea siderea coral from the Jardínes de la Reina in southern Cuba was drilled to obtain a 220 year long archive of environmental change. The genus Siderastrea has not been extensively studied as an SST archive, yet Sr/Ca ratios in the Cuban core show a clear seasonal signal and strong correlation to instrumental SST data (r2 = 0.86 and 0.36 for monthly and interannual (winter season) timescales, respectively). Annual growth rates (linear extension) of the coral are observed to have a minor influence on Sr/Ca variability, but do not show a direct correlation to SST on timescales from annual to multidecadal. Sr/Ca measurements from the Cuban coral are used to reconstruct monthly and seasonal (winter, summer) SST extending back more than two centuries. Wintertime SST in southern Cuba is compared to other coral Sr/Ca records of winter-season SST from locations sensitive to the NAO in order to investigate the stationarity of the NAO SST 'fingerprint' through time.

The North Atlantic Oscillation and the ITCZ in a climate simulation

NASA Astrophysics Data System (ADS)

Cavalcanti, I. F. A.; Souza, P.

2009-04-01

The North Atlantic Oscillation (NAO) and the Atlantic Intertropical Convergence Zone (ITCZ) features are analyzed in a climate simulation with the CPTEC/COLA AGCM. The CPTEC/COLA AGCM reproduces the ITCZ seasonal north-south displacement as well as the seasonal east-west intensity, but the model overestimates the convection. The two phases of NAO are well simulated in the four seasons and also the largest intensity in DJF. The main mode of atmospheric variability considering the North and South Atlantic region, which displays a shifting of the NAO centers and a center of action over South Atlantic to the south of Africa is also reproduced. This mode, in DJF, is associated with the north-south ITCZ displacement in April, in the observed data. The displacement of the NAO centers southwestward allows the increase of pressure over the tropical North Atlantic Ocean and the increase of trade winds and displacement of the confluence and convergence zone southwards. The opposite occurs when the centers are displaced northeastward. The model Atlantic ITCZ position in April is associated with the anomalous (observed) Atlantic SST and the southward displacement of the confluence zone, but the simulated atmospheric features in DJF does not display the main mode of variability, as in the observations. This occurs due to the lack of interaction between the atmosphere and ocean in the atmospheric model. While in the observations the physical mechanism that links the NAO centers of action to the ITCZ position is the ocean-atmosphere interaction, from DJF to April, the atmospheric model responds to the prescribed SST at the same month, in April.

On the dynamic forcing of short-term climate fluctuations by feedback mechanisms

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reiter, E.R.

1979-09-01

The energies involved in the general circulation of the atmosphere, especially the zonal available potential energy, show considerable interannual variability, suggesting the presence of various internal feedback mechanisms in the ocean-atmosphere system. Sea-surface temperature (SST) variations appear to have some effect on the hydrological cycle. The possible existence of feedback mechanisms between ocean and atmosphere seem to be evident in some of the data from the North Pacific and North Atlantic. One of these proposed mechanisms involves the variation in the convergence between the North and South Pacific trade-wind systems and is strongly reflected in rainfall variability within the drymore » region of the equatorial Pacific. Similar variations appear in low-latitude SST anomalies. The convergence between the two trade-wind systems in the Atlantic region also undergoes marked interannual variations. This quasi-biennial oscillation (QBO) in trade-wind convergence over the Atlantic appears to be tied to the global QBO of equatorial stratospheric winds and to regional rainfall regimes in the dry region of northeastern Brazil. A variability pattern of SST's with a QBO has been detected off the coast of Senegal, in the Gulf of Guinea and even in the Gulf Stream as it leaves the North American continental shelf. Possible physical connections between some of these QBO's are pointed out by a hypothetical feedback model. It is also suggested that interaction of a QBO with the annual cycle may lead to beating frequencies resembling climatic trends of a duration of several years.« less

Decadal climate predictability in the southern Indian Ocean captured by SINTEX-F using a simple SST-nudging scheme.

PubMed

Morioka, Yushi; Doi, Takeshi; Behera, Swadhin K

2018-01-26

Decadal climate variability in the southern Indian Ocean has great influences on southern African climate through modulation of atmospheric circulation. Although many efforts have been made to understanding physical mechanisms, predictability of the decadal climate variability, in particular, the internally generated variability independent from external atmospheric forcing, remains poorly understood. This study investigates predictability of the decadal climate variability in the southern Indian Ocean using a coupled general circulation model, called SINTEX-F. The ensemble members of the decadal reforecast experiments were initialized with a simple sea surface temperature (SST) nudging scheme. The observed positive and negative peaks during late 1990s and late 2000s are well reproduced in the reforecast experiments initiated from 1994 and 1999, respectively. The experiments initiated from 1994 successfully capture warm SST and high sea level pressure anomalies propagating from the South Atlantic to the southern Indian Ocean. Also, the other experiments initiated from 1999 skillfully predict phase change from a positive to negative peak. These results suggest that the SST-nudging initialization has the essence to capture the predictability of the internally generated decadal climate variability in the southern Indian Ocean.

A study on bulk and skin temperature difference using observations from Atlantic and Pacific Coastal regions of United States

NASA Astrophysics Data System (ADS)

Alappattu, Denny P.; Wang, Qing; Yamaguchi, Ryan; Lind, Richard; Reynolds, Mike; Christman, Adam

2017-05-01

Analysis of bulk-skin sea surface temperature (SST) difference form the west and east coasts of United States is presented using the data collected from three field experiments. These experiments were conducted at offshore Duck, North Carolina and in the Monterey Bay of the California coastal region. Bulk SST measurements were made using conventional thermistors from a depth of one meter below the sea level. Infrared radiometers were used to measure the surface skin SST. Depending on measurement depth and prevailing conditions, the bulk SST can differ from skin SST by few tenths of a degree to O(1°C). Difference between bulk and skin SST arise from cools skin and warm layer effects. Bulk-skin SST difference (ΔSST) estimated from east coast observations varied from -0.46°C to 1.24°C. Here, the bulk SST was higher than skin SST most of the time during the observations. This indicates cool skin effect was the dominant factor determining the ΔSST in the east coast. For wind speeds less than 4 m s-1, we also noticed an increase in ΔSST. Additionally, for low winds (<4 m s-1) ΔSST also varied diurnally with the occurrence of generally higher ΔSST in the nighttime in comparison with daytime. Moreover, increase in downwelling longwave radiation reduced the bulk-skin SST difference. ΔSST calculated from the observation in the Monterey bay varied between 2.3° and -2.3°C. This was higher than the variability ΔSST observed at the east coast. Moreover, ΔSST variability observed at west coast was independent of wind speed.

Seasonal Evolution and Variability Associated with the West African Monsoon System

NASA Technical Reports Server (NTRS)

Gu, Guojun; Adler, Robert F.

2003-01-01

In this study, we investigate the seasonal variations in surface rainfall and associated large-scale processes in the tropical eastern Atlantic and West African region. The 5-yr (1998-2002) high-quality TRMM rainfall, sea surface temperature (SST), water vapor and cloud liquid water observations are applied along with the NCEP/NCAR reanalysis wind components and a 3-yr (2000-2002) Quickscat satellite-observed surface wind product. Major mean rainfall over West Africa tends to be concentrated in two regions and is observed in two different seasons, manifesting an abrupt shift of the mean rainfall zone during June-July. (i) Near the Gulf of Guinea (about 5 degN), intense convection and rainfall are seen during April-June and roughly follow the seasonality of SST in the tropical eastern Atlantic. (ii) Along the latitudes of about 10 deg. N over the interior West African continent, a second intense rain belt begins to develop from July and remains there during the later summer season. This belt co-exists with a northwardmoved African Easterly Jet (AEJ) and its accompanying horizonal and vertical shear zones, the appearance and intensification of an upper tropospheric Tropical Easterly Jet (TEJ), and a strong low-level westerly flow. Westward-propagating wave signals [ i e . , African easterly waves (AEWs)] dominate the synoptic-scale variability during July-September, in contrast to the evident eastward-propagating wave signals during May- June. The abrupt shift of mean rainfall zone thus turns out to be a combination of two different physical processes: (i) Evident seasonal cycles in the tropical eastern Atlantic ocean which modulate convection and rainfall in the Gulf of Guinea by means of SST thermal forcing and SST-related meridional gradient; (ii) The interaction among the AEJ, TEJ, low-level westerly flow, moist convection and AEWs during July-September which modulates rainfall variability in the interior West Africa, primarily within the ITCZ rain band. Evident seasonality in synoptic-scale wave signals is shown to be a good evidence for this seasonal evolution.

Decadal-timescale changes of the Atlantic overturning circulation and climate in a coupled climate model with a hybrid-coordinate ocean component

NASA Astrophysics Data System (ADS)

Persechino, A.; Marsh, R.; Sinha, B.; Megann, A. P.; Blaker, A. T.; New, A. L.

2012-08-01

A wide range of statistical tools is used to investigate the decadal variability of the Atlantic Meridional Overturning Circulation (AMOC) and associated key variables in a climate model (CHIME, Coupled Hadley-Isopycnic Model Experiment), which features a novel ocean component. CHIME is as similar as possible to the 3rd Hadley Centre Coupled Model (HadCM3) with the important exception that its ocean component is based on a hybrid vertical coordinate. Power spectral analysis reveals enhanced AMOC variability for periods in the range 15-30 years. Strong AMOC conditions are associated with: (1) a Sea Surface Temperature (SST) anomaly pattern reminiscent of the Atlantic Multi-decadal Oscillation (AMO) response, but associated with variations in a northern tropical-subtropical gradient; (2) a Surface Air Temperature anomaly pattern closely linked to SST; (3) a positive North Atlantic Oscillation (NAO)-like pattern; (4) a northward shift of the Intertropical Convergence Zone. The primary mode of AMOC variability is associated with decadal changes in the Labrador Sea and the Greenland Iceland Norwegian (GIN) Seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency NAO that may be associated with a rapid atmospheric teleconnection from the tropics to the extratropics. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to processes in the Labrador Sea. A secondary mode of AMOC variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density.

A USCLIVAR Project to Assess and Compare the Responses of Global Climate Models to Drought-Related SST Forcing Patterns: Overview and Results

NASA Technical Reports Server (NTRS)

Schubert, Siegfried; Wang, Hailan; Koster, Randal; Weaver, Scott; Gutzler, David; Dai, Aiguo; Delworth, Tom; Deser, Clara; Findell, Kristen; Fu, Rong;

Understanding the interdecadal variabilities of East Asian summer precipitation: from a perspective of joint influence of oceanic forcings

NASA Astrophysics Data System (ADS)

Zhang, Z.; Sun, X.; Yang, X. Q.

2017-12-01

East Asian summer precipitation (EASP) is highly complicated in both temporal and spatial variabilities at interdecadal time scales, with various time periods and anomalous spatial distribution patterns. The joint influences of three dominant interdecadal signals, i.e., Pacific Decadal Oscillation (PDO), Atlantic Multidecadal Oscillation (AMO) and Indian Ocean Basin Mode (IOBM), are revealed to be responsible for most of the interdecadal variabilities of EASP in this study, which, however, are not the simply linear combinations of their individual climate effects. Specifically, when PDO and AMO are in antiphase, SST anomalies of the same signs appear in both North Pacific and North Atlantic, the Asian westerly jet (AWJ) is accelerated and acts as a waveguide, favoring a zonally orientated Rossby wave train from North Atlantic to northern East Asia across the mid-high latitude Eurasia. Correspondingly, interdecadal precipitation anomalies exhibit a meridional tripole mode over East China. When PDO and AMO are in phase with oppositely signed SST anomalies in North Pacific and North Atlantic, the waveguide mechanism doesn't work since AWJ is significantly reduced, and the Rossby wave train from North Atlantic travels to South Asia along the great circle path, causing anomalous Indian summer monsoon precipitation (ISMP). In turn, by triggering another Rossby wave trains along both the mid-latitudes and coastal regions of East Asia, the ISMP anomalies induce a meridional dipole mode of interdecadal precipitation anomalies over East China. Through the ISMP and the same dynamical processes, IOBM is more important for the interdecadal precipitation anomalies over northern East Asia.

The Atmospheric Response to a Future Warming Deficit in North Atlantic SSTs

NASA Astrophysics Data System (ADS)

Gervais, M.; Shaman, J. L.; Kushnir, Y.

2017-12-01

As SSTs increase globally over the 21st century, global climate models project a significant deficit in warming within the subpolar gyre of the North Atlantic Ocean. This study investigates the impact of this warming deficit on atmosphere circulation. A series of large ensemble experiments are conducted using the Community Atmosphere Model 5 forced with specified sea ice and SSTs for the early (2010-2019), mid (2050-2059), and late (2090-2099) 21stcentury. SST and sea ice fields from the Community Earth System Model Large Ensemble experiment are used as boundary conditions for the control simulations. Experiments with either a filled or deepened warming hole are conducted by adding a SST perturbation field to these time-varying SST boundary conditions. Results from these experiments demonstrate that the warming hole has significant local and remote impacts on the atmosphere. Filling (deepening) the warming hole results in a local increase (decrease) in turbulent heat fluxes relative to the control run and consequentially an increase (decrease) in temperature in the overlying lower troposphere that spreads over Europe. There are significant impacts on the location and strength of both the North Atlantic and North Pacific jets as well as on the North Atlantic Oscillation. These impacts of the warming hole on both the mean state and variability of the atmosphere have important implications for sensible weather in the Northern Hemisphere and in particular over Europe.

The role of local sea surface temperature pattern changes in shaping climate change in the North Atlantic sector

NASA Astrophysics Data System (ADS)

Hand, Ralf; Keenlyside, Noel S.; Omrani, Nour-Eddine; Bader, Jürgen; Greatbatch, Richard J.

2018-03-01

Beside its global effects, climate change is manifested in many regionally pronounced features mainly resulting from changes in the oceanic and atmospheric circulation. Here we investigate the influence of the North Atlantic SST on shaping the winter-time response to global warming. Our results are based on a long-term climate projection with the Max Planck Institute Earth System Model (MPI-ESM) to investigate the influence of North Atlantic sea surface temperature pattern changes on shaping the atmospheric climate change signal. In sensitivity experiments with the model's atmospheric component we decompose the response into components controlled by the local SST structure and components controlled by global/remote changes. MPI-ESM simulates a global warming response in SST similar to other climate models: there is a warming minimum—or "warming hole"—in the subpolar North Atlantic, and the sharp SST gradients associated with the Gulf Stream and the North Atlantic Current shift northward by a few a degrees. Over the warming hole, global warming causes a relatively weak increase in rainfall. Beyond this, our experiments show more localized effects, likely resulting from future SST gradient changes in the North Atlantic. This includes a significant precipitation decrease to the south of the Gulf Stream despite increased underlying SSTs. Since this region is characterised by a strong band of precipitation in the current climate, this is contrary to the usual case that wet regions become wetter and dry regions become drier in a warmer climate. A moisture budget analysis identifies a complex interplay of various processes in the region of modified SST gradients: reduced surface winds cause a decrease in evaporation; and thermodynamic, modified atmospheric eddy transports, and coastal processes cause a change in the moisture convergence. The changes in the the North Atlantic storm track are mainly controlled by the non-regional changes in the forcing. The impact of the local SST pattern changes on regions outside the North Atlantic is small in our setup.

A reconstruction of sea surface temperature variability in the southeastern Gulf of Mexico from 1734 to 2008 C.E. using cross-dated Sr/Ca records from the coral Siderastrea siderea

NASA Astrophysics Data System (ADS)

DeLong, Kristine L.; Flannery, Jennifer A.; Poore, Richard Z.; Quinn, Terrence M.; Maupin, Christopher R.; Lin, Ke; Shen, Chuan-Chou

2014-05-01

This study uses skeletal variations in coral Sr/Ca from three Siderastrea siderea coral colonies within the Dry Tortugas National Park in the southeastern Gulf of Mexico (24°42'N, 82°48'W) to reconstruct monthly sea surface temperature (SST) variations from 1734 to 2008 Common Era (C.E.). Calibration and verification of the replicated coral Sr/Ca-SST reconstruction with local, regional, and historical temperature records reveals that this proxy-temperature relationship is stable back to 1879 C.E. The coral SST reconstruction contains robust interannual ( 2.0°C) and multidecadal variability ( 1.5°C) for the past 274 years, the latter of which does not covary with the Atlantic Multidecadal Oscillation. Winter SST extremes are more variable than summer SST extremes (±2.2°C versus ±1.6°C, 2σ) suggesting that Loop Current transport in the winter dominates variability on interannual and longer time scales. Summer SST maxima are increasing (+1.0°C for 274 years, σMC = ±0.5°C, 2σ), whereas winter SST minima contain no significant trend. Colder decades ( 1.5°C) during the Little Ice Age (LIA) do not coincide with decades of sunspot minima. The coral SST reconstruction contains similar variability to temperature reconstructions from the northern Gulf of Mexico (planktic foraminifer Mg/Ca) and the Caribbean Sea (coral Sr/Ca) suggesting areal reductions in the Western Hemisphere Warm Pool during the LIA. Mean summer coral SST extremes post-1985 C.E. (29.9°C) exceeds the long-term summer average (29.2°C for 1734-2008 C.E.), yet the warming trend after 1985 C.E. (0.04°C for 24 years, σMC = ±0.5, 2σ) is not significant, whereas Caribbean coral Sr/Ca studies contain a warming trend for this interval.

Subtropical Gyre Variability as Seen from Satellites

NASA Technical Reports Server (NTRS)

Signorini, Sergio R.; McClain, Charles R.

2011-01-01

A satellite multi-sensor approach is used to analyse the biological response of open ocean regions of the subtropical gyres to changes in physical forcing. Thirteen years (1998-2010) of SeaWiFS chlorophyll a (Chl-a), combined with concurrent satellite records of sea-surface temperature (SST) and sea level height, were analysed to investigate the seasonal and interannual variability of Chl-a concentration within these immense so-called ocean deserts. The seasonal variability of Chl-a within the gyres is driven mostly by the warming/cooling of surface waters. Summer warming promotes shallower mixed layers and lower Chl-a due to a reduction of vertical mixing and consequently a decrease in nutrient supply. The opposite happens during the winter cooling period. Therefore, long-term trends in SST have the potential to cause an impact on the interannual variability of Chl-a. Our analyses show that, during the 13 whole years of SeaWiFS data record, the North Pacific, Indian Ocean, and North Atlantic gyres experienced a decrease in Chl-a of 9%, 12%, and 11%, respectively, with corresponding SST increases of 0.27 C, 0.42 C, and 0.32 C. The South Pacific and South Atlantic gyres also showed warming trends but with weak positive trends in Chl-a that are not statistically significant. We hypothesize that the warming of surface waters in these two gyres are counterbalanced by other interacting physical and biological driving mechanisms, as indicated in previous studies.

Variation in the Norwegian gyre and its links to the termohaline circulation (THC).

NASA Astrophysics Data System (ADS)

Gunnarson, B. E.; Linderholm, H. W.; Wilson, R.; Rydval, M.

2017-12-01

Summer temperature patterns in Scandinavia are partly governed by variations in the North Atlantic drift (being part of the Gulf Stream) causing northern Europe to be warmer than similar latitudes. Observation show that northwestern European climate is strongly link to sea surface temperature (SST) and the ocean circulation (the Norwegian gyre, NG) in the Norwegian Sea. On decadal- multidecadal time scales, there is also positive association with the sub-tropical gyre, but also a weaker (and negative) connection to the sub Polar gyre (SPG) which is linked to the thermohaline circulation (THC). The negative correlations occur only during the April-June and July-September (JAS) seasons, when the ocean mixed layer is shallow in the North Atlantic. A network of Maximum Latewood Density (MXD) tree-ring chronologies from 7 sites in Northern Scandinavia, 1 in central Scotland and 1 in Labrador was used to identifying SST influences on local to regional summer temperatures patterns during 1901-20XX. The sites represent tree growth strongly correlated with mean JAS temperatures (Fennoscandia r > 0.7, Scotland r > 0.6, Labrador r > 0.5). Both the Scotland and Labrador chronologies correlates only with SST from adjacent coastal areas. The Fennoscandian chronologies showed strong and temporally consistent correlations with SST across the NG (r > 0.5), but also positive correlations of the same magnitude across the sub-tropical gyre. In addition, a negative, but weaker, correlation was found over the SPG domain. Climate models (PMIP5) were not able to reproduce the correlation patterns evident in both observations and tree-ring data. The tripolar correlation pattern suggests that North Atlantic SST influences summer temperature variability in Northern Fennoscandia, illustrating the potential for using tree-rings to reconstruct the THC and the heat transport towards the North Atlantic region and atmosphere- ocean interaction back in time.

Hemispheric Symmetries of Plio-Pleistocene Surface Ocean Conditions: Insights from Southern Hemisphere ODP Sites 1125 and 1088

NASA Astrophysics Data System (ADS)

Lawrence, K. T.; Peterson, L.; Kelly, C.; Miller, H.; Seidenstein, J.

2013-12-01

For decades, most studies of Plio-Pleistocene climate and of the transition from the warmth of the Pliocene to the colder and more variable conditions of the Pleistocene have focused solely on northern hemisphere climate processes and responses. Here, we explore the southern hemisphere response to this major climate transition by documenting ocean surface conditions at Ocean Drilling Program Sites 1125 (42οS, 178οW, 1360m) and 1088 (40οS, 15οE, 2082m) through the Plio-Pleistocene. Secular trends in alkenone-derived sea surface temperature (SST) records indicate that these mid-latitude southern hemisphere sites cooled ~3-4οC over the past 3 Myrs, a magnitude comparable to sites located at similar latitudes in both the North Atlantic and North Pacific. This observation suggests that contraction of the low latitude warm pool was hemispherically symmetric. Our highly resolved (3 kyr resolution) Site 1125 SST record bears considerable structural similarity to SST records from nearby site 1123 (42οS,171οW) as well as sites 846 (3οS, 91οW) in the eastern equatorial Pacific and U1313 (41οN, 33οW) in the North Atlantic. Most of these SST records are dominated by 100k power and contain strong secondary 41k peaks throughout the past 3 million years. North Atlantic site U1313 is the exception, mirroring the shift in dominant periodicity from 41k to 100k associated with the mid-Pleistocene transition, that has long been observed in benthic oxygen isotope records. Finally, in southern hemisphere SST records as well as at site U1313 from the north Atlantic we observe weak precessional power that is not evident in benthic oxygen isotope record. These results suggest a fairly hemispherically-coordinated response of ocean surface temperature to changing global climate conditions during the Plio-Pleistocene in terms of both secular trends and dominant orbital frequencies.

Optimal Spectral Decomposition (OSD) for Ocean Data Assimilation

DTIC Science & Technology

2015-01-01

tropical North Atlantic from the Argo float data (Chu et al. 2007 ), and temporal and spatial variability of global upper-ocean heat content (Chu 2011...O. V. Melnichenko, and N. C. Wells, 2007 : Long baro- clinic Rossby waves in the tropical North Atlantic observed fromprofiling floats. J...Harrison, and D. Stammer , D., Eds., Vol. 2, ESA Publ. WPP- 306, doi:10.5270/OceanObs09.cwp.86. Tang, Y., and R. Kleeman, 2004: SST assimilation

On the nonlinear forced response of the North Atlantic atmosphere to meridional shifts of the Gulf Stream path

NASA Astrophysics Data System (ADS)

Seo, H.; Kwon, Y. O.; Joyce, T. M.; Ummenhofer, C.

2016-12-01

This study examines the North Atlantic atmospheric circulation response to the meridional shift of Gulf Stream path using a large-ensemble, high-resolution, and hemispheric-scale WRF simulations. The model is forced with wintertime SST anomalies derived from a wide range of Gulf Stream shift scenarios. The key result of the model experiments, supported in part by an independent analysis of a reanalysis data set, is that the large-scale, quasi-steady North Atlantic circulation response is unambiguously nonlinear about the sign and amplitude of chosen SST anomalies. This nonlinear response prevails over the weak linear response and resembles the negative North Atlantic Oscillation, the leading intrinsic mode of variability in the model and the observations. Further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the anomalous southward shift of the North Atlantic eddy-driven jet stream, which is reinforced nearly equally by the high-frequency transient eddy feedback and the low-frequency high-latitude wave breaking events. The result highlights the importance of the intrinsically nonlinear transient eddy dynamics and eddy-mean flow interactions in generating the nonlinear forced response to the meridional shift in the Gulf Stream.

The potential of the coral species Porites astreoides as a paleoclimate archive for the Tropical South Atlantic Ocean

NASA Astrophysics Data System (ADS)

Pereira, N. S.; Sial, A. N.; Frei, R.; Ullmann, C. V.; Korte, C.; Kikuchi, R. K. P.; Ferreira, V. P.; Kilbourne, K. H.

2017-08-01

The aragonitic skeletons of corals are unique archives of geochemical tracers that can be used as proxies for environmental conditions with high fidelity and sub-annual resolution. Such records have been extensively used for reconstruction of climatic conditions in the Pacific and Indian Oceans, Red Sea and Caribbean, but lack for the Equatorial South Atlantic. Here we present coral-based records of Sr/Ca, δ18O and δ13C and the first δ18O-SST calibration for the scleractinian coral species Porites astreoides from the Rocas Atoll, Equatorial South Atlantic. The investigated geochemical proxies for P. astreoides presented a very well-developed seasonal cyclicity in all proxies. We use the monthly means of δ18O and SST from the period of 2001-2013 to propose a calibration for a paleothermometer based on Porites, which gives T(°C) = -8.69(±0.79)* δ18O -7.05(±3.14), and yielded a SST δ18O-depended reconstruction with fidelity better than 0.5 °C for most of the record. Biases of up to 2 °C might be associated with reduced growth rate periods of the coral record. The Sr/Ca data show systematic, annual fluctuations but analyses are too imprecise to propose a Sr/Ca-SST calibration. The δ13C values are found to vary in phase with δ18O and Sr/Ca and are interpreted to be controlled by solar irradiation-modulated photosynthetic activity on the annual level. Our findings extend the global data base of coral records, contributing to further investigations using coral skeleton as environmental archives. In particular, the present study helps to better understand the climate variability of the South Atlantic tropical ocean-atmosphere system.

Covariability of Climate and Streamflow in the Upper Rio Grande from Interannual to Interdecadal Timescales

NASA Technical Reports Server (NTRS)

Pascolini-Campbell, M.; Seager, Richard; Pinson, Ariane; Cook, Benjamin I.

2017-01-01

Study region: The Upper Rio Grande (URG) flows from its headwaters in Colorado, U.S., and provides an important source of water to millions of people in the U.S. states of Colorado, New Mexico, Texas, and also Mexico. Study focus: We reassess the explanatory power of the relationship of sea surface temperatures (SST) on URG streamflow variability on interannual to interdecadal timescales. We find a significant amount of the variance of spring-summer URG streamflow cannot be fully explained by SST. New hydrological insights: We find that the interdecadal teleconnection between SST and streamflow is more clear than on interannual timescales. The highest ranked years tend to be clustered during positive phases of the Pacific Decadal Oscillation (PDO). During the periods of decadal high flow (1900-1920, and 1979-1995), Pacific SST resembles a positive PDO pattern and the Atlantic a negative Atlantic Multidecadal Oscillation (AMO) pattern; an interbasin pattern shown in prior studies to be conducive to high precipitation and streamflow. To account for the part of streamflow variance not explained by SST, we analyze atmospheric Reanalysis data for the months preceding the highest spring-summer streamflow events. A variety of atmospheric configurations are found to precede the highest flow years through anomalous moisture convergence. This lack of consistency suggests that, on interannual timescales, weather and not climate can dominate the generation of high streamflow events.

Multi-decadal Variability of the Indian Monsoon Rainfall for the last 14 kyr

NASA Astrophysics Data System (ADS)

Panmei, C.; Pothuri, D.

2017-12-01

Precise reconstruction of Indian monsoon fluctuation events and variability trends over the last 14 kyr has great implications for understanding the dynamics and possible forcing/feedback mechanisms associated with it. We have carried out high-resolution Indian monsoon variability studies of multi-decadal to sub-centennial timescales for the past 14 kyr through oxygen isotopes and Mg/Ca-derived sea surface temperatures (SST) from a western Bay of Bengal sediment core MD 161/17, using planktonic foraminifera Globigerinoides ruber. Indian summer monsoon (ISM) intensity was low during the Younger Dryas (YD) as evidenced by enriched δ18Osw coincides with a striking warming of 1.5°C. We observed ISM intensification from 12-9 kyr, followed by a milder period from 9-7.2 kyr. ISM gradually weakened from 7.2-2.5 kyr, after which there were two very prominent shifts in both ISM and SST; abrupt decrease at 2.4 kyr and increase at 1.4 kyr for ISM, while SST exhibited opposite trend. The contrasting trend continued from 1.4 kyr to the present wherein ISM precipitation has been decreasing and SST has been increasing. In addition, spectral analysis was done using Redfit and the ISM precipitation records reveal statistically significant periodicities at 2118, 411, 344, 144, 101 and 90 yrs. Furthermore, we compared our results with other existing records from the Northern Indian Ocean and adjacent regions, and found that the records share similarities suggesting regional dynamics being expressed coherently. Our results suggest that ISM precipitation and warming/cooling of the Northern Indian Ocean is directly associated with the southward/northward shift of the Intertropical Convergence Zone, which in turn is influenced by Atlantic Meridional Overturning Circulation, North Atlantic climate, and solar insolation interplaying differently at different timescales.

The Roles of Climate Change and Climate Variability in the 2017 Atlantic Hurricane Season

NASA Technical Reports Server (NTRS)

Lim, Young-Kwon; Schubert, Siegfried D.; Kovach, Robin; Molod, Andrea M.; Pawson, Steven

2018-01-01

The 2017 hurricane season was extremely active with six major hurricanes, the third most on record. The sea-surface temperatures (SSTs) over the eastern Main Development Region (EMDR), where many tropical cyclones (TCs) developed during active months of August/September, were approximately 0.96 degrees Centigrade above the 1901-2017 average (warmest on record): about 0.42 degrees Centigrade from a long-term upward trend and the rest (around 80 percent) attributed to the Atlantic Meridional Mode (AMM). The contribution to the SST from the North Atlantic Oscillation over the EMDR was a weak warming, while that from ENSO was negligible. Nevertheless, ENSO, the NAO, and the AMM all contributed to favorable wind shear conditions, while the AMM also produced enhanced atmospheric instability. Compared with the strong hurricane years of 2005-2010, the ocean heat content (OHC) during 2017 was larger across the tropics, with higher SST anomalies over the EMDR and Caribbean Sea. On the other hand, the dynamical/thermodynamical atmospheric conditions, while favorable for enhanced TC activity, were less prominent than in 2005-2010 across the tropics. The results suggest that unusually warm SST in the EMDR together with the long fetch of the resulting storms in the presence of record-breaking OHC were key factors in driving the strong TC activity in 2017.

Multidecadal climate variability of global lands and oceans

USGS Publications Warehouse

McCabe, G.J.; Palecki, M.A.

2006-01-01

Principal components analysis (PCA) and singular value decomposition (SVD) are used to identify the primary modes of decadal and multidecadal variability in annual global Palmer Drought Severity Index (PDSI) values and sea-surface temperature (SSTs). The PDSI and SST data for 1925-2003 were detrended and smoothed (with a 10-year moving average) to isolate the decadal and multidecadal variability. The first two principal components (PCs) of the PDSI PCA explained almost 38% of the decadal and multidecadal variance in the detrended and smoothed global annual PDSI data. The first two PCs of detrended and smoothed global annual SSTs explained nearly 56% of the decadal variability in global SSTs. The PDSI PCs and the SST PCs are directly correlated in a pairwise fashion. The first PDSI and SST PCs reflect variability of the detrended and smoothed annual Pacific Decadal Oscillation (PDO), as well as detrended and smoothed annual Indian Ocean SSTs. The second set of PCs is strongly associated with the Atlantic Multidecadal Oscillation (AMO). The SVD analysis of the cross-covariance of the PDSI and SST data confirmed the close link between the PDSI and SST modes of decadal and multidecadal variation and provided a verification of the PCA results. These findings indicate that the major modes of multidecadal variations in SSTs and land-surface climate conditions are highly interrelated through a small number of spatially complex but slowly varying teleconnections. Therefore, these relations may be adaptable to providing improved baseline conditions for seasonal climate forecasting. Published in 2006 by John Wiley & Sons, Ltd.

Long-term changes in the Northwestern Atlantic and Mediterranean SST from 1982 to 2016: A contribution of the Operational Oceanography to the determination of the present day Climate

NASA Astrophysics Data System (ADS)

Artale, Vincenzo; Buongiorno Nardelli, Bruno; Marullo, Salvatore; Pisano, Andrea; Santoleri, Rosalia; Tronconi, Cristina

2017-04-01

Estimating long-term SST changes is crucial to evaluate global warming impact at regional scales. Here, we analyze the Mediterranean (MED) and the Northwestern Atlantic Box (NWA) SST changes over the last 34 years (1982 - 2016) by combining reprocessed (REP) and near-real-time (NRT) data. Actually, the Italian National Research Council (CNR) has recently produced daily (nighttime), 4 km resolution REP MED level 4 datasets (REP L4), also covering the adjacent Atlantic region, based on the latest Pathfinder v5.2 AVHRR dataset (1982-2012). These data represent the longest satellite MED SST L4 time series and are freely distributed through the European Copernicus Marine Environment Monitoring Service (CMEMS). However, as Pathfinder has not yet released an update of its product, the REP data end in 2012. To fill in the gap between 2013 and 2016, we investigated the possibility to extend the time series by using the Mediterranean near real time (NRT), multi-sensor L4 SST data at Ultra-High spatial Resolution (UHR) produced by CNR, which are distributed through CMEMS and now mirrored at GHRSST. Since this product is available since 2008, the consistency with the REP has been assessed. Combining the REP L4 data (1982-2012) and a bias-corrected version of the NRT L4 data (2013-2016), we built the SST time series and provided updated estimates of the MED and NWA SST trends. The analysis shows that The Atlantic Box and The Mediterranean Sea have similar trend behavior until 2008. Afterward the Mediterranean Sea SST continued to increase while the Atlantic Box persisted in its warming pause.

The climate of the Common Era off the Iberian Peninsula

NASA Astrophysics Data System (ADS)

Abrantes, Fátima; Rodrigues, Teresa; Rufino, Marta; Salgueiro, Emília; Oliveira, Dulce; Gomes, Sandra; Oliveira, Paulo; Costa, Ana; Mil-Homens, Mário; Drago, Teresa; Naughton, Filipa

2017-12-01

The Mediterranean region is a climate hot spot, sensitive not only to global warming but also to water availability. In this work we document major temperature and precipitation changes in the Iberian Peninsula and margin during the last 2000 years and propose an interplay of the North Atlantic internal variability with the three atmospheric circulation modes (ACMs), (North Atlantic Oscillation (NAO), east atlantic (EA) and Scandinavia (SCAND)) to explain the detected climate variability. We present reconstructions of sea surface temperature (SST derived from alkenones) and on-land precipitation (estimated from higher plant n-alkanes and pollen data) in sedimentary sequences recovered along the Iberian Margin between the south of Portugal (Algarve) and the northwest of Spain (Galiza) (36 to 42° N). A clear long-term cooling trend, from 0 CE to the beginning of the 20th century, emerges in all SST records and is considered to be a reflection of the decrease in the Northern Hemisphere summer insolation that began after the Holocene optimum. Multi-decadal/centennial SST variability follows other records from Spain, Europe and the Northern Hemisphere. Warm SSTs throughout the first 1300 years encompass the Roman period (RP), the Dark Ages (DA) and the Medieval Climate Anomaly (MCA). A cooling initiated at 1300 CE leads to 4 centuries of colder SSTs contemporary with the Little Ice Age (LIA), while a climate warming at 1800 CE marks the beginning of the modern/Industrial Era. Novel results include two distinct phases in the MCA: an early period (900-1100 years) characterized by intense precipitation/flooding and warm winters but a cooler spring-fall season attributed to the interplay of internal oceanic variability with a positive phase in the three modes of atmospheric circulation (NAO, EA and SCAND). The late MCA is marked by cooler and relatively drier winters and a warmer spring-fall season consistent with a shift to a negative mode of the SCAND. The Industrial Era reveals a clear difference between the NW Iberia and the Algarve records. While off NW Iberia variability is low, the Algarve shows large-amplitude decadal variability with an inverse relationship between SST and river input. Such conditions suggest a shift in the EA mode, from negative between 1900 and 1970 CE to positive after 1970, while NAO and SCAND remain in a positive phase. The particularly noticeable rise in SST at the Algarve site by the mid-20th century (±1970), provides evidence for a regional response to the ongoing climate warming. The reported findings have implications for decadal-scale predictions of future climate change in the Iberian Peninsula.

Decadal time scale variability recorded in the Quelccaya summit ice core δ18O isotopic ratio series and its relation with the sea surface temperature

NASA Astrophysics Data System (ADS)

Mélice, J. L.; Roucou, P.

The spectral characteristics of the δ18O isotopic ratio time series of the Quelccaya ice cap summit core are investigated with the multi taper method (MTM), the singular spectrum analysis (SSA) and the wavelet transform (WT) techniques for the 500 y long 1485-1984 period. The most significant (at the 99.8% level) cycle according to the MTM F-test has a period centered at 14.4 y while the largest variance explaining oscillation according to the SSA technique has a period centered at 12.9 y. The stability over time of these periods is investigated by performing evolutive MTM and SSA on the 500 y long δ18O series with a 100 y wide moving window. It is shown that the cycles with largest amplitude and that the oscillations with largest extracting variance have corresponding periods aggregated around 13.5 y that are very stable over the period between 1485 and 1984. The WT of the same isotopic time series reveals the existence of a main oscillation around 12 y which are also very stable in time. The relation between the isotopic data at Quelccaya and the annual sea surface temperature (SST) field anomalies is then evaluated for the overlapping 1919-1984 period. Significant global correlation and significant coherency at 12.1 y are found between the isotopic series and the annual global sea surface temperature (GSST) series. Moreover, the correlation between the low (over 8 y) frequency component of the isotopic time series and the annual SST field point out significant values in the tropical North Atlantic. This region is characterized by a main SST variability at 12.8 y. The Quelccaya δ18O isotopic ratio series may therefore be considered as a good recorder of the tropical North Atlantic SSTs. This may be explained by the following mechanism: the water vapor amount evaporated by the tropical North Atlantic is function of the SST. So is the water vapor δ18O isotopic ratio. This water vapor is advected during the rainy season by northeast winds and precipitates at the Quelccaya summit with its tropical North Atlantic isotopic signature. It is also suggested from this described stability of the decadal time scale variability observed in the Quelccaya isotopic series, that the decadal time scale GSST variability was also stable during the last five centuries.

Reevaluation of mid-Pliocene North Atlantic sea surface temperatures

USGS Publications Warehouse

Robinson, Marci M.; Dowsett, Harry J.; Dwyer, Gary S.; Lawrence, Kira T.

2008-01-01

Multiproxy temperature estimation requires careful attention to biological, chemical, physical, temporal, and calibration differences of each proxy and paleothermometry method. We evaluated mid-Pliocene sea surface temperature (SST) estimates from multiple proxies at Deep Sea Drilling Project Holes 552A, 609B, 607, and 606, transecting the North Atlantic Drift. SST estimates derived from faunal assemblages, foraminifer Mg/Ca, and alkenone unsaturation indices showed strong agreement at Holes 552A, 607, and 606 once differences in calibration, depth, and seasonality were addressed. Abundant extinct species and/or an unrecognized productivity signal in the faunal assemblage at Hole 609B resulted in exaggerated faunal-based SST estimates but did not affect alkenone-derived or Mg/Ca–derived estimates. Multiproxy mid-Pliocene North Atlantic SST estimates corroborate previous studies documenting high-latitude mid-Pliocene warmth and refine previous faunal-based estimates affected by environmental factors other than temperature. Multiproxy investigations will aid SST estimation in high-latitude areas sensitive to climate change and currently underrepresented in SST reconstructions.

Tropical Forcing of the Summer East Atlantic Pattern

NASA Astrophysics Data System (ADS)

Wulff, C. Ole; Greatbatch, Richard J.; Domeisen, Daniela I. V.; Gollan, Gereon; Hansen, Felicitas

2017-11-01

The Summer East Atlantic (SEA) mode is the second dominant mode of summer low-frequency variability in the Euro-Atlantic region. Using reanalysis data, we show that SEA-related circulation anomalies significantly influence temperatures and precipitation over Europe. We present evidence that part of the interannual SEA variability is forced by diabatic heating anomalies of opposing signs in the tropical Pacific and Caribbean that induce an extratropical Rossby wave train. This precipitation dipole is related to SST anomalies characteristic of the developing El Niño-Southern Oscillation phases. Seasonal hindcast experiments forced with observed sea surface temperatures (SSTs) exhibit skill at capturing the interannual SEA variability corroborating the proposed mechanism and highlighting the possibility for improved prediction of boreal summer variability. Our results indicate that tropical forcing of the SEA likely played a role in the dynamics of the 2015 European heat wave.

On the Influence of Global Warming on Atlantic Hurricane Frequency

NASA Astrophysics Data System (ADS)

Hosseini, S. R.; Scaioni, M.; Marani, M.

2018-04-01

In this paper, the possible connection between the frequency of Atlantic hurricanes to the climate change, mainly the variation in the Atlantic Ocean surface temperature has been investigated. The correlation between the observed hurricane frequency for different categories of hurricane's intensity and Sea Surface Temperature (SST) has been examined over the Atlantic Tropical Cyclogenesis Regions (ACR). The results suggest that in general, the frequency of hurricanes have a high correlation with SST. In particular, the frequency of extreme hurricanes with Category 5 intensity has the highest correlation coefficient (R = 0.82). In overall, the analyses in this work demonstrates the influence of the climate change condition on the Atlantic hurricanes and suggest a strong correlation between the frequency of extreme hurricanes and SST in the ACR.

An 800-Year Record of Sediment-Derived, Instrumentally-Calibrated Foraminiferal Mg/Ca SST Estimates From the Tropical North Atlantic

NASA Astrophysics Data System (ADS)

Black, D. E.; Abahazi, M. A.; Thunell, R. C.; Tappa, E. J.

2005-12-01

Most geochemical paleoclimate proxies are calibrated to different climate variables using laboratory culture, surface sediment, or sediment trap experiments. The varved, high-deposition rate sediments of the Cariaco Basin (Venezuela) provide the nearly unique opportunity to compare and calibrate paleoceanographic proxy data directly against true oceanic historical instrumental climate records. Here we present one of the first sediment-derived foraminiferal-Mg/Ca to SST calibrations spanning A. D. 1870-1990. The record of Mg/Ca-estimated tropical North Atlantic SSTs is then extended back to approximately A. D. 1200. Box core PL07-73 BC, recovered from the northeastern slope of Cariaco Basin, was sampled at consecutive 1 mm increments and processed for foraminiferal population, stable isotope, and Mg/Ca (by ICP-AES) analyses. The age model for this core was established by correlating faunal population records from PL07-73 to a nearby very well-dated Cariaco Basin box core, PL07-71 BC. The resulting age model yields consecutive sample intervals of one to two years. Mg/Ca ratios measured on Globigerina bulloides in samples deposited between A. D. 1870 and 1990 were calibrated to monthly SSTs from the Met Office Hadley Centre's SST data set for the Cariaco Basin grid square. Annual correlations between G. bulloides Mg/Ca and instrumental SST were highest (r=0.6, p<.0001, n=120) for the months of March, April, and May, the time when sediment trap studies indicate G. bulloides is most abundant in the basin. The full-length Mg/Ca-estimated SST record is characterized by decadal- and centennial-scale variability. The tropical western North Atlantic does not appear to have experienced a pronounced Medieval Warm Period relative to the complete record. However, strong Little Ice Age cooling of as much as 3 ° C occurred between A. D. 1525 and 1625. Spring SSTs gradually rose between A. D. 1650 and 1900 followed by a 2.5 ° C warming over the 20th century.

Influence of SST anomalies in low latitudes on atmospheric heat transport to the Arctic

NASA Astrophysics Data System (ADS)

Alekseev, Genrikh; Kuzmina, Svetlana; Glok, Natalia

2017-04-01

The purpose of the study is to assess the influence of SST anomalies in the low latitudes of the Atlantic, Indian and Pacific oceans to climatic change of the winter atmospheric meridional heat transport (MAHT) to the Arctic and to propose the mechanisms of this influence. Estimates of sensible and latent heat transport to the Arctic through the "Atlantic Gate" at 70 ° N in winter (December-February) 1980-2015 fulfilled on base ERA / Interim and monthly SST from HadISST were used. Multi-dimensional cross-correlation analysis was applied. The area and month in each ocean were found with maximal correlations between SST and winter MAHT. Mean SST in selected areas for each month of 1980-2015 were calculated and its correlations with MAHT were estimated. The correlation coefficients equal from 0.57 to 0.42, and after removing the noise increased up to 0.75 with MAHT lag from 27 to 30 months. The SST and MAHT series include together with positive trend the 5-7 years fluctuations. The mechanism of SST anomalies influence on winter MAHT to the Arctic includes the interaction of atmospheric (Hadley and Ferrel circulations, jet streams, NAO) and oceanic (Gulf Stream, the North Atlantic, the Norwegian currents) circulation patterns. To justify the proposed scheme the evaluation of the links between SST anomalies, the NAO index, the Atlantic water inflow to the Barents Sea, are investigated. The study is supported with RFBR project 15-05-03512.

Predicting the spatiotemporal distributions of marine fish species utilizing earth system data in a maximum entropy modeling framework

NASA Astrophysics Data System (ADS)

Wang, L.; Kerr, L. A.; Bridger, E.

2016-12-01

Changes in species distributions have been widely associated with climate change. Understanding how ocean conditions influence marine fish distributions is critical for elucidating the role of climate in ecosystem change and forecasting how fish may be distributed in the future. Species distribution models (SDMs) can enable estimation of the likelihood of encountering species in space or time as a function of environmental conditions. Traditional SDMs are applied to scientific-survey data that include both presences and absences. Maximum entropy (MaxEnt) models are promising tools as they can be applied to presence-only data, such as those collected from fisheries or citizen science programs. We used MaxEnt to relate the occurrence records of marine fish species (e.g. Atlantic herring, Atlantic mackerel, and butterfish) from NOAA Northeast Fisheries Observer Program to environmental conditions. Environmental variables from earth system data, such as sea surface temperature (SST), sea bottom temperature (SBT), Chlorophyll-a, bathymetry, North Atlantic oscillation (NAO), and Atlantic multidecadal oscillation (AMO), were matched with species occurrence for MaxEnt modeling the fish distributions in Northeast Shelf area. We developed habitat suitability maps for these species, and assessed the relative influence of environmental factors on their distributions. Overall, SST and Chlorophyll-a had greatest influence on their monthly distributions, with bathymetry and SBT having moderate influence and climate indices (NAO and AMO) having little influence. Across months, Atlantic herring distribution was most related to SST 10th percentile, and Atlantic mackerel and butterfish distributions were most related to previous month SST. The fish distributions were most affected by previous month Chlorophyll-a in summer months, which may indirectly indicate the accumulative impact of primary productivity. Results highlighted the importance of spatial and temporal scales when using SDMs to investigate the habitat suitability and distributions of a focal species. MaxEnt models have the potential to provide hindcasts of where species might have been in the past in relation to historical environmental conditions, nowcasts in relation to current conditions, or forecasts of future species distributions.

Climatic records of the last and penultimate deglaciations in the South Atlantic and South Indian Ocean

NASA Astrophysics Data System (ADS)

Michel, Elisabeth; Waelbroeck, Claire; Govin, Aline; Skinner, Luke; Vàzquez Riveiros, Natalia; Dewilde, Fabien; Isguder, Gulay; Rebaubier, Hélène

2013-04-01

Surface and deep-water records of Termination I and II in two twin South Atlantic deep-sea cores (44°09' S, 14°14' W, 3770 m depth) and one South Indian core (46°29' S, 88°01' E, 3420 m depth) are presented. Sea surface temperature has been reconstructed based on planktonic foraminifera census counts in all cases, as well as Mg/Ca of G. bulloides and N. pachyderma s. over the last deglaciation. The uncertainty on reconstructed SST using different statistical methods and different faunal databases is assessed. Over the last deglaciation, combined 14C dating and correlation of the SST record with the air temperature signal recorded in Antarctic ice cores allowed us to correct for variable surface reservoir ages in the South Atlantic core (Skinner et al., 2010). Preliminary dating of the South Indian core over the last termination has been done by correlation of its magnetic signal with those of a neighboring 14C dated core (Smart et al., 2010). We have refined the later age scale using the Atlantic core age scale as reference. Benthic isotopic signals in the South Atlantic and South Indian cores over the last deglaciation exhibit the same amplitude and timing. Our results thus indicate that bottom waters at the South Indian site remained isolated from better ventilated deep waters of northern origin until ~15 ka (Waelbroeck et al., 2011). Over Termination II, the two cores have been dated by correlation of their SST records with the air temperature signal recorded in EDC versus the EDC3 age scale (Govin et al., 2009; 2012). A careful examination of the various sources of uncertainty on the derived dating has been performed. Benthic and planktonic isotopic signals reveal analogies but also differences with respect to the last termination. SST was significantly warmer during the Last Interglacial than during the Holocene in both sites. South Atlantic deep waters were also significantly better ventilated during the Last Interglacial than during the Holocene, whereas bottom water ventilation was similar during these two interglacials at the South Indian site.

How Robust Are the Surface Temperature Fingerprints of the Atlantic Overturning Meridional Circulation on Monthly Time Scales?

NASA Astrophysics Data System (ADS)

Alexander-Turner, R.; Ortega, P.; Robson, J. I.

2018-04-01

It has been suggested that changes in the Atlantic Meridional Overturning Circulation (AMOC) can drive sea surface temperature (SST) on monthly time scales (Duchez et al., 2016, https://doi.org/10.1002/2017GB005667). However, with only 11 years of continuous observations, the validity of this result over longer, or different, time periods is uncertain. In this study, we use a 120 yearlong control simulation from a high-resolution climate model to test the robustness of the AMOC fingerprints. The model reproduces the observed AMOC seasonal cycle and its variability, and the observed 5-month lagged AMOC-SST fingerprints derived from 11 years of data. However, the AMOC-SST fingerprints are very sensitive to the particular time period considered. In particular, both the Florida current and the upper mid-ocean transport produce highly inconsistent fingerprints when using time periods shorter than 30 years. Therefore, several decades of RAPID observations will be necessary to determine the real impact of the AMOC on SSTs at monthly time scales.

Climatic Constraints on Growth Rate and Geochemistry (Sr/Ca and U/Ca) of the Coral Siderastrea stellata in the Southwest Equatorial Atlantic (Rocas Atoll, Brazil)

NASA Astrophysics Data System (ADS)

Evangelista, H.; Sifeddine, A.; Corrège, T.; Servain, J.; Dassié, E. P.; Logato, R.; Cordeiro, R. C.; Shen, C.-C.; Le Cornec, F.; Nogueira, J.; Segal, B.; Castagna, A.; Turcq, B.

2018-03-01

Although relatively rare compared to similar latitudes in the Pacific or Indian Oceans, massive coral colonies are present in the Tropical/Equatorial Southwestern Atlantic Ocean. However, detailed geochemical compositions of these corals are still largely unknown. In this work, we present growth rates, Sr/Ca, and U/Ca ratios of the coral colony (Siderastrea stellata) sampled at Rocas Atoll, off the Brazilian coast. These variables are primarily affected by sea surface temperature (SST) at seasonal scale, and by wind stress at interannual scale, these results represent a broad new finding. A lower significance at the interannual time scale between Sr/Ca and U/Ca with respect to SST is attributed to the low SST amplitude closed to Equator. An investigation on the dependence of coral growth rates with respect to the "cloud shading effect" promoted by the Intertropical Convergence Zone (ITCZ) does not show significant influence. Additionally, rain seems to act on local geochemistry of Sr/Ca ratios and growth rate at the decadal scale.

Assessing millennial-scale variability during the Holocene: A perspective from the western tropical Pacific

NASA Astrophysics Data System (ADS)

Khider, D.; Jackson, C. S.; Stott, L. D.

2014-03-01

We investigate the relationship between tropical Pacific and Southern Ocean variability during the Holocene using the stable oxygen isotope and magnesium/calcium records of cooccurring planktonic and benthic foraminifera from a marine sediment core collected in the western equatorial Pacific. The planktonic record exhibits millennial-scale sea surface temperature (SST) oscillations over the Holocene of 0.5°C while the benthic δ18Oc document 0.10‰ millennial-scale changes of Upper Circumpolar Deep Water (UCDW), a water mass which outcrops in the Southern Ocean. Solar forcing as an explanation for millennial-scale SST variability requires (1) a large climate sensitivity and (2) a long 400 year delayed response, suggesting that if solar forcing is the cause of the variability, it would need to be considerably amplified by processes within the climate system at least at the core location. We also explore the possibility that SST variability arose from volcanic forcing using a simple red noise model. Our best estimates of volcanic forcing falls short of reproducing the amplitude of observed SST variations although it produces power at low-frequency similar to that observed in the MD81 record. Although we cannot totally discount the volcanic and solar forcing hypotheses, we are left to consider that the most plausible source for Holocene millennial-scale variability lies within the climate system itself. In particular, UCDW variability coincided with deep North Atlantic changes, indicating a role for the deep ocean in Holocene millennial-scale variability.

Effects of Northern Hemisphere Sea Surface Temperature Changes on the Global Air Quality

NASA Astrophysics Data System (ADS)

Yi, K.; Liu, J.

2017-12-01

The roles of regional sea surface temperature (SST) variability on modulating the climate system and consequently the air quality are investigated using the Community Earth System Model (CESM). Idealized, spatially uniform SST anomalies of +/- 1 °C are superimposed onto the North Pacific, North Atlantic, and North Indian Oceans individually. Ignoring the response of natural emissions, our simulations suggest large seasonal and regional variability of surface O3 and PM2.5 concentrations in response to SST anomalies, especially during boreal summers. Increasing the SST by 1 °C in one of the oceans generally decreases the surface O3 concentrations from 1 to 5 ppbv while increases the anthropogenic PM2.5 concentrations from 0.5 to 3 µg m-3. We implement the integrated process rate (IPR) analysis in CESM and find that meteorological transport in response to SST changes is the key process causing air pollutant perturbations in most cases. During boreal summers, the increase in tropical SST over different ocean basins enhances deep convection, which significantly increases the air temperature over the upper troposphere and trigger large-scale subsidence over nearby and remote regions. These processes tend to increase tropospheric stability and suppress rainfall at lower mid-latitudes. Consequently, it reduces the vertical transport of O3 to the surface while facilitating the accumulation of PM2.5 concentrations over most regions. In addition, this regional SST warming may also considerably suppress intercontinental transport of air pollution as confirmed with idealized CO-like tracers. Our findings indicate a robust linkage between basin-scale SST variability and regional air quality, which can help local air quality management.

Simulated variability of the Atlantic meridional overturning circulation

NASA Astrophysics Data System (ADS)

Bentsen, M.; Drange, H.; Furevik, T.; Zhou, T.

To examine the multi-annual to decadal scale variability of the Atlantic Meridional Overturning Circulation (AMOC) we conducted a four-member ensemble with a daily reanalysis forced, medium-resolution global version of the isopycnic coordinate ocean model MICOM, and a 300-years integration with the fully coupled Bergen Climate Model (BCM). The simulations of the AMOC with both model systems yield a long-term mean value of 18 Sv and decadal variability with an amplitude of 1-3 Sv. The power spectrum of the inter-annual to decadal scale variability of the AMOC in BCM generally follows the theoretical red noise spectrum, with indications of increased power near the 20-years period. Comparison with observational proxy indices for the AMOC, e.g. the thickness of the Labrador Sea Water, the strength of the baroclinic gyre circulation in the North Atlantic Ocean, and the surface temperature anomalies along the mean path of the Gulf Stream, shows similar trends and phasing of the variability, indicating that the simulated AMOC variability is robust and real. Mixing indices have been constructed for the Labrador, the Irminger and the Greenland-Iceland-Norwegian (GIN) seas. While convective mixing in the Labrador and the GIN seas are in opposite phase, and linked to the NAO as observations suggest, the convective mixing in the Irminger Sea is in phase with or leads the Labrador Sea. Newly formed deep water is seen as a slow, anomalous cold and fresh, plume flowing southward along the western continental slope of the Atlantic Ocean, with a return flow of warm and saline water on the surface. In addition, fast-travelling topographically trapped waves propagate southward along the continental slope towards equator, where they go east and continue along the eastern rim of the Atlantic. For both types of experiments, the Northern Hemisphere sea level pressure and 2 m temperature anomaly patterns computed based on the difference between climate states with strong and weak AMOC yields a NAO-like pattern with intensified Icelandic low and Azores high, and a warming of 0.25-0.5 °C of the central North Atlantic sea-surface temperature (SST). The reanalysis forced simulations indicate a coupling between the Labrador Sea Water production rate and an equatorial Atlantic SST index in accordance with observations. This coupling is not identified in the coupled simulation.

On the relationship between satellite-retrieved surface temperature fronts and chlorophyll a in the western South Atlantic

NASA Astrophysics Data System (ADS)

Saraceno, Martin; Provost, Christine; Piola, Alberto R.

2005-11-01

The time-space distribution of chlorophyll a in the southwestern Atlantic is examined using 6 years (1998-2003) of sea surface color images from Sea-viewing Wide Field of View Sensor (SeaWiFS). Chlorophyll a (chl a) distribution is confronted with sea surface temperature (SST) fronts retrieved from satellite imagery. Histogram analysis of the color, SST, and SST gradient data sets provides a simple procedure for pixel classification from which eight biophysical regions in the SWA are identified, including three new regions with regard to Longhurst (1998) work: Patagonian Shelf Break (PSB), Brazil Current Overshoot, and Zapiola Rise region. In the PSB region, coastal-trapped waves are suggested as a possible mechanism leading to the intraseasonal frequencies observed in SST and chl a. Mesoscale activity associated with the Brazil Current Front and, in particular, eddies drifting southward is probably responsible for the high chl a values observed throughout the Brazil Current Overshoot region. The Zapiola Rise is characterized by a local minimum in SST gradient magnitudes and shows chl a maximum values in February, 3 months later than the austral spring bloom of the surroundings. Significant interannual variability is present in the color imagery. In the PSB, springs and summers with high chl a concentrations seem associated with stronger local northerly wind speed, and possible mechanisms are discussed. Finally, the Brazil-Malvinas front is detected using both SST gradient and SeaWiFS images. The time-averaged position of the front at 54.2°W is estimated at 38.9°S and its alongshore migration of about 300 km.

Thermal evolution of the western South Atlantic and the adjacent continent during Termination 1

NASA Astrophysics Data System (ADS)

Chiessi, C. M.; Mulitza, S.; Mollenhauer, G.; Silva, J. B.; Groeneveld, J.; Prange, M.

2015-06-01

During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SSTs) of the western South Atlantic, and in mean air temperatures (MATs) over southeastern South America. It has been suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is so far largely unknown. Here we address this issue, presenting high-temporal-resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling-Allerød, in phase with an existing SST record from the NBC. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental in driving the Earth out of the last glacial.

Thermal evolution of the western South Atlantic and the adjacent continent during Termination 1

NASA Astrophysics Data System (ADS)

Chiessi, C. M.; Mulitza, S.; Mollenhauer, G.; Silva, J. B.; Groeneveld, J.; Prange, M.

2014-12-01

During Termination 1, millennial-scale weakening events of the Atlantic meridional overturning circulation (AMOC) supposedly produced major changes in sea surface temperatures (SST) of the western South Atlantic, and in mean air temperatures (MAT) over southeastern South America. It was suggested, for instance, that the Brazil Current (BC) would strengthen (weaken) and the North Brazil Current (NBC) would weaken (strengthen) during slowdown (speed-up) events of the AMOC. This anti-phase pattern was claimed to be a necessary response to the decreased North Atlantic heat piracy during periods of weak AMOC. However, the thermal evolution of the western South Atlantic and the adjacent continent is largely unknown and a compelling record of the BC-NBC anti-phase behavior remains elusive. Here we address this issue, presenting high temporal resolution SST and MAT records from the BC and southeastern South America, respectively. We identify a warming in the western South Atlantic during Heinrich Stadial 1 (HS1), which is followed first by a drop and then by increasing temperatures during the Bølling-Allerød, in-phase with an existing NBC record. Additionally, a similar SST evolution is shown by a southernmost eastern South Atlantic record, suggesting a South Atlantic-wide pattern in SST evolution during most of Termination 1. Over southeastern South America, our MAT record shows a two-step increase during Termination 1, synchronous with atmospheric CO2 rise (i.e., during the second half of HS1 and during the Younger Dryas), and lagging abrupt SST changes by several thousand years. This delay corroborates the notion that the long duration of HS1 was fundamental to drive the Earth out of the last glacial.

A reconstruction of sea surface temperature variability in the southeastern Gulf of Mexico from 1734 to 2008 C.E. using cross-dated Sr/Ca records from the coral Siderastrea siderea

USGS Publications Warehouse

DeLong, Kristine L.; Maupin, Christopher R.; Flannery, Jennifer A.; Quinn, Terrence M.; Shen, CC

2014-01-01

This study uses skeletal variations in coral Sr/Ca from three Siderastrea siderea coral colonies within the Dry Tortugas National Park in the southeastern Gulf of Mexico (24°42′N, 82°48′W) to reconstruct monthly sea surface temperature (SST) variations from 1734 to 2008 Common Era (C.E.). Calibration and verification of the replicated coral Sr/Ca-SST reconstruction with local, regional, and historical temperature records reveals that this proxy-temperature relationship is stable back to 1879 C.E. The coral SST reconstruction contains robust interannual (~2.0°C) and multidecadal variability (~1.5°C) for the past 274 years, the latter of which does not covary with the Atlantic Multidecadal Oscillation. Winter SST extremes are more variable than summer SST extremes (±2.2°C versus ±1.6°C, 2σ) suggesting that Loop Current transport in the winter dominates variability on interannual and longer time scales. Summer SST maxima are increasing (+1.0°C for 274 years, σMC = ±0.5°C, 2σ), whereas winter SST minima contain no significant trend. Colder decades (~1.5°C) during the Little Ice Age (LIA) do not coincide with decades of sunspot minima. The coral SST reconstruction contains similar variability to temperature reconstructions from the northern Gulf of Mexico (planktic foraminifer Mg/Ca) and the Caribbean Sea (coral Sr/Ca) suggesting areal reductions in the Western Hemisphere Warm Pool during the LIA. Mean summer coral SST extremes post-1985 C.E. (29.9°C) exceeds the long-term summer average (29.2°C for 1734–2008 C.E.), yet the warming trend after 1985 C.E. (0.04°C for 24 years, σMC = ±0.5, 2σ) is not significant, whereas Caribbean coral Sr/Ca studies contain a warming trend for this interval.

Malaria epidemics and the influence of the tropical South Atlantic on the Indian monsoon

NASA Astrophysics Data System (ADS)

Cash, B. A.; Rodó, X.; Ballester, J.; Bouma, M. J.; Baeza, A.; Dhiman, R.; Pascual, M.

2013-05-01

The existence of predictability in the climate system beyond the relatively short timescales of synoptic weather has provided significant impetus to investigate climate variability and its consequences for society. In particular, relationships between the relatively slow changes in sea surface temperature (SST) and climate variability at widely removed points across the globe provide a basis for statistical and dynamical efforts to predict numerous phenomena, from rainfall to disease incidence, at seasonal to decadal timescales. We describe here a remote influence, identified through observational analysis and supported through numerical experiments with a coupled atmosphere-ocean model, of the tropical South Atlantic (TSA) on both monsoon rainfall and malaria epidemics in arid northwest India. Moreover, SST in the TSA is shown to provide the basis for an early warning of anomalous hydrological conditions conducive to malaria epidemics four months later, therefore at longer lead times than those afforded by rainfall. We find that the TSA is not only significant as a modulator of the relationship between the monsoon and the El Niño/Southern Oscillation, as has been suggested by previous work, but for certain regions and temporal lags is in fact a dominant driver of rainfall variability and hence malaria outbreaks.

Changing characteristics of streamflow in the Midwest and its relation to oceanic-atmospheric oscillations

NASA Astrophysics Data System (ADS)

Thakur, B.; Pathak, P.; Kalra, A.; Ahmad, S.

2016-12-01

The identification of primary drivers of streamflow may prove beneficial in forecasting streamflow in the Midwestern U.S. In the past researches, streamflow in the region have been strongly correlated with El Niño-Southern Oscillation (ENSO), Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO). The present study takes in to account the pre-defined Pacific and Atlantic Ocean regions (e.g., ENSO, PDO, AMO) along with new regions with an intent to identify new significantly correlated regions. This study assesses the interrelationship between sea surface temperatures (SST) anomalies in the Pacific and Atlantic Ocean and seasonal streamflow in the Midwestern U.S. Average Pacific and Atlantic Ocean SST anomalies, were calculated for 2 different 3 month series: September-November and December-February so as to create a lead time varying from 3 to 9 months. Streamflow were averaged for three seasons: spring (April-June), spring-summer (April-August) and summer (June-August). The correlation between streamflow and SST is analyzed using singular value decomposition for a period of 1960-2013. The result of the study showed several regions-other than the known Pacific and Atlantic Ocean regions- that were significantly correlated with streamflow stations. Higher correlation between the climate indices and streamflow were observed as the lead time decreased. The identification of the associations between SST and streamflow and significant SST regions in the Pacific and Atlantic Ocean may enhance the skill of streamflow predictability and water management in the region.

A comparison between general circulation model simulations using two sea surface temperature datasets for January 1979

NASA Technical Reports Server (NTRS)

Ose, Tomoaki; Mechoso, Carlos; Halpern, David

1994-01-01

Simulations with the UCLA atmospheric general circulation model (AGCM) using two different global sea surface temperature (SST) datasets for January 1979 are compared. One of these datasets is based on Comprehensive Ocean-Atmosphere Data Set (COADS) (SSTs) at locations where there are ship reports, and climatology elsewhere; the other is derived from measurements by instruments onboard NOAA satellites. In the former dataset (COADS SST), data are concentrated along shipping routes in the Northern Hemisphere; in the latter dataset High Resolution Infrared Sounder (HIRS SST), data cover the global domain. Ensembles of five 30-day mean fields are obtained from integrations performed in the perpetual-January mode. The results are presented as anomalies, that is, departures of each ensemble mean from that produced in a control simulation with climatological SSTs. Large differences are found between the anomalies obtained using COADS and HIRS SSTs, even in the Northern Hemisphere where the datasets are most similar to each other. The internal variability of the circulation in the control simulation and the simulated atmospheric response to anomalous forcings appear to be linked in that the pattern of geopotential height anomalies obtained using COADS SSTs resembles the first empirical orthogonal function (EOF 1) in the control simulation. The corresponding pattern obtained using HIRS SSTs is substantially different and somewhat resembles EOF 2 in the sector from central North America to central Asia. To gain insight into the reasons for these results, three additional simulations are carried out with SST anomalies confined to regions where COADS SSTs are substantially warmer than HIRS SSTs. The regions correspond to warm pools in the northwest and northeast Pacific, and the northwest Atlantic. These warm pools tend to produce positive geopotential height anomalies in the northeastern part of the corresponding oceans. Both warm pools in the Pacific produce large-scale circulation anomalies with a pattern that resembles that obtained using COADS SSTs as well as EOF 1 of the control simulation; the warm pool in the Atlantic does not. These results suggest that the differences obtained with COADS SSTs and HIRS SSTs are mostly due to the differences in the datasets over the northern Pacific. There was a blocking episode near Greenland in late January 1979. Both simulations with warm SST anomalies over the northwest and northeast Pacific show a tendency toward increased incidence of North Atlantic blocking; the simulation with warm SST anomalies over the northwest Atlantic shows a tendency toward decreased incidence. These results suggest that features in both SST datasets that do not have a counterpart in the other dataset contribute signficantly to the differences between the simulated and observed fields. The results of this study imply that uncertainties in current SST distributions for the world oceans can be as important as the SST anomalies themselves in terms of their impact on the atmospheric circulation. Caution should be exercised, therefore, when linking anomalous circulation and SST patterns, especially in long-range prediction.

Operational use of high-resolution sst in a coupled sea ice-ocean model

NASA Astrophysics Data System (ADS)

Albretsen, A.

2003-04-01

A high-latitude, near real time, sea surface temperature (SST) product with 10 km resolution is developed at the Norwegian Meteorological Institute (met.no) through the EUMETSAT project OSI-SAF (Ocean and Sea Ice Satellite Application Facility). The product covers the Atlantic Ocean from 50N to 90N and is produced twice daily. A digitized SST and sea ice map is produced manually once a week at the Ice Mapping Service at met.no using all available information from the previous week. This map is the basis for a daily SST analysis, in which the most recent OSI-SAF SST products are successively overlaid. The resulting SST analysis field is then used in a simple data assimilation scheme in a coupled ice-ocean model to perform daily 10 days forecasts of ocean and sea ice variables. Also, the associated OSI-SAF sea ice concentration product, built from different polar orbiting satellites, is assimilated into the sea ice model. Preliminary estimates of impact on forecast skill and error statistics will be presented.

Atlantic water variability on the SE Greenland continental shelf and its relationship to SST

NASA Astrophysics Data System (ADS)

Sutherland, D. A.; Straneo, F.; Rosing-Asvid, A.; Stenson, G.; Davidson, F. J.; Hammill, M.

2012-12-01

Interaction of warm, Atlantic-origin water (AW) and colder, polar origin water (PW) advecting southward in the East Greenland Current (EGC) influences the heat content of water entering Greenland's outlet glacial fjords. Here we use depth and temperature data derived from deep-diving seals to map out water mass variability across the continental shelf and to augment existing bathymetric products. We find two dominant modes in the vertical temperature structure: a cold mode, with the typical AW/PW layering observed in the EGC, and a warm mode, where AW is present throughout the water column. The prevalence of these modes varies seasonally and spatially across the continental shelf, implying distinct AW pathways. In addition, we find that satellite sea surface temperatures (SST) correlate significantly with temperatures in the upper 50 m (R=0.54), but this correlation decreases with depth (R=0.22 at 200 m), and becomes insignificant below 250 m. Thus, care must be taken in using SST as a proxy for heat content, as AW mainly resides in these deeper layers. Regional map showing the location of all seal tracks originating from Canada and Greenland (stars). Tracks passing inside (red) or outside (blue) the SE Greenland region (black) were subdivided into continental shelf regions (green boxes) near Sermilik Fjord (SF), Cape Farewell (CF) and Kangerdlugssuaq Fjord (KG). GEBCO bathymetry is contoured at 200, 1000, 2000, and 3000 m.

NorTropical Warm Pool variability and its effects on the climate of Colombia

NASA Astrophysics Data System (ADS)

Ricaurte Villota, Constanza; Romero-Rodriguez, Deisy; Coca-Domínguez, Oswaldo

2015-04-01

Much has been said about the effects of El Niño Southern Oscillation (ENSO) on oceanographic and climatic conditions in Colombia, but little is known about the influence of the Atlantic Warm Pool (AWP), which includes the gulf of Mexico, the Caribbean and the western tropical North Atlantic. The AWP has been identified by some authors as an area that influences the Earth's climate, associated with anomalous summer rainfall and hurricane activity in the Atlantic. The aim of this study was to understand the variation in the AWP and its effects on the climate of Colombia. An annual average of sea surface temperature (SST) was obtained from the composition of monthly images of the Spectroradiometer Moderate Resolution Imaging Spectroradiometer (MODIS), with resolution of 4 km, for one area that comprises the marine territory of Colombia, Panama, Costa Rica both the Pacific and the Caribbean, and parts of the Caribbean coast of Nicaragua, for the period between 2007 and 2013. The results suggest that warm pool is not restricted to the Caribbean, but it also covers a strip Pacific bordering Central America and the northern part of the Colombian coast, so it should be called the Nor-Tropical Warm pool (NTWP). Within the NTWP higher SST correspond to a marine area extending about 1 degree north and south of Central and out of the Colombian Caribbean coast. The NTWP also showed large interannual variability, with the years 2008 and 2009 with lower SST in average, while 2010, 2011 and 2013 years with warmer conditions, matching with greater precipitation. It was also noted that during warmer conditions (high amplitude NTWP) the cold tongue from the south Pacific has less penetration on Colombian coast. Finally, the results suggest a strong influence of NTWP in climatic conditions in Colombia.

Atlantic-induced pan-tropical climate change over the past three decades

NASA Astrophysics Data System (ADS)

Li, Xichen; Xie, Shang-Ping; Gille, Sarah T.; Yoo, Changhyun

2016-03-01

During the past three decades, tropical sea surface temperature (SST) has shown dipole-like trends, with warming over the tropical Atlantic and Indo-western Pacific but cooling over the eastern Pacific. Competing hypotheses relate this cooling, identified as a driver of the global warming hiatus, to the warming trends in either the Atlantic or Indian Ocean. However, the mechanisms, the relative importance and the interactions between these teleconnections remain unclear. Using a state-of-the-art climate model, we show that the Atlantic plays a key role in initiating the tropical-wide teleconnection, and the Atlantic-induced anomalies contribute ~55-75% of the tropical SST and circulation changes during the satellite era. The Atlantic warming drives easterly wind anomalies over the Indo-western Pacific as Kelvin waves and westerly anomalies over the eastern Pacific as Rossby waves. The wind changes induce an Indo-western Pacific warming through the wind-evaporation-SST effect, and this warming intensifies the La Niña-type response in the tropical Pacific by enhancing the easterly trade winds and through the Bjerknes ocean dynamical processes. The teleconnection develops into a tropical-wide SST dipole pattern. This mechanism, supported by observations and a hierarchy of climate models, reveals that the tropical ocean basins are more tightly connected than previously thought.

Suborbital Holocene Climate Variability over Continental Western Eurasia Coupled with Poleward Heat Transport to the Northeastern Atlantic Ocean

NASA Astrophysics Data System (ADS)

Baker, J. L.; Lachniet, M. S.; Asmerom, Y.; Polyak, V. J.

2016-12-01

The centennial-scale coupling between the Holocene paleoclimate of Eurasia and ocean-atmosphere dynamics in the North Atlantic sector remains weakly understood, due to a paucity of high-resolution data from the continental interior. To investigate these links, we detrended a composite record of stalagmite δ18O from Kinderlinskaya Cave (southern Urals Mountains), which exhibits long-term warming from 11.7 ka to present. The chronologies of two stalagmites were constrained by 29 U-Th dates obtained through MC-ICP-MS analysis. Stable-isotope analysis at 0.5-mm resolution along the growth axes resulted in an average sampling frequency of 12.5 years. Stalagmite δ18O reflects multidecadal changes in the δ18O of winter half-year precipitation, which is highly sensitive to AO/NAO-like shifts in the strength and position of mid-latitude westerlies. Spectral density and wavelet analysis of the detrended record revealed significant periodicities near 2.4 ka, 1.4 ka, and 1.0 ka, which are common in northern hemispheric paleoclimate records and possibly related to solar and oceanic forcing during the Holocene. Coherent hemispheric coupling of continental and oceanic paleoclimate at suborbital timescales is demonstrated by comparison of our record with reconstructions of sea-surface temperature (SST) and meridional flow strength in the North Atlantic sector. Specifically, SST at cores MD-23258 and LO09-14 in the Barents Sea and Reykjanes Ridge, respectively, exhibit opposite phasing during the Holocene, due to alternating strength between the eastern and western branches of the North Atlantic Current, a major component of AMOC. Estimating the SST gradient between these sites as a proxy for poleward heat transport to the northeastern Atlantic Ocean, we find a strong covariance with detrended stalagmite δ18O. This relationship suggests that persistent strengthening (weakening) of wintertime westerlies, analogous to positive (negative) phases of the AO/NAO, was forced by enhanced (reduced) poleward heat transport along the Norwegian Current. Our record complements existing reconstructions of Holocene AO/NAO variability and provides a paleoanalog for the oceanographic response to rapid melting of the Greenland Ice Sheet under modern anthropogenic warming.

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

NASA Astrophysics Data System (ADS)

Kawakubo, Y.; Alibert, C.; Yokoyama, Y.

2017-12-01

We present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally 0.5°C, making Sr/Ca-derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long-lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe "Tenmei" famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low-frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio-Oyashio variability to the large-scale atmosphere at decadal timescale.

Associations of multi-decadal sea-surface temperature variability with US drought

USGS Publications Warehouse

McCabe, G.J.; Betancourt, J.L.; Gray, S.T.; Palecki, M.A.; Hidalgo, H.G.

2008-01-01

Recent research suggests a link between drought occurrence in the conterminous United States (US) and sea surface temperature (SST) variability in both the tropical Pacific and North Atlantic Oceans on decadal to multidecadal (D2M) time scales. Results show that the Atlantic Multidecadal Oscillation (AMO) is the most consistent indicator of D2M drought variability in the conterminous US during the 20th century, but during the 19th century the tropical Pacific is a more consistent indicator of D2 M drought. The interaction between El Nin??o-Southern Oscillation (ENSO) and the AMO explain a large part of the D2M drought variability in the conterminous US. More modeling studies are needed to reveal possible mechanisms linking low-frequency ENSO variability and the AMO with drought in the conterminous US. ?? 2007 Elsevier Ltd and INQUA.

Interdecadal variability in pan-Pacific and global SST, revisited

NASA Astrophysics Data System (ADS)

Tung, Ka-Kit; Chen, Xianyao; Zhou, Jiansong; Li, King-Fai

2018-05-01

Interest in the "Interdecadal Pacific Oscillation (IPO)" in the global SST has surged recently on suggestions that the Pacific may be the source of prominent interdecadal variations observed in the global-mean surface temperature possibly through the mechanism of low-frequency modulation of the interannual El Nino-Southern Oscillation (ENSO) phenomenon. IPO was defined by performing empirical orthogonal function (EOF) analysis of low-pass filtered SST. The low-pass filtering creates its unique set of mathematical problems—in particular, mode mixing—and has led to some questions, many unanswered. To understand what these EOFs are, we express them first in terms of the recently developed pairwise rotated EOFs of the unfiltered SST, which can largely separate the high and low frequency bands without resorting to filtering. As reported elsewhere, the leading rotated dynamical modes (after the global warming trend) of the unfiltered global SST are: ENSO, Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). IPO is not among them. The leading principal component (PC) of the low-pass filtered global SST is usually defined as IPO and it is seen to comprise of ENSO, PDO and AMO in various proportions depending on the filter threshold. With decadal filtering, the contribution of the interannual ENSO is understandably negligible. The leading dynamical mode of the filtered global SST is mostly AMO, and therefore should not have been called the Interdecadal "Pacific" Oscillation. The leading dynamical mode of the filtered pan-Pacific SST is mostly PDO. This and other low-frequency variability that have the action center in the Pacific, from either the pan-Pacific or global SST, have near zero global mean.

Variability and Predictability of West African Droughts. A Review in the Role of Sea Surface Temperature Anomalies

NASA Technical Reports Server (NTRS)

Rodriguez-Fonseca, Belen; Mohino, Elsa; Mechoso, Carlos R.; Caminade, Cyril; Biasutti, Michela; Gaetani, Marco; Garcia-Serrano, J.; Vizy, Edward K.; Cook, Kerry; Xue, Yongkang;

Recruitment success of different fish stocks in the North Sea in relation to climate variability

NASA Astrophysics Data System (ADS)

Dippner, Joachim W.

1997-09-01

Long-term data of year class strengths of different commercially harvested fish stocks based on a virtual population analysis (VPA) are available from ICES. The anomalies of these long-term data sets of year class strength are analyzed using Empirical Orthogonal Functions (EOFs) and are related to climate variability: the anomalies of the sea surface temperature (SST) in the northern North Sea and the North Atlantic Oscillation (NAO) index. A Canonical Correlation Analysis (CCA) between the leading eigenmodes is performed. The results suggest that the variability in the fish recruitment of western mackerel and three gadoids, namely North Sea cod, North Sea saithe, and North Sea whiting is highly correlated to the variability of the North Sea SST which is directly influenced by the NAO. For North Sea haddock and herring no meaningful correlation exists to North Sea SST and NAO. The results allow the conclusion that is seems possible to predict long-term changes in the fish recruitment from climate change scenarios for North Sea cod, North Sea saithe and western mackerel. Furthermore, the results indicate the possibility of recruitment failure for North Sea cod, North Sea whiting, and western mackerel in the case of global warming.

Modes of North Atlantic Decadal Variability in the ECHAM1/LSG Coupled Ocean-Atmosphere General Circulation Model.

NASA Astrophysics Data System (ADS)

Zorita, Eduardo; Frankignoul, Claude

1997-02-01

The climate variability in the North Atlantic sector is investigated in a 325-yr integration of the ECHAM1/ LSG coupled ocean-atmosphere general circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies arise as a response of the oceanic surface layer to the stochastic forcing by the atmosphere, with the heat exchanges both generating and damping the SST anomalies. In the ocean interior, the temperature spectra are red up to a period of about 20 years, and substantial decadal fluctuations are found in the upper kilometer or so of the water column. Using extended empirical orthogonal function analysis, two distinct quasi-oscillatory modes of ocean-atmosphere variability are identified, with dominant periods of about 20 and 10 years, respectively. The oceanic changes in both modes reflect the direct forcing by the atmosphere through anomalous air-sea fluxes and Ekman pumping, which after some delay affects the intensity of the subtropical and subpolar gyres. The SST is also strongly modulated by the gyre currents. In the thermocline, the temperature and salinity fluctuations are in phase, as if caused by thermocline displacements, and they have no apparent connection with the thermohaline circulation. The 20-yr mode is the most energetic one; it is easily seen in the thermocline and can be found in SST data, but it is not detected in the atmosphere alone. As there is no evidence of positive ocean-atmosphere feedback, the 20-yr mode primarily reflects the passive response of the ocean to atmospheric fluctuations, which may be in part associated with climate anomalies appearing a few years earlier in the North Pacific. The 10-yr mode is more surface trapped in the ocean. Although the mode is most easily seen in the temperature variations of the upper few hundred meters of the ocean, it is also detected in the atmosphere alone and thus appears to be a coupled ocean-atmosphere mode. In both modes, the surface heat flux acts neutrally on the associated SST anomalies once they have been generated, so that their persistence appears to be due in part to an overall adjustment of the air-sea heat exchanges to the SST patterns.

Shrubs tracing sea surface temperature--Calluna vulgaris on the Faroe Islands.

PubMed

Beil, Ilka; Buras, Allan; Hallinger, Martin; Smiljanić, Marko; Wilmking, Martin

2015-11-01

The climate of Central and Northern Europe is highly influenced by the North Atlantic Ocean due to heat transfer from lower latitudes. Detailed knowledge about spatio-temporal variability of sea surface temperature (SST) in that region is thus of high interest for climate and environmental research. Because of the close relations between ocean and coastal climate and the climate sensitivity of plant growth, annual rings of woody plants in coastal regions might be used as a proxy for SST. We show here for the first time the proxy potential of the common and widespread evergreen dwarf shrub Calluna vulgaris (heather), using the Faroe Islands as our case study. Despite its small and irregular ring structure, the species seems suitable for dendroecological investigations. Ring width showed high and significant correlations with summer and winter air temperatures and SST. The C. vulgaris chronology from the Faroe Islands, placed directly within the North Atlantic Current, clearly reflects variations in summer SSTs over an area between Iceland and Scotland. Utilising shrubs like C. vulgaris as easy accessible and annually resolved proxies offers an interesting possibility for reconstruction of the coupled climate-ocean system at high latitudes.

Is a changing climate affecting the tropical cyclone behavior of Cape Verde?

NASA Astrophysics Data System (ADS)

Emmenegger, T. W.; Mann, M. E.; Evans, J. L.

2016-12-01

An existing dataset of synthetic tropical cyclone (TC) tracks derived from climate change simulations were used to explore TC variability within a 250 km radius of the Cape Verde Islands (16.5388N, 23.0418W). The synthetic sets were examined according to genesis point location, track projection, intensity, frequency, and seasonality within the observational era (1851 AD to present). These factors of TC variability have been shown to be strongly related to climate oscillations, thus the historical era was grouped by the increasing and decreasing regimes of sea surface temperature (SST) in the main development region (MDR) of the Atlantic Ocean. Numerous studies have examined Atlantic Basin activity throughout this era; the goal of this study is to investigate possible variations in TC behavior around Cape Verde, ultimately determining whether Cape Verde experiences similar fluctuations in activity as observed basin-wide. We find that several facets of TC variability such as intensity, seasonality, and genesis point location around Cape Verde are not significantly different to that of the entire basin, thus forecasts of the entire basin in these respects may also apply to our site. A long-term trend of increasing TC frequency can be identified basin-wide within the observed set, yet activity around Cape Verde does not display this same behavior observably or in any synthetic set. A relationship between the location of genesis points and the regimes of SST fluctuation is shown to be existent. We find both more observed and synthetic genesis points within the vicinity of Cape Verde during cool periods, and an eastward and equatorward shift in cyclogenesis is evident during warm regimes. This southeastern shift in genesis points attributes to the increased intensities of TCs seen during periods of warmer SST. Years of increased SST are additionally linked to an earlier seasonality in Cape Verde.

Separating the Effects of Tropical Atlantic and Pacific SST-driven Climate Variability on Amazon Carbon Exchange

NASA Astrophysics Data System (ADS)

Liptak, J.; Keppel-Aleks, G.

2016-12-01

Amazon forests store an estimated 25% percent of global terrestrial carbon per year1, 2, but the responses of Amazon carbon uptake to climate change is highly uncertain. One source of this uncertainty is tropical sea surface temperature variability driven by teleconnections. El Nino-Southern Oscillation (ENSO) is a key driver of year-to-year Amazon carbon exchange, with associated temperature and precipitation changes favoring net carbon storage in La Nina years, and net carbon release during El Nino years3. To determine how Amazon climate and terrestrial carbon fluxes react to ENSO alone and in concert with other SST-driven teleconnections such as the Atlantic Multidecadal Oscillation (AMO), we force the atmosphere (CAM5) and land (CLM4) components of the CESM(BGC) with prescribed monthly SSTs over the period 1950—2014 in a Historical control simulation. We then run an experiment (PAC) with time-varying SSTs applied only to the tropical equatorial Pacific Ocean, and repeating SST seasonal cycle climatologies elsewhere. Limiting SST variability to the equatorial Pacific indicates that other processes enhance ENSO-driven Amazon climate anomalies. Compared to the Historical control simulation, warming, drying and terrestrial carbon loss over the Amazon during El Nino periods are lower in the PAC simulation, especially prior to 1990 during the cool phase of the AMO. Cooling, moistening, and net carbon uptake during La Nina periods are also reduced in the PAC simulation, but differences are greater after 1990 during the warm phase of the AMO. By quantifying the relationships among climate drivers and carbon fluxes in the Historical and PAC simulations, we both assess the sensitivity of these relationships to the magnitude of ENSO forcing and quantify how other teleconnections affect ENSO-driven Amazon climate feedbacks. We expect that these results will help us improve hypotheses for how Atlantic and Pacific climate trends will affect future Amazon carbon carbon cycling. Pan, Y. et al. A large and persistent carbon sink in the world's forests. Science 333, 988-993 (2011) Brienen, Roel J. W. et al. Long-term decline of the Amazon carbon sink. Nature 519, 344-348 (2015) Botta, A. et al. Long-term variations of climate and carbon fluxes over the Amazon basin. Geophys. Res. Lett. 29 (2002)

No inter-gyre pathway for sea-surface temperature anomalies in the North Atlantic.

PubMed

Foukal, Nicholas P; Lozier, M Susan

2016-04-22

Recent Lagrangian analyses of surface drifters have questioned the existence of a surface current connecting the Gulf Stream (GS) to the subpolar gyre (SPG) and have cast doubt on the mechanism underlying an apparent pathway for sea-surface temperature (SST) anomalies between the two regions. Here we use modelled Lagrangian trajectories to determine the fate of surface GS water and satellite SST data to analyse pathways of GS SST anomalies. Our results show that only a small fraction of the surface GS water reaches the SPG, the water that does so mainly travels below the surface mixed layer, and GS SST anomalies do not propagate into the SPG on interannual timescales. Instead, the inter-gyre heat transport as part of the Atlantic Meridional Overturning Circulation must be accomplished via subsurface pathways. We conclude that the SST in the SPG cannot be predicted by tracking SST anomalies along the GS.

No inter-gyre pathway for sea-surface temperature anomalies in the North Atlantic

PubMed Central

Foukal, Nicholas P.; Lozier, M. Susan

2016-01-01

Recent Lagrangian analyses of surface drifters have questioned the existence of a surface current connecting the Gulf Stream (GS) to the subpolar gyre (SPG) and have cast doubt on the mechanism underlying an apparent pathway for sea-surface temperature (SST) anomalies between the two regions. Here we use modelled Lagrangian trajectories to determine the fate of surface GS water and satellite SST data to analyse pathways of GS SST anomalies. Our results show that only a small fraction of the surface GS water reaches the SPG, the water that does so mainly travels below the surface mixed layer, and GS SST anomalies do not propagate into the SPG on interannual timescales. Instead, the inter-gyre heat transport as part of the Atlantic Meridional Overturning Circulation must be accomplished via subsurface pathways. We conclude that the SST in the SPG cannot be predicted by tracking SST anomalies along the GS. PMID:27103496

African aerosols and Atlantic tropical cyclone activities

NASA Astrophysics Data System (ADS)

Kafatos, M.; Sun, D.; Sahoo, A.

2006-12-01

Previous studies have shown that the Atlantic basin major hurricane (MH) activity is associated with western Sahelian monsoon rainfall, while rainfall in the Sahel is found to be highly anti-correlated with the African dust storms. So if the Atlantic basin MH activity may be anti-correlated with the African dust aerosols? In order to investigate the relationship between the African dust and the tropical cyclone (including both tropical storms and hurricanes) activities in the Atlantic basin, we explore how the African dust may link to Atlantic TC activity by using the long-term (1982-2005) NCEP Reynolds sea surface temperature (SST) product, and tropical cyclone (TC) data from the National Hurricane Center Best Track Files, and the TOMS aerosol index (AI) data, because the TOMS AI positive values are associated with UV-absorbing aerosols, like dust and smoke. Although no significant negative correlation between the TOMS AI and the Atlantic TC or MH frequency and duration is found, the initial locations of the Atlantic tropical cyclones did occur over the ocean where the aerosol loading was low. Our analysis shows that SST over the north tropical Atlantic ocean is anti-correlated with the TOMS aerosol index. This may be due to the radiative forcing of the aerosols. The effects of the dust aerosols carried across the West African region led to a lowering of SST and therefore inhibited tropical cyclogenesis. During 2005, the aerosol loading along the western African coast was unusually low, while the SST over the main development region (MDR) was abnormally high, and the Atlantic TC/hurricane activities became record strong. We propose future observations to test these results.

Breakdown of NAO reproducibility into internal versus externally-forced components: a two-tier pilot study

NASA Astrophysics Data System (ADS)

Douville, Hervé; Ribes, A.; Tyteca, S.

2018-03-01

Assessing the ability of atmospheric models to capture observed climate variations when driven by observed sea surface temperature (SST), sea ice concentration (SIC) and radiative forcings is a prerequisite for the feasibility of near term climate predictions. Here we achieve ensembles of global atmospheric simulations to assess and attribute the reproducibility of the boreal winter atmospheric circulation against the European Centre for Medium Range Forecasts (ECMWF) twentieth century reanalysis (ERA20C). Our control experiment is driven by the observed SST/SIC from the Atmospheric Model Intercomparison Project. It is compared to a similar ensemble performed with the ECMWF model as a first step toward ERA20C. Moreover, a two-tier methodology is used to disentangle externally-forced versus internal variations in the observed SST/SIC boundary conditions and run additional ensembles allowing us to attribute the observed atmospheric variability. The focus is mainly on the North Atlantic Oscillation (NAO) variability which is more reproducible in our model than in the ECMWF model. This result is partly due to the simulation of a positive NAO trend across the full 1920-2014 integration period. In line with former studies, this trend might be mediated by a circumglobal teleconnection mechanism triggered by increasing precipitation over the tropical Indian Ocean (TIO). Surprisingly, this response is mainly related to the internal SST variability and is not found in the ECMWF model driven by an alternative SST dataset showing a weaker TIO warming in the first half of the twentieth century. Our results may reconcile the twentieth century observations with the twenty-first century projections of the NAO. They should be however considered with caution given the limited size of our ensembles, the possible influence of other sources of NAO variability, and the uncertainties in the tropical SST trend and breakdown between internal versus externally-forced variability.

Temperature Calibration of a Northern Gulf of Mexico Siderastrea siderea Coral

NASA Astrophysics Data System (ADS)

Wagner, A. J.; DeLong, K. L.; Kilbourne, K. H.; Richey, J. N.; Jelinek, K.; Hickerson, E.; Slowey, N. C.

2015-12-01

The Gulf of Mexico (GOM) is sensitive to oceanic and atmospheric variability in both the Atlantic and Pacific Oceans (i.e., Atlantic Multidecadal Oscillation (AMO), El Niño Southern Oscillation (ENSO), Pacific North American Pattern (PNA), and Pacific Decadal Oscillation (PDO)). The major GOM current, the Loop Current, feeds the Gulf Stream as it transports oceanic heat to the northern Atlantic Ocean. The northern GOM is the northernmost summer extent of the western hemisphere warm pool (WHWP) that drives oceanic moisture flux and precipitation into the Americas. Decadally-resolved foraminifera reconstructions from the northern GOM indicates SST was 2 to 4ºC colder on average than today during the Little Ice Age (LIA, ~1850), whereas a subannually-resolved coral reconstruction from the southeastern GOM find 1.5 to 2ºC colder intervals and reduced areal extent of the WHWP on interannual time scales during some intervals of the LIA. However, records capable of resolving annual and subannual SST variability from the northern GOM, necessary for investigating WHWP northern extent, are still lacking. Here we present a new temperature reconstruction for the northern GOM derived from strontium-to-calcium (Sr/Ca) ratios of approximately monthly samples milled from a Siderastrea siderea coral core collected from the Flower Garden Banks National Marine Sanctuary (FGBNMS; 27° 52.5'N, 93° 49'W) growing at a water depth of 20 m. Coral Sr/Ca is calibrated to reef temperature data from FGBNMS Hobotemp data loggers near the reef cap in ~22 m water depth (1986-2004) and to NOAA OISST (1981-2004), which co-varies with the reef temperature (r=0.95, p<0.05, n=146) and consistently captures winter values in reef temperature with slightly warmer summers (0.9ºC on average). The Sr/Ca-SST calibration slope (-0.043, r=-0.89, n=136, p<0.01 for reef temperature; -0.039, r=-0.94, n=275, p<0.01 for OISST) agrees well with published coral Sr/Ca-SST calibrations for S. siderea in the southeastern GOM from shallower water depths.

The Tropical Western Hemisphere Warm Pool

NASA Astrophysics Data System (ADS)

Wang, C.; Enfield, D. B.

2002-12-01

The paper describes and examines variability of the tropical Western Hemisphere warm pool (WHWP) of water warmer than 28.5oC. The WHWP is the second-largest tropical warm pool on Earth. Unlike the Eastern Hemisphere warm pool in the western Pacific, which straddles the equator, the WHWP is entirely north of the equator. At various stages of development the WHWP extends over parts of the eastern North Pacific, the Gulf of Mexico, the Caribbean, and the western tropical North Atlantic. It has a large seasonal cycle and its interannual fluctuations of area and intensity are significant. Surface heat fluxes warm the WHWP through the boreal spring to an annual maximum of SST and WHWP area in the late summer/early fall, associated with eastern North Pacific and Atlantic hurricane activities and rainfall from northern South America to the southern tier of the United States. Observations suggest that a positive ocean-atmosphere feedback operating through longwave radiation and associated cloudiness seems to operate in the WHWP. During winter preceding large warm pool, there is an alteration of the Walker and Hadley circulation cells that serves as a "tropospheric bridge" for transferring Pacific ENSO effects to the Atlantic sector and inducing initial warming of warm pool. Associated with the warm SST anomalies is a decrease in sea level pressure anomalies and an anomalous increase in atmospheric convection and cloudiness. The increase in convective activity and cloudiness results in less net longwave radiation loss from the sea surface, which then reinforces SST anomalies.

Coupled dynamics that determine the position and variability of the ITCZ

NASA Astrophysics Data System (ADS)

Xie, S.; Miyama, T.; Wang, Y.; Xu, H.; de Szoeke, S.

2006-05-01

The intertropical convergence zone (ITCZ) is displaced north of the equator in the eastern Pacific and Atlantic Oceans, as a result of asymmetry in continental geometry and air-sea interactions. This latitudinal asymmetry plays an important role in shaping the equatorial annual cycle, the seasonality of the equatorial mode in both the ocean basins, and the tropical Atlantic meridional mode. Despite its climatic importance, the northward- displaced ITCZ is poorly simulated in state-of-the-art global climate models, casting doubts on their simulations of the past and current climate and projection of future climate. A regional ocean-atmosphere model has been developed to study the effects of external influences (e.g., high- latitude cooling in the northern North Atlantic) and internal feedback on the Pacific ITCZ. The regional ocean- atmosphere model (ROAM) reproduces salient features of eastern Pacific climate, including a northward- displaced intertropical convergence zone (ITCZ) collocated with a zonal band of high SSTs, a low-cloud deck in the Southeast Pacific, the equatorial cold tongue and its annual cycle. The model climate - such as the position of the ITCZ, equatorial annual cycle and maximum SST - is sensitive to the treatment of low cloud. In another experiment where tropical North Atlantic SST is lowered by 2C, equatorial Pacific SST decreases by up to 3C in January-April but changes much less in other seasons, resulting in a weakened equatorial annual cycle. Central American mountains, poorly resolved in global models, appear to play an important role in this cross-basin interaction. The coupled dynamics of the ITCZ in the model and its utility to downscale coarse- resolution paleoclimate simulations will be discussed.

About the seasonal variability of the Alboran Sea circulation

NASA Astrophysics Data System (ADS)

Vargas-Yáñez, M.; Plaza, F.; García-Lafuente, J.; Sarhan, T.; Vargas, J. M.; Vélez-Belchi, P.

2002-07-01

Data from a mooring line deployed midway between the Alboran Island and Cape Tres Forcas are used to study the time variability of the Alboran Sea from May 1997 to May 1998. The upper layer salinity and zonal velocity present annual and semiannual cycles characterised by a minimum in spring and autumn and a maximum in summer and winter. Temperature has the opposite behaviour to that of salinity indicating changes in the presence of the Atlantic water within the Alboran Passage. A large set of SST images is used to study these cycles. The decrease of salinity and velocity in our mooring location in spring and autumn seems to be related to the eastward drifting of the Western Alboran Gyre (WAG). The increase of salinity and velocity is caused by the Atlantic current flowing south of the Alboran Island and its associated thermohaline front. Conductivity-temperature-depth (CTD) data from two cruises along the 3°W are coherent with current meters and SST interpretations. During the period analysed, summer months are characterised by the stability of the two-gyre system, while in winter, the circulation is characterised by a coastal jet flowing close to the African shore. We use sea level differences across the Strait of Gibraltar for studying the variability of the Atlantic inflow. We discuss the changes in the Alboran Sea circulation and its relation with the variability of the inertial radius of the Atlantic inflow. Though our results are speculative, we find a possible relation between the disappearance of the two-gyre system and a reversal of the circulation in Gibraltar. Longer time series are needed to conclude, but comparison with previous works makes us think that the seasonal cycle described from May 1997 to May 1998 could be the most likely one for the Alboran Sea upper layer.

Global Meteorological Drought: A Synthesis of Current Understanding with a Focus on SST Drivers of Precipitation Deficits

NASA Technical Reports Server (NTRS)

Schubert, S.; Stewart, R.; Wang, H.; Barlow, M.; Berbery, H.; Cai, W.; Hoerling, M.; Kanikicharla, K.; Koster, R.; Lyon, B.;

The global warming in the North Atlantic Sector and the role of the ocean

NASA Astrophysics Data System (ADS)

Hand, R.; Keenlyside, N. S.; Greatbatch, R. J.; Omrani, N. E.

2014-12-01

This work presents an analysis of North Atlantic ocean-atmosphere interaction in a warming climate, based on a long-term earth system model experiment forced by the RCP 8.5 scenario, the strongest greenhouse gas forcing used in the climate projections for the 5th Assessement report of the Intergovernmental Panel on Climate Change). In addition to a global increase in SSTs as a direct response to the radiative forcing, the model shows a distinct change of the local sea surface temperature (SST hereafter) patterns in the Gulf Stream region: The SST front moves northward by several hundred kilometers, likely as a response of the wind-driven part of the oceanic surface circulation, and becomes more zonal. As a consequence of a massive slowdown of the Atlantic Meridional Overturning Circulation, the northeast North Atlantic only shows a moderate warming compared to the rest of the ocean. The feedback of these changes on the atmosphere was studied in a set of sensitivity experiments based on the SST climatology of the coupled runs. The set consists of a control run based on the historical run, a run using the full SST from the coupled RCP 8.5 run and two runs, where the SST signal was deconstructed into a homogenous mean warming part and a local pattern change. In the region of the precipitation maximum in the historical run the future scenario shows an increase of absolute SSTs, but a significant decrease in local precipitation, low-level convergence and upward motion. Since warmer SSTs usually cause the opposite, this indicates that the local response in that region is connected to the (with respect to the historical run) weakened SST gradients rather than to the absolute SST. Consistently, the model shows enhanced precipitation north of this region, where the SST gradients are enhanced. However, the signal restricts to the low and mid-troposphere and does not reach the higher model levels. There is little evidence for a large-scale response to the changes in the Gulf Stream region; instead, the large scale signal is mainly controlled by the warmer background state and the AMOC slowdown and influenced by tropical SSTs. In a warmer climate the same change in SST gradient has a stronger effect on precipitation and the model produces a slightly enhanced North Atlantic storm track.

Causes of Upper-Ocean Temperature Anomalies in the Tropical North Atlantic

NASA Astrophysics Data System (ADS)

Rugg, A.; Foltz, G. R.; Perez, R. C.

2016-02-01

Hurricane activity and regional rainfall are strongly impacted by upper ocean conditions in the tropical North Atlantic, defined as the region between the equator and 20°N. A previous study analyzed a strong cold sea surface temperature (SST) anomaly that developed in this region during early 2009 and was recorded by the Pilot Research Array in the Tropical Atlantic (PIRATA) moored buoy at 4°N, 23°W (Foltz et al. 2012). The same mooring shows a similar cold anomaly in the spring of 2015 as well as a strong warm anomaly in 2010, offering the opportunity for a more comprehensive analysis of the causes of these events. In this study we examine the main causes of the observed temperature anomalies between 1998 and 2015. Basin-scale conditions during these events are analyzed using satellite SST, wind, and rain data, as well as temperature and salinity profiles from the NCEP Global Ocean Data Assimilation System. A more detailed analysis is conducted using ten years of direct measurements from the PIRATA mooring at 4°N, 23°W. Results show that the cooling and warming anomalies were caused primarily by wind-driven changes in surface evaporative cooling, mixed layer depth, and upper-ocean vertical velocity. Anomalies in surface solar radiation acted to damp the wind-driven SST anomalies in the latitude bands of the ITCZ (3°-8°N). Basin-scale analyses also suggest a strong connection between the observed SST anomalies and the Atlantic Meridional Mode, a well-known pattern of SST and surface wind anomalies spanning the tropical Atlantic.

Interannual and low-frequency variability of Upper Indus Basin winter/spring precipitation in observations and CMIP5 models

NASA Astrophysics Data System (ADS)

Greene, Arthur M.; Robertson, Andrew W.

2017-12-01

An assessment is made of the ability of general circulation models in the CMIP5 ensemble to reproduce observed modes of low-frequency winter/spring precipitation variability in the region of the Upper Indus basin (UIB) in south-central Asia. This season accounts for about two thirds of annual precipitation totals in the UIB and is characterized by "western disturbances" propagating along the eastward extension of the Mediterranean storm track. Observational data are utilized for for spatiotemporal characterization of the precipitation seasonal cycle, to compute seasonalized spectra and finally, to examine teleconnections, in terms of large-scale patterns in sea-surface temperature (SST) and atmospheric circulation. Annual and lowpassed variations are found to be associated primarily with SST modes in the tropical and extratropical Pacific. A more obscure link to North Atlantic SST, possibly related to the North Atlantic Oscillation, is also noted. An ensemble of 31 CMIP5 models is then similarly assessed, using unforced preindustrial multi-century control runs. Of these models, eight are found to reproduce well the two leading modes of the observed seasonal cycle. This model subset is then assessed in the spectral domain and with respect to teleconnection patterns, where a range of behaviors is noted. Two model families each account for three members of this subset. The degree of within-family similarity in behavior is shown to reflect underlying model differences. The results provide estimates of unforced regional hydroclimate variability over the UIB on interannual and decadal scales and the corresponding far-field influences, and are of potential relevance for the estimation of uncertainties in future water availability.

Teleconnected ocean forcing of Western North American droughts and pluvials during the last millennium

USGS Publications Warehouse

Routson, Cody C.; Woodhouse, Connie A.; Overpeck, Jonathan T.; Betancourt, Julio L.; McKay, Nicholas P.

2016-01-01

Western North America (WNA) is rich in hydroclimate reconstructions, yet questions remain about the causes of decadal-to-multidecadal hydroclimate variability. Teleconnection patterns preserved in annually-resolved tree-ring reconstructed drought maps, and anomalies in a global network of proxy sea surface temperature (SST) reconstructions, were used to reassess the evidence linking ocean forcing to WNA hydroclimate variability over the past millennium. Potential forcing mechanisms of the Medieval Climate Anomaly (MCA) and individual drought and pluvial events—including two multidecadal-length MCA pluvials—were evaluated. We show strong teleconnection patterns occurred during the driest (wettest) years within persistent droughts (pluvials), implicating SSTs as a potent hydroclimate forcing mechanism. The role of the SSTs on longer timescales is more complex. Pacific teleconnection patterns show little long-term change, whereas low-resolution SST reconstructions vary over decades to centuries. While weaker than the tropical Pacific teleconnections, North Atlantic teleconnection patterns and SST reconstructions also show links to WNA droughts and pluvials, and may in part account for longer-term WNA hydroclimate changes. Nonetheless, evidence linking WNA hydroclimate to SSTs still remains sparse and nuanced—especially over long-timescales with a broader range of hydroclimatic variability than characterized during the 20th century.

Shaping climate change in the North Atlantic sector: The role of the atmospheric response to local SST changes vs. large-scale changes

NASA Astrophysics Data System (ADS)

Hand, Ralf; Keenlyside, Noel; Omrani, Nour-Eddine; Greatbatch, Richard; Bader, Jürgen

2017-04-01

Climate change simulations robustly show a warming hole in the sub-polar North Atlantic that results from slowing of the AMOC countering the global warming signal. Here we investigate how the distinct SST spatial structures, which include a sharpening of the Gulf Stream SST gradients, influence climate change in the NA sector in winter. For this we analyse the RCP8.5 scenario simulation of the MPI Earth System Model. Additional sensitivity experiments with the atmospheric model component, ECHAM5, are performed to deconstruct the effect of the local spatial structure of the SST change from those arising from large-scale warming of the ocean, remote SST pattern changes and changed radiative forcings. The MPI model simulation shows a signifcant decrease in precipitation to the south of the GS extension region in the future, despite a strong increase in underlying SST. While directly to the north there is a significant increase in precipitation. These distinct features in the precipitation response over the North Atlantic result from the local SST. Over the Gulf Stream, the differential structure of the precipitation changes reflects the changes of the local SST gradients there. Over the subpolar gyre the increase in precipitation is partly suppressed. In this region the Subpolar Gyre the weakened AMOC causes a SST warming, that is much weaker than the warming other regions of the ocean show at the same latitude. The large-scale response, which includes the overall increase in precipitation over the NA is due to the overall warming, remote SSTs and/or directly connected to the radiative forcing.

Teasing Apart Regional Climate and Meltwater Influences on Florida Straits Sea Surface Temperature and Salinity over the past 40 kyr

NASA Astrophysics Data System (ADS)

Schmidt, M. W.; Lynch-Stieglitz, J.

2008-12-01

Recent reconstructions of North Atlantic salinity variability over the last glacial cycle show that abrupt climate events are linked to major reorganizations in the low-latitude hydrologic cycle, affecting large-scale changes in evaporation minus precipitation (E-P) patterns. Although there is general agreement that the Intertropical Convergence Zone (ITCZ) migrates southward during cold stadials, it remains unclear how this shift affects the net E-P budget in the North Atlantic. In order to reconstruct a high resolution record of past sea surface temperature (SST) and salinity (SSS) in the Florida Straits across abrupt climate events of the last 40 kyr, we combine Mg/Ca paleothermometry and δ18O measurements in shells from the surface-dwelling foraminifera Globigerinoides ruber in cores KNR166-2-JPC29 (24°17'N, 83°16'W; 648 m depth; 8-20 cm/kyr sed. rate) and JPC26 (24°19.61'N, 83°15.14'W; 546 m depth; 18-240 cm/kyr sed. rate) and calculate δ18OSEAWATER (δ18OSW) variability. Removal of the δ18OSW signal due to continental ice volume variation results in the ice volume-free (IVF) δ18OSW record (a proxy for SSS variability). Although most waters flowing through the Florida Straits today originate in the tropical western Atlantic, major meltwater discharges from the Mississippi River across the last deglacial period also influenced SST and SSS in the Florida Straits. To constrain periods of increased meltwater discharge, we measured Ba/Ca ratios in G. ruber from select intervals. Because riverine waters have a much higher dissolved Ba+2 concentration relative to seawater, foraminifera Ba/Ca ratios can be used as an additional proxy to constrain periods of increase riverine discharge. Initial results suggest the hydrographic history of the Florida Straits is influenced by both meltwater discharge and regional climate variability linked to the high-latitude North Atlantic. Both the IVF- δ18OSW and Ba/Ca records reveal a prolonged period from 16.0-13.0 kyr when elevated meltwater discharge was the dominant influence on surface water conditions in the Florida Straits. It is likely that SSS in the Florida Straits was significantly fresher than today during this interval. In contrast, periods of minimal meltwater influence (such as the Younger Dryas and across D-O cycles of MIS 3) are characterized by abrupt SST and SSS shifts that covary with the NGRIP δ18Oice record. SSTs in the Florida Straits cool by 1.5-2.0 °C and regional salinity increases (IVF-δ18OSW increase of 0.5-0.7‰) at the initiation of cold stadial events as the ITCZ shifts south. The most likely explanation for these rapid shifts in IVF-δ18OSW values is that moisture transport out of the North Atlantic increases when the North Atlantic cools and the ITCZ shifts southward.

Predicting summer monsoon of Bhutan based on SST and teleconnection indices

NASA Astrophysics Data System (ADS)

Dorji, Singay; Herath, Srikantha; Mishra, Binaya Kumar; Chophel, Ugyen

2018-02-01

The paper uses a statistical method of predicting summer monsoon over Bhutan using the ocean-atmospheric circulation variables of sea surface temperature (SST), mean sea-level pressure (MSLP), and selected teleconnection indices. The predictors are selected based on the correlation. They are the SST and MSLP of the Bay of Bengal and the Arabian Sea and the MSLP of Bangladesh and northeast India. The Northern Hemisphere teleconnections of East Atlantic Pattern (EA), West Pacific Pattern (WP), Pacific/North American Pattern, and East Atlantic/West Russia Pattern (EA/WR). The rainfall station data are grouped into two regions with principal components analysis and Ward's hierarchical clustering algorithm. A support vector machine for regression model is proposed to predict the monsoon. The model shows improved skills over traditional linear regression. The model was able to predict the summer monsoon for the test data from 2011 to 2015 with a total monthly root mean squared error of 112 mm for region A and 33 mm for region B. Model could also forecast the 2016 monsoon of the South Asia Monsoon Outlook of World Meteorological Organization (WMO) for Bhutan. The reliance on agriculture and hydropower economy makes the prediction of summer monsoon highly valuable information for farmers and various other sectors. The proposed method can predict summer monsoon for operational forecasting.

A Decadal-scale Air-sea Interaction Theory for North Atlantic Multidecadal Variability: the NAT-NAO-AMOC-AMO Coupled Mode and Its Remote Influences

NASA Astrophysics Data System (ADS)

Li, Jianping; Sun, Cheng; Jin, Fei-Fei

2017-04-01

ABSTRACT North Atlantic region shows prominent multidecadal variability. Observational analysis shows that the North Atlantic Oscillation (NAO) leads the oceanic Atlantic Multidecadal Oscillation (AMO) by 15-20 years and the latter also leads the former by around 15 years. The mechanisms are investigated using simulations from a fully coupled model, and a NATNAO-AMOC-AMO Coupled Mode is proposed to explain the multidecadal variability in North Atlantic region. The NAT-NAO-AMO-AMOC coupled mode has important remote influences on regional climates. Observational analysis identifies a significant in-phase relationship between the AMV and Siberian warm season (May to October) precipitation. The physical mechanism for this relationship is investigated using both observations and numerical simulations. North Atlantic sea surface temperature (SST) warming associated with the positive AMV phase can excite an eastward propagating wave train response across the entire Eurasian continent, which includes an east-west dipole structure over Siberia. The dipole then leads to anomalous southerly winds bringing moisture northward to Siberia; the precipitation increases correspondingly. Furthermore, a prominent teleconnection pattern of multidecadal variability of cold season (November to April) upper-level atmospheric circulation over North Africa and Eurasia (NA-EA) is revealed by empirical orthogonal function analysis of the Twentieth Century Reanalysis data, and this teleconnection pattern is referred to as the Africa-Asia multidecadal teleconnection pattern (AAMT). A strong inphase relationship is observed between the AAMT and Atlantic multidecadal variability (AMV) and this connection is mainly due to Rossby wave dynamics. The AAMT acts as an atmospheric bridge conveying the influence of AMV onto the downstream multidecadal climate variability.

Changes to Yucatán Peninsula precipitation associated with salinity and temperature extremes of the Caribbean Sea during the Maya civilization collapse.

PubMed

Wu, Henry C; Felis, Thomas; Scholz, Denis; Giry, Cyril; Kölling, Martin; Jochum, Klaus P; Scheffers, Sander R

2017-11-20

Explanations of the Classic Maya civilization demise on the Yucatán Peninsula during the Terminal Classic Period (TCP; ~CE 750-1050) are controversial. Multiyear droughts are one likely cause, but the role of the Caribbean Sea, the dominant moisture source for Mesoamerica, remains largely unknown. Here we present bimonthly-resolved snapshots of reconstructed sea surface temperature (SST) and salinity (SSS) variability in the southern Caribbean from precisely dated fossil corals. The results indicate pronounced interannual to decadal SST and SSS variability during the TCP, which may be temporally coherent to precipitation anomalies on the Yucatán. Our results are best explained by changed Caribbean SST gradients affecting the Caribbean low-level atmospheric jet with consequences for Mesoamerican precipitation, which are possibly linked to changes in Atlantic Meridional Overturning Circulation strength. Our findings provide a new perspective on the anomalous hydrological changes during the TCP that complement the oft-suggested southward displacement of the Intertropical Convergence Zone. We advocate for a strong role of Caribbean SST and SSS condition changes and related ocean-atmosphere interactions that notably influenced the propagation and transport of precipitation to the Yucatán Peninsula during the TCP.

Development of Sr/Ca-d18O Temperature Calibrations of a Siderastrea siderea Coral from the Gulf of Mexico

NASA Astrophysics Data System (ADS)

Wagner, A. J.; DeLong, K. L.; Kilbourne, H.; Slowey, N. C.

2016-12-01

The Gulf of Mexico (GOM) is sensitive to oceanic and atmospheric variability in both the Atlantic and Pacific Oceans (i.e., Atlantic Multidecadal Oscillation (AMO), El Niño Southern Oscillation (ENSO), Pacific North American pattern (PNA), and Pacific Decadal Oscillation (PDO)). The major GOM current, the Loop Current, feeds the Gulf Stream as it transports oceanic heat to the northern Atlantic Ocean. The northern GOM is the northernmost summer extent of the western hemisphere warm pool (WHWP) that drives oceanic moisture flux and precipitation into the Americas. Decadally-resolved foraminifera reconstructions from the northern GOM indicates SST was 2 to 4ºC colder on average than today during the Little Ice Age (LIA, 1850), whereas a subannually-resolved coral reconstruction from the southeastern GOM find 1.5 to 2ºC colder intervals and reduced areal extent of the WHWP on interannual time scales during some intervals of the LIA. However, records capable of resolving annual and subannual SST variability from the northern GOM, necessary for investigating WHWP northern extent, are still lacking. Here we present a new temperature reconstruction for the northern GOM derived from strontium-to-calcium (Sr/Ca) ratios of approximately monthly samples milled from a Siderastrea siderea coral core collected from the Flower Garden Banks National Marine Sanctuary (FGBNMS; 27° 52.5'N, 93° 49'W) growing at a water depth of 20 m. Coral Sr/Ca and δ18O is calibrated to reef temperature data from FGBNMS Hobotemp data loggers near the reef cap in 22 m water depth (1986-2004) and to NOAA OISST (1981-2004). Coral Sr/Ca co-varies with the reef temperature (r=0.95, p<0.05, n=146) and consistently captures winter values in reef temperature with slightly warmer summers (0.9ºC on average). Pseudocoral analysis is used to assess the relationships between SST and SSS in coral δ18O.

Reconstructing medieval climate in the tropical North Atlantic with corals from Anegada, British Virgin Islands

NASA Astrophysics Data System (ADS)

Kilbourne, K. H.; Xu, Y. Y.

2014-12-01

Resolving the patterns of climate variability during the Medieval Climate Anomaly (MCA) is key for exploring forced versus unforced variability during the last 1000 years. Tropical Atlantic climate is currently not well resolved during the MCA despite it being an important source of heat and moisture to the climate system today. To fill this data gap, we collected cores from Diploria strigosa corals brought onto the low-lying island of Anegada, British Virgin Islands (18.7˚N, 64.3˚S) during an overwash event and use paired analysis of Sr/Ca and δ18O in the skeletal aragonite to explore climate in the tropical Atlantic at the end of the MCA. The three sub-fossil corals used in this analysis overlap temporally and together span the years 1256-1372 C.E. An assessment of three modern corals from the study site indicates that the most robust features of climate reconstructions using Sr/Ca and δ18O in this species are the seasonal cycle and inter-annual variability. The modern seasonal temperature range is 2.8 degrees Celsius and the similarity between the modern and sub-fossil coral Sr/Ca indicates a similar range during the MCA. Today seasonal salinity changes locally are driven in large part by the migration of a regional salinity front. The modern corals capture the related large seasonal seawater δ18O change, but the sub-fossil corals indicate stable seawater δ18O throughout the year, supporting the idea that this site remained on one side of the salinity front continuously throughout the year. Inter-annual variability in the region is influenced by the cross-equatorial SST gradient, the North Atlantic Oscillation and ENSO. Gridded instrumental SST from the area surrounding Anegada and coral geochemical records from nearby Puerto Rico demonstrate concentrations of variance in specific frequency bands associated with these phenomena. The sub-fossil coral shows no concentration of variance in the modern ENSO frequency band, consistent with reduced ENSO variability found in central Pacific corals growing at the same time.

Molluscan sclerochronology on the Faroese Shelf and its potential to obtain closer insights into the climate variability of North Atlantic water masses

NASA Astrophysics Data System (ADS)

Bonitz, F. G. W.; Andersson Dahl, C.; Trofimova, T.

2016-12-01

In this study, we investigate the climate variability in the North Atlantic during the last 350 years by applying sclerochronological methods. The inflow of North Atlantic water masses into the Arctic and the Norwegian Sea is important for the climate in these regions. A better understanding of the climate variability on highly resolved time scales is needed to obtain a better fundament for climate predictions for these areas. However, highly resolved paleoclimate records are sparse in the North Atlantic and instrumental data cover only the last 50 - 150 years. Bivalve shells provide highly resolved climate archives, especially the shells of the long-lived bivalve species Arctica islandica. This widely occurring species forms annual growth increments, which can be analyzed similarly to tree rings. Climatic and oceanographic changes are recorded population-wide in the shell`s growth rate and in the isotopic composition of the shell. Hence, multi-centennial absolutely dated chronologies can be built by cross-matching live-collected and sub-fossil specimens. Our chronology building effort has led to the first multi-centennial absolutely dated chronology from the Faroese Shelf covering the time period from AD 1642 - 2013. The growth indices of the chronology anti-correlate with April - September sea surface temperatures (SST) for the last 100 years indicating favorable conditions for growth when temperatures are lower. This also suggests that the main growing season of A. islandica around the Faroe Islands occurs in this time period; a hypothesis supported by δ18O-based temperature reconstructions from growth increments representing the years 2001 - 2013. The RBAR, which is an indicator for the signal strength throughout the chronology shows an inverse relationship with Atlantic Multi-decadal Oscillation (AMO) data indicating that periods of higher AMO indexes result in a weakened signal strength in the chronology for the same time period. In conclusion, our results suggest that a combination of the growth increment variability and δ18O measurements of the growth increments can provide a tool to obtain information about the year-to-year SST variability beyond instrumental observations and the signal strength throughout the chronology may provide information about the timing of major AMO shifts.

Historical analysis of interannual rainfall variability and trends in southeastern Brazil based on observational and remotely sensed data

NASA Astrophysics Data System (ADS)

Vásquez P., Isela L.; de Araujo, Lígia Maria Nascimento; Molion, Luiz Carlos Baldicero; de Araujo Abdalad, Mariana; Moreira, Daniel Medeiros; Sanchez, Arturo; Barbosa, Humberto Alves; Rotunno Filho, Otto Corrêa

2018-02-01

The Brazilian Southeast is considered a humid region. It is also prone to landslides and floods, a result of significant increases in rainfall during spring and summer caused by the South Atlantic Convergence Zone (SACZ). Recently, however, the region has faced a striking rainfall shortage, raising serious concerns regarding water availability. The present work endeavored to explain the meteorological drought that has led to hydrological imbalance and water scarcity in the region. Hodrick-Prescott smoothing and wavelet transform techniques were applied to long-term hydrologic and sea surface temperature (SST)—based climate indices monthly time series data in an attempt to detect cycles and trends that could help explain rainfall patterns and define a framework for improving the predictability of extreme events in the region. Historical observational hydrologic datasets available include monthly precipitation amounts gauged since 1888 and 1940 and stream flow measured since the 1930s. The spatial representativeness of rain gauges was tested against gridded rainfall satellite estimates from 2000 to 2015. The analyses revealed variability in four time scale domains—infra-annual, interannual, quasi-decadal and inter-decadal or multi-decadal. The strongest oscillations periods revealed were: for precipitation—8 months, 2, 8 and 32 years; for Pacific SST in the Niño-3.4 region—6 months, 2, 8 and 35.6 years, for North Atlantic SST variability—6 months, 2, 8 and 32 years and for Pacific Decadal Oscillation (PDO) index—6.19 months, 2.04, 8.35 and 27.31 years. Other periodicities less prominent but still statistically significant were also highlighted.

On the impact of the resolution on the surface and subsurface Eastern Tropical Atlantic warm bias

NASA Astrophysics Data System (ADS)

Martín-Rey, Marta; Lazar, Alban

2016-04-01

The tropical variability has a great importance for the climate of adjacent areas. Its sea surface temperature anomalies (SSTA) affect in particular the Brazilian Nordeste and the Sahelian region, as well as the tropical Pacific or the Euro-Atlantic sector. Nevertheless, the state-of the art climate models exhibits very large systematic errors in reproducing the seasonal cycle and inter-annual variability in the equatorial and coastal Africa upwelling zones (up to several °C for SST). Theses biases exist already, in smaller proportions though, in forced ocean models (several 1/10th of °C), and affect not only the mixed layer but also the whole thermocline. Here, we present an analysis of the impact of horizontal and vertical resolution changes on these biases. Three different DRAKKAR NEMO OGCM simulations have been analysed, associated to the same forcing set (DFS4.4) with different grid resolutions: "REF" for reference (1/4°, 46 vertical levels), "HH" with a finer horizontal grid (1/12°, 46 v.l.) and "HV" with a finer vertical grid (1/4°, 75 v.l.). At the surface, a more realistic seasonal SST cycle is produced in HH in the three upwellings, where the warm bias decreases (by 10% - 20%) during boreal spring and summer. A notable result is that increasing vertical resolution in HV causes a shift (in advance) of the upwelling SST seasonal cycles. In order to better understand these results, we estimate the three upwelling subsurface temperature errors, using various in-situ datasets, and provide thus a three-dimensional view of the biases.

The North Atlantic Oscillation Reconstructed at Bermuda for 220 Years Using Sr/Ca Ratios in Diploria labyrinthiformis (brain coral)

NASA Astrophysics Data System (ADS)

Goodkin, N. F.; Hughen, K. A.; Cohen, A. L.; Curry, W. B.; Doney, S. C.

2006-12-01

The North Atlantic Oscillation (NAO) is a meridional oscillation in atmospheric mass measured by pressure anomalies between Iceland (65°N, 23°W) and the Azores (38°N, 26°W) (Hurrell, 1995). Changes between the positive and negative phase of the NAO strongly influence weather patterns across the US, Europe and the Middle East. A shift in recent decades toward a sustained positive NAO has raised questions about the influence of greenhouse gas emissions on this system. Unfortunately, instrumental records are too short to identify the natural baseline variability of the NAO, and NAO reconstructions generally encompass only land-based proxies, excluding ocean processes. Winter-time sea surface temperatures (SST) in the Sargasso Sea have previously been shown to correlate to the NAO (Visbeck et al., 2001), and thus a long winter SST record based on proxy data could be used to reconstruct NAO variability back in time. Here we present an annually resolved winter-time strontium to calcium ratio (Sr/Ca) record from a 220-year old brain coral (Diploria labyrinthiformis) collected from the south shore of Bermuda. Brain coral is prevalent in Bermuda and shows distinct annual banding in its skeleton providing precise age models. Winter-time coral Sr/Ca has previously been shown to accurately record winter SST free from growth rate influences (Goodkin et al., 2005), and that relationship is confirmed here. Cross-spectral analysis between winter-time coral Sr/Ca and four instrumental and proxy records of the NAO (Hurrell, 1995, Jones et al., 1997, Luterbacher et al., 2001, Cook et al., 2002) show two frequencies of coherence with >95% confidence. At periods greater than 20 years and between 3 and 5 years, the coral Sr/Ca effectively captures the NAO variability. Filtering the coral record to these frequencies and comparing to the instrumental and proxy records, including another marine-based NAO reconstruction from the North and Norwegian Seas (Schoene et al., 2003), show strong agreement and provide information about the differences between high and low frequency responses to the NAO. At high frequencies, SST at Bermuda shows a positive correlation to the NAO, as predicted by the tri-pole SST response (Visbeck et al., 2001), and succeeds well at capturing amplitude variability. At low frequencies, however, Bermuda SST shows a negative correlation to the NAO, different than the response predicted by the high frequency tri-pole pattern. One possible explanation for this shift is a response to changes in the meridional overturning circulation (MOC), which is believed to show variability at lower frequencies (Curry et al., 2003) and which may be driving changes in both SST and the NAO. Over 50 years during the late 20th century warming (1950-1999), the amplitude of the Sr/Ca-based NAO record at 3-5 year periods is 20% greater than that found during an equivalent interval at the end of the LIA (1800- 1849). Low-frequency (20-50 year) variability also appears larger during the second half of the 20th century, compared to the LIA. These results indicate a change in NAO variability at different mean temperatures, with larger amplitude changes during warmer climates. However, a sustained positive NAO during the late LIA does not appear to support the hypothesis of a linear relationship between mean NAO and mean hemispheric temperature, as observed during the late 20th century warming.

A Skilful Marine Sclerochronological Network Based Reconstruction of North Atlantic Subpolar Gyre Dynamics

NASA Astrophysics Data System (ADS)

Reynolds, D.; Hall, I. R.; Slater, S. M.; Scourse, J. D.; Wanamaker, A. D.; Halloran, P. R.; Garry, F. K.

2017-12-01

Spatial network analyses of precisely dated, and annually resolved, tree-ring proxy records have facilitated robust reconstructions of past atmospheric climate variability and the associated mechanisms and forcings that drive it. In contrast, a lack of similarly dated marine archives has constrained the use of such techniques in the marine realm, despite the potential for developing a more robust understanding of the role basin scale ocean dynamics play in the global climate system. Here we show that a spatial network of marine molluscan sclerochronological oxygen isotope (δ18Oshell) series spanning the North Atlantic region provides a skilful reconstruction of basin scale North Atlantic sea surface temperatures (SSTs). Our analyses demonstrate that the composite marine series (referred to as δ18Oproxy_PC1) is significantly sensitive to inter-annual variability in North Atlantic SSTs (R=-0.61 P<0.01) and surface air temperatures (SATs; R=-0.67, P<0.01) over the 20th century. Subpolar gyre (SPG) SSTs dominates variability in the δ18Oproxy_PC1 series at sub-centennial frequencies (R=-0.51, P<0.01). Comparison of the δ18Oproxy_PC1 series against variability in the strength of the European Slope Current and maximum North Atlantic meridional overturning circulation derived from numeric climate models (CMIP5), indicates that variability in the SPG region, associated with the strength of the surface currents of the North Atlantic, are playing a significant role in shaping the multi-decadal scale SST variability over the industrial era. These analyses demonstrate that spatial networks developed from sclerochronological archives can provide powerful baseline archives of past ocean variability that can facilitate the development of a quantitative understanding for the role the oceans play in the global climate systems and constraining uncertainties in numeric climate models.

ENSO related SST anomalies and relation with surface heat fluxes over south Pacific and Atlantic

NASA Astrophysics Data System (ADS)

Chatterjee, S.; Nuncio, M.; Satheesan, K.

2017-07-01

The role of surface heat fluxes in Southern Pacific and Atlantic Ocean SST anomalies associated with El Nino Southern Oscillation (ENSO) is studied using observation and ocean reanalysis products. A prominent dipole structure in SST anomaly is found with a positive (negative) anomaly center over south Pacific (65S-45S, 120W-70W) and negative (positive) one over south Atlantic (50S-30S, 30W-0E) during austral summer (DJF) of El Nino (LaNina). During late austral spring-early summer (OND) of El Nino (LaNina), anomalous northerly (southerly) meridional moisture transport and a positive (negative) sea level pressure anomaly induces a suppressed (enhanced) latent heat flux from the ocean surface over south Pacific. This in turn results in a shallower than normal mixed layer depth which further helps in development of the SST anomaly. Mixed layer thins further due to anomalous shortwave radiation during summer and a well developed SST anomaly evolves. The south Atlantic pole exhibits exactly opposite characteristics at the same time. The contribution from the surface heat fluxes to mixed layer temperature change is found to be dominant over the advective processes over both the basins. Net surface heat fluxes anomaly is also found to be maximum during late austral spring-early summer period, with latent heat flux having a major contribution to it. The anomalous latent heat fluxes between atmosphere and ocean surface play important role in the growth of observed summertime SST anomaly. Sea-surface height also shows similar out-of-phase signatures over the two basins and are well correlated with the ENSO related SST anomalies. It is also observed that the magnitude of ENSO related anomalies over the southern ocean are weaker in LaNina years than in El Nino years, suggesting an intensified tropics-high latitude tele-connection during warm phases of ENSO.

Response of the global surface ozone distribution to Northern Hemisphere sea surface temperature changes: implications for long-range transport

NASA Astrophysics Data System (ADS)

Yi, Kan; Liu, Junfeng; Ban-Weiss, George; Zhang, Jiachen; Tao, Wei; Cheng, Yanli; Tao, Shu

2017-07-01

The response of surface ozone (O3) concentrations to basin-scale warming and cooling of Northern Hemisphere oceans is investigated using the Community Earth System Model (CESM). Idealized, spatially uniform sea surface temperature (SST) anomalies of ±1 °C are individually superimposed onto the North Pacific, North Atlantic, and North Indian oceans. Our simulations suggest large seasonal and regional variability in surface O3 in response to SST anomalies, especially in the boreal summer. The responses of surface O3 associated with basin-scale SST warming and cooling have similar magnitude but are opposite in sign. Increasing the SST by 1 °C in one of the oceans generally decreases the surface O3 concentrations from 1 to 5 ppbv. With fixed emissions, SST increases in a specific ocean basin in the Northern Hemisphere tend to increase the summertime surface O3 concentrations over upwind regions, accompanied by a widespread reduction over downwind continents. We implement the integrated process rate (IPR) analysis in CESM and find that meteorological O3 transport in response to SST changes is the key process causing surface O3 perturbations in most cases. During the boreal summer, basin-scale SST warming facilitates the vertical transport of O3 to the surface over upwind regions while significantly reducing the vertical transport over downwind continents. This process, as confirmed by tagged CO-like tracers, indicates a considerable suppression of intercontinental O3 transport due to increased tropospheric stability at lower midlatitudes induced by SST changes. Conversely, the responses of chemical O3 production to regional SST warming can exert positive effects on surface O3 levels over highly polluted continents, except South Asia, where intensified cloud loading in response to North Indian SST warming depresses both the surface air temperature and solar radiation, and thus photochemical O3 production. Our findings indicate a robust linkage between basin-scale SST variability and continental surface O3 pollution, which should be considered in regional air quality management.

Interannual-to-multidecadal hydroclimate variability and its sectoral impacts in northeastern Argentina

NASA Astrophysics Data System (ADS)

Lovino, Miguel A.; Müller, Omar V.; Müller, Gabriela V.; Sgroi, Leandro C.; Baethgen, Walter E.

2018-06-01

This study examines the joint variability of precipitation, river streamflow and temperature over northeastern Argentina; advances the understanding of their links with global SST forcing; and discusses their impacts on water resources, agriculture and human settlements. The leading patterns of variability, and their nonlinear trends and cycles are identified by means of a principal component analysis (PCA) complemented with a singular spectrum analysis (SSA). Interannual hydroclimatic variability centers on two broad frequency bands: one of 2.5-6.5 years corresponding to El Niño Southern Oscillation (ENSO) periodicities and the second of about 9 years. The higher frequencies of the precipitation variability (2.5-4 years) favored extreme events after 2000, even during moderate extreme phases of the ENSO. Minimum temperature is correlated with ENSO with a main frequency close to 3 years. Maximum temperature time series correlate well with SST variability over the South Atlantic, Indian and Pacific oceans with a 9-year frequency. Interdecadal variability is characterized by low-frequency trends and multidecadal oscillations that have induced a transition from dryer and cooler climate to wetter and warmer decades starting in the mid-twentieth century. The Paraná River streamflow is influenced by North and South Atlantic SSTs with bidecadal periodicities. The hydroclimate variability at all timescales had significant sectoral impacts. Frequent wet events between 1970 and 2005 favored floods that affected agricultural and livestock productivity and forced population displacements. On the other hand, agricultural droughts resulted in soil moisture deficits that affected crops at critical growth stages. Hydrological droughts affected surface water resources, causing water and food scarcity and stressing the capacity for hydropower generation. Lastly, increases in minimum temperature reduced wheat and barley yields.

Interannual Modulation of Subtropical Atlantic Boreal Summer Dust Variability by ENSO

DOE Office of Scientific and Technical Information (OSTI.GOV)

DeFlorio, Mike; Goodwin, Ian D.; Cayan, Dan

2016-01-01

Dust variability in the climate system has been studied for several decades, yet there remains an incomplete understanding of the dynamical mechanisms controlling interannual and decadal variations in dust transport. The sparseness of multi-year observational datasets has limited our understanding of the relationship between climate variations and atmospheric dust. We use available observations and a century-length fully coupled Community Earth System Model (CESM) simulation to show that the El Niño- Southern Oscillation (ENSO) exerts a control on North African dust transport during boreal summer. In CESM, this relationship is stronger over the dusty tropical North Atlantic than near Barbados, onemore » of the few sites having a multi-decadal observed record. During strong La Niña summers in CESM, a statistically significant increase in lower tropospheric easterly wind is associated with an increase in North African dust transport over the Atlantic. Barbados dust and Pacific SST variability are only weakly correlated in both observations and CESM, suggesting that other processes are controlling the crossbasin variability of dust. We also use our CESM simulation to show that the relationship between downstream North African dust transport and ENSO fluctuates on multidecadal timescales and may be modulated by the North Atlantic Oscillation (NAO). Our findings indicate that existing observations of dust over the tropical North Atlantic are not extensive enough to completely describe the variability of dust and dust transport, and demonstrate the importance of global models to supplement and interpret observational records.« less

The Little Ice Age was 1.0-1.5 °C cooler than current warm period according to LOD and NAO

NASA Astrophysics Data System (ADS)

Mazzarella, Adriano; Scafetta, Nicola

2018-02-01

We study the yearly values of the length of day (LOD, 1623-2016) and its link to the zonal index (ZI, 1873-2003), the Northern Atlantic oscillation index (NAO, 1659-2000) and the global sea surface temperature (SST, 1850-2016). LOD is herein assumed to be mostly the result of the overall circulations occurring within the ocean-atmospheric system. We find that LOD is negatively correlated with the global SST and with both the integral function of ZI and NAO, which are labeled as IZI and INAO. A first result is that LOD must be driven by a climatic change induced by an external (e.g. solar/astronomical) forcing since internal variability alone would have likely induced a positive correlation among the same variables because of the conservation of the Earth's angular momentum. A second result is that the high correlation among the variables implies that the LOD and INAO records can be adopted as global proxies to reconstruct past climate change. Tentative global SST reconstructions since the seventeenth century suggest that around 1700, that is during the coolest period of the Little Ice Age (LIA), SST could have been about 1.0-1.5 °C cooler than the 1950-1980 period. This estimated LIA cooling is greater than what some multiproxy global climate reconstructions suggested, but it is in good agreement with other more recent climate reconstructions including those based on borehole temperature data.

Advances in Understanding Decadal Climate Variability

NASA Technical Reports Server (NTRS)

Busalaacchi, Antonio J.

1998-01-01

Recently, a joint Brazil-France-U.S. program, known as PIRATA (Pilot Research moored Array in the Tropical Atlantic), was proposed to begin the deployment of moored measurement platforms in the tropical Atlantic in order to enhance the existing observational data base and subsequent understanding of the processes by which the ocean and atmosphere couple in key regions of the tropical Atlantic Ocean. Empirical studies have suggested that there are strong relationships between tropical Atlantic upper ocean variability, SST, ocean-atmosphere coupling and regional climate variability. During the early 1980's a coordinated set of surface wind, subsurface thermal structure, and subsurface current observations were obtained as part of the U.S.-France SEQUAL- FOCAL process experiment designed to observe the seasonal response of the tropical Atlantic Ocean to surface forcing. Since that time, however, the observational data base for the tropical Atlantic Ocean has disintegrated to a few shiptracks measuring ocean temperatures and a small collection of tide gauge stations measuring sea level. A more comprehensive set of observations, modeling and empirical studies is now in order to make progress on understanding the regional climate variability. The proposed PIRATA program will use mooring platforms similar to the tropical Pacific Ocean TAO array to measure surface fluxes of momentum and heat and the corresponding changes in the upper ocean thermal structure. It is anticipated that the oceanic data from this monitoring array will also be used in a predictive mode for initialization studies of regional coupled climate models. Of particular interest are zonal and meridional modes of ocean-atmosphere variability within the tropical Atlantic basin that have significant impacts on the regional climate of the bordering continents.

Advances in Understanding Decadal Climate Variability

NASA Technical Reports Server (NTRS)

Busalacchi, Antonio J.

1999-01-01

Recently, a joint Brazil-France-U.S. program, known as PIRATA (Pilot Research moored Array in the Tropical Atlantic), was proposed to begin the deployment of moored measurement platforms in the tropical Atlantic in order to enhance the existing observational data base and subsequent understanding of the processes by which the ocean and atmosphere couple in key regions of the tropical Atlantic Ocean. Empirical studies have suggested that there are strong relationships between tropical Atlantic upper ocean variability, SST, ocean-atmosphere coupling and regional climate variability. During the early 1980's a coordinated set of surface wind, subsurface thermal structure, and subsurface current observations were obtained as part of the U.S.-France SEQUAL-FOCAL process experiment designed to observe the seasonal response of the tropical Atlantic Ocean to surface forcing. Since that time, however, the observational data base for the tropical Atlantic Ocean has disintegrated to a few ship-tracks measuring ocean temperatures and a small collection of tide gauge stations measuring sea level. A more comprehensive set of observations, modeling and empirical studies is now in order to make progress on understanding the regional climate variability. The proposed PIRATA program will use mooring platforms similar to the tropical Pacific Ocean TAO array to measure surface fluxes of momentum and heat and the corresponding changes in the upper ocean thermal structure. It is anticipated that the oceanic data from this monitoring array will also be used in a predictive mode for initialization studies of regional coupled climate models. Of particular interest are zonal and meridional modes of ocean-atmosphere variability within the tropical Atlantic basin that have significant impacts on the regional climate of the bordering continents.

The long-term variability of Changma in the East Asian summer monsoon system: A review and revisit

NASA Astrophysics Data System (ADS)

Lee, June-Yi; Kwon, MinHo; Yun, Kyung-Sook; Min, Seung-Ki; Park, In-Hong; Ham, Yoo-Geun; Jin, Emilia Kyung; Kim, Joo-Hong; Seo, Kyong-Hwan; Kim, WonMoo; Yim, So-Young; Yoon, Jin-Ho

2017-05-01

Changma, which is a vital part of East Asian summer monsoon (EASM) system, plays a critical role in modulating water and energy cycles in Korea. Better understanding of its long-term variability and change is therefore a matter of scientific and societal importance. It has been indicated that characteristics of Changma have undergone significant interdecadal changes in association with the mid-1970s global-scale climate shift and the mid-1990s EASM shift. This paper reviews and revisits the characteristics on the long-term changes of Changma focusing on the underlying mechanisms for the changes. The four important features are manifested mainly during the last few decades: 1) mean and extreme rainfalls during Changma period from June to September have been increased with the amplification of diurnal cycle of rainfall, 2) the dry spell between the first and second rainy periods has become shorter, 3) the rainfall amount as well as the number of rainy days during August have significantly increased, probably due to the increase in typhoon landfalls, and 4) the relationship between the Changma rainfall and Western Pacific Subtropical High on interannual time scale has been enhanced. The typhoon contribution to the increase in heavy rainfall is attributable to enhanced interaction between typhoons and midlatitude baroclinic environment. It is noted that the change in the relationship between Changma and the tropical sea surface temperature (SST) over the Indian, Pacific, and Atlantic Oceans is a key factor in the long-term changes of Changma and EASM. Possible sources for the recent mid-1990s change include 1) the tropical dipole-like SST pattern between the central Pacific and Indo-Pacific region (the global warming hiatus pattern), 2) the recent intensification of tropical SST gradients among the Indian Ocean, the western Pacific, and the eastern Pacific, and 3) the tropical Atlantic SST warming.

Short-finned Pilot Whales (Globicephala macrorhynchus) Target Gulf Stream and Shelf Break Waters in the U.S. Mid-Atlantic Bight

NASA Astrophysics Data System (ADS)

Thorne, L. H.; Foley, H.; Webster, D.; Baird, R.; Swaim, Z.; Read, A.

2016-02-01

Short-finned pilot whales (Globicephala macrorhynchus) are deep-diving predators that feed on squid and regularly exploit prey at depths of more than 500 m. Detailed information on the habitat use of pilot whales in the Northwest Atlantic is lacking, which complicates management of the species, particularly for efforts to mitigate bycatch and depredation in the pelagic longline fishery. To address this limitation, we tracked the horizontal and vertical movements of short-finned pilot whales with LIMPET satellite-linked transmitters off Cape Hatteras, North Carolina, in 2014. We deployed 14 satellite tags and 4 satellite-linked depth recording tags, with deployments of 2 to 194 days (mean=57 days). Using randomly-generated temporally-matched pseudo-absences with modeled distance constraints and mixed-effects generalized additive models (GAMMs), we evaluated pilot whale movement relative to environmental variables (distance to shelf break, sea surface temperature SST), location of Gulf Stream, bathymetric slope, and depth). Pilot whales showed two types of behavior, showing a strong affinity for either the shelf break or waters of the Gulf Stream. Slope, distance to shelf break, and SST were significant predictors of habitat use (p<<0.001 for all variables, R2=0.40). Pilot whales demonstrated a preference for waters close to the shelf break, with warmer SST values (peak preference 25°C) and medium to high bathymetric slopes (peak preference 40 percent rise). We observed seasonal patterns in pilot whale movements, with whales diving to deeper depths in late summer and fall months than in spring months (Wilcoxon test, p<<0.001). Diving behavior was also significantly influenced by SST; pilot whales took longer and deeper dives in warmer waters (Pearson's correlation coefficients >0.40, p<<0.001). We use these results to develop spatial maps of pilot whale habitat relative to seasonal and environmental factors in order to identify areas and times of high risk for interactions between pilot whales and longline gear.

Advancing decadal-scale climate prediction in the North Atlantic sector.

PubMed

Keenlyside, N S; Latif, M; Jungclaus, J; Kornblueh, L; Roeckner, E

2008-05-01

The climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences. Prominent examples include hurricane activity in the Atlantic, and surface-temperature and rainfall variations over North America, Europe and northern Africa. Although these multidecadal variations are potentially predictable if the current state of the ocean is known, the lack of subsurface ocean observations that constrain this state has been a limiting factor for realizing the full skill potential of such predictions. Here we apply a simple approach-that uses only sea surface temperature (SST) observations-to partly overcome this difficulty and perform retrospective decadal predictions with a climate model. Skill is improved significantly relative to predictions made with incomplete knowledge of the ocean state, particularly in the North Atlantic and tropical Pacific oceans. Thus these results point towards the possibility of routine decadal climate predictions. Using this method, and by considering both internal natural climate variations and projected future anthropogenic forcing, we make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.

Interannual Rainfall Variability in the Tropical Atlantic Region

NASA Technical Reports Server (NTRS)

Gu, Guojun

2005-01-01

Rainfall variability on seasonal and interannual-to-interdecadal time scales in the tropical Atlantic is quantified using a 25-year (1979-2003) monthly rainfall dataset from the Global Precipitation Climatology Project (GPCP). The ITCZ measured by monthly rainfall between 15-37.5 deg W attains its peak as moving to the northernmost latitude (4-10 deg N) during July-September in which the most total rainfall is observed in the tropical Atlantic basin (17.5 deg S-22.5 deg N, 15 deg-37.5 deg W); the ITCZ becomes weakest during January-February with the least total rainfall as it moves to the south. In contrast, rainfall variability on interannual to interdecadal time scales shows a quite different seasonal preference. The most intense interannual variability occurs during March-May when the ITCZ tends to be near the equator and becomes weaker. Significant, negative correlations between the ITCZ strength and latitude anomalies are observed during boreal spring and early summer. The ITCZ strength and total rainfall amount in the tropical Atlantic basin are significantly modulated by the Pacific El Nino and the Atlantic equatorial mode (or Atlantic Nino) particularly during boreal spring and summer; whereas the impact of the Atlantic interhemispheric mode is considerably weaker. Regarding the anomalous latitudes of the ITCZ, the influence can come from both local, i.e., the Atlantic interhemispheric and equatorial modes, and remote forcings, i. e., El Nino; however, a direct impact of El Nino on the latitudes of the ITCZ can only be found during April-July, not in winter and early spring in which the warmest SST anomalies are usually observed in the equatorial Pacific.

Identifying and Investigating the Late-1960s Interhemispheric SST Shift

NASA Astrophysics Data System (ADS)

Friedman, A. R.; Lee, S. Y.; Liu, Y.; Chiang, J. C. H.

2014-12-01

The global north-south interhemispheric sea surface temperature (SST) difference experienced a pronounced and rapid decrease in the late 1960s, which has been linked to drying in the Sahel, South Asia, and East Asia. However, some basic questions about the interhemispheric SST shift remain unresolved, including its scale and whether the constituent changes in different basins were coordinated. In this study, we systematically investigate the spatial and temporal behavior of the late-1960s interhemispheric SST shift using ocean surface and subsurface observations. We also evaluate potential mechanisms using control and specific-forcing CMIP5 simulations. Using a regime shift detection technique, we identify the late-1960s shift as the most prominent in the historical observational SST record. We additionally examine the corresponding changes in upper-ocean heat content and salinity associated with the shift. We find that there were coordinated upper-ocean cooling and freshening in the subpolar North Atlantic, the region of the largest-magnitude SST decrease during the interhemispheric shift. These upper-ocean changes correspond to a weakened North Atlantic thermohaline circulation (THC). However, the THC decrease does not fully account for the rapid global interhemispheric SST shift, particularly the warming in the extratropical Southern Hemisphere.

Structure and dynamics of the Benguela low-level coastal jet

NASA Astrophysics Data System (ADS)

Patricola, Christina M.; Chang, Ping

2017-10-01

Generations of coupled atmosphere-ocean general circulation models have been plagued by persistent warm sea surface temperature (SST) biases in the southeastern tropical Atlantic. The SST biases are most severe in the eastern boundary coastal upwelling region and are sensitive to surface wind stress and wind stress curl associated with the Benguela low-level coastal jet (BLLCJ), a southerly jet parallel to the Angola-Namibia coast. However, little has been documented about this atmospheric source of oceanic bias. Here we investigate the characteristics and dynamics of the BLLCJ using observations, reanalyses, and atmospheric model simulations. Satellite wind products and high-resolution reanalyses and models represent the BLLCJ with two near-shore maxima, one near the Angola-Benguela front (ABF) at 17.5°S, and the other near 25-27.5°S, whereas coarse resolution reanalyses and models represent the BLLCJ poorly with a single, broad, more offshore maximum. Model experiments indicate that convex coastal geometry near the ABF supports the preferred location of the BLLCJ northern maximum by supporting conditions for a hydraulic expansion fan. Intraseasonal variability of the BLLCJ is associated with large-scale variability in intensity and location of the South Atlantic subtropical high through modulation of the low-level zonal pressure gradient.

Seasonal and Interannual Variability of the Brazil - Malvinas Front: an Altimetry Perspective

NASA Astrophysics Data System (ADS)

Saraceno, M.; Valla, D.; Pelegrí, J. L.; Piola, A. R.

2016-02-01

The Brazil and Malvinas Confluence in the Southwestern Atlantic is one of the most energetic regions of the world ocean. Using recent measurements of sub-surface velocity currents, collected along 2348 nautical miles with a vessel mounted acoustic Doppler profiler onboard R/V BIO Hespérides, we validate geostrophic velocities derived from gridded fields of sea surface height (SSH). A remarkable correspondence between in-situ surface hydrographic data collected from the vessel and satellite sea surface temperature (SST), color and altimetry data allows selecting a specific SSH contour to track the position of the Brazil-Malvinas front. We then use 22 years of SSH data distributed by AVISO to show that the Brazil-Malvinas front shows a NS orientation in winter and a NE-SW orientation in summer, in good agreement with results based on the analysis of SST gradients. Furthermore, a clear southward migration of the front during the 22 year period is observed. The migration is associated with the southward shift of the South Atlantic high-pressure system that is in turn related to large climate changes in the southern portion of the South American continent. The seasonal variability in the orientation of the front is related to the Brazil and Malvinas encountering currents.

EnOI-IAU Initialization Scheme Designed for Decadal Climate Prediction System IAP-DecPreS

NASA Astrophysics Data System (ADS)

Wu, Bo; Zhou, Tianjun; Zheng, Fei

2018-02-01

A decadal climate prediction system named as IAP-DecPreS was constructed in the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, based on a fully coupled model FGOALS-s2 and a newly developed initialization scheme, referred to as EnOI-IAU. In this paper, we introduce the design of the EnOI-IAU scheme, assess the accuracies of initialization integrations using the EnOI-IAU and preliminarily evaluate hindcast skill of the IAP-DecPreS. The EnOI-IAU scheme integrates two conventional assimilation approaches, ensemble optimal interpolation (EnOI) and incremental analysis update (IAU). The EnOI and IAU were applied to calculate analysis increments and incorporate them into the model, respectively. Three continuous initialization (INIT) runs were conducted for the period of 1950-2015, in which observational sea surface temperature (SST) from the HadISST1.1 and subsurface ocean temperature profiles from the EN4.1.1 data set were assimilated. Then nine-member 10 year long hindcast runs initiated from the INIT runs were conducted for each year in the period of 1960-2005. The accuracies of the INIT runs are evaluated from the following three aspects: upper 700 m ocean temperature, temporal evolution of SST anomalies, and dominant interdecadal variability modes, Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Finally, preliminary evaluation of the ensemble mean of the hindcast runs suggests that the IAP-DecPreS has skill in the prediction of the PDO-related SST anomalies in the midlatitude North Pacific and AMO-related SST anomalies in the tropical North Atlantic.

Simulation of Rainfall Variability Over West Africa

NASA Astrophysics Data System (ADS)

Bader, J.; Latif, M.

The impact of sea surface temperature (SST) and vegetation on precipitation over West Africa is investigated with the atmospheric general circulation model ECHAM4.x/T42. Ensemble experiments -driven with observed SST- show that At- lantic SST has a significant influence on JJA precipitation over West Africa. Four- teen experiments were performed in which the climatological SST was enhanced or decreased by one Kelvin in certain ocean areas. Changing SST in the eastern tropi- cal Atlantic only caused significant changes along the Guinea Coast, with a positive SSTA increasing rainfall and a negative reducing it. The response was nearly linear. Changing SST in other ocean areas caused significant changes over West Africa, es- pecially in the Sahel area. The response is found to be non linear, with only negative SSTA leading to significant reduction in Sahel rainfall. Also, the impact of the SSTAs from the different ocean regions was not additive with respect to the rainfall. Four simulations with a coupled model (the simple dynamic vegetation model (SVege) and the ECHAM4-AGCM were coupled) were also performed, driven with observed SST from 1945 to 1998. The standard ECHAM-AGCM -forced by the same observed SST- was able to reproduce the drying trend from the fifties to the mid-eighties in the Sahel, but failed to mirror the magnitude of the rainfall anomalies. The coupled model was not only able to reproduce this drying trend, but was also able to better reproduce the amplitudes of the rainfall anomalies. The dynamic vegetation acted like an amplifier, increasing the SST induced rainfall anomalies.

Sea Surface Temperature Influence on Terrestrial Gross Primary Production along the Southern California Current.

PubMed

Reimer, Janet J; Vargas, Rodrigo; Rivas, David; Gaxiola-Castro, Gilberto; Hernandez-Ayon, J Martin; Lara-Lara, Ruben

2015-01-01

Some land and ocean processes are related through connections (and synoptic-scale teleconnections) to the atmosphere. Synoptic-scale atmospheric (El Niño/Southern Oscillation [ENSO], Pacific Decadal Oscillation [PDO], and North Atlantic Oscillation [NAO]) decadal cycles are known to influence the global terrestrial carbon cycle. Potentially, smaller scale land-ocean connections influenced by coastal upwelling (changes in sea surface temperature) may be important for local-to-regional water-limited ecosystems where plants may benefit from air moisture transported from the ocean to terrestrial ecosystems. Here we use satellite-derived observations to test potential connections between changes in sea surface temperature (SST) in regions with strong coastal upwelling and terrestrial gross primary production (GPP) across the Baja California Peninsula. This region is characterized by an arid/semiarid climate along the southern California Current. We found that SST was correlated with the fraction of photosynthetic active radiation (fPAR; as a proxy for GPP) with lags ranging from 0 to 5 months. In contrast ENSO was not as strongly related with fPAR as SST in these coastal ecosystems. Our results show the importance of local-scale changes in SST during upwelling events, to explain the variability in GPP in coastal, water-limited ecosystems. The response of GPP to SST was spatially-dependent: colder SST in the northern areas increased GPP (likely by influencing fog formation), while warmer SST at the southern areas was associated to higher GPP (as SST is in phase with precipitation patterns). Interannual trends in fPAR are also spatially variable along the Baja California Peninsula with increasing secular trends in subtropical regions, decreasing trends in the most arid region, and no trend in the semi-arid regions. These findings suggest that studies and ecosystem process based models should consider the lateral influence of local-scale ocean processes that could influence coastal ecosystem productivity.

Sea Surface Temperature Influence on Terrestrial Gross Primary Production along the Southern California Current

PubMed Central

Reimer, Janet J.; Vargas, Rodrigo; Rivas, David; Gaxiola-Castro, Gilberto; Hernandez-Ayon, J. Martin; Lara-Lara, Ruben

2015-01-01

Some land and ocean processes are related through connections (and synoptic-scale teleconnections) to the atmosphere. Synoptic-scale atmospheric (El Niño/Southern Oscillation [ENSO], Pacific Decadal Oscillation [PDO], and North Atlantic Oscillation [NAO]) decadal cycles are known to influence the global terrestrial carbon cycle. Potentially, smaller scale land-ocean connections influenced by coastal upwelling (changes in sea surface temperature) may be important for local-to-regional water-limited ecosystems where plants may benefit from air moisture transported from the ocean to terrestrial ecosystems. Here we use satellite-derived observations to test potential connections between changes in sea surface temperature (SST) in regions with strong coastal upwelling and terrestrial gross primary production (GPP) across the Baja California Peninsula. This region is characterized by an arid/semiarid climate along the southern California Current. We found that SST was correlated with the fraction of photosynthetic active radiation (fPAR; as a proxy for GPP) with lags ranging from 0 to 5 months. In contrast ENSO was not as strongly related with fPAR as SST in these coastal ecosystems. Our results show the importance of local-scale changes in SST during upwelling events, to explain the variability in GPP in coastal, water-limited ecosystems. The response of GPP to SST was spatially-dependent: colder SST in the northern areas increased GPP (likely by influencing fog formation), while warmer SST at the southern areas was associated to higher GPP (as SST is in phase with precipitation patterns). Interannual trends in fPAR are also spatially variable along the Baja California Peninsula with increasing secular trends in subtropical regions, decreasing trends in the most arid region, and no trend in the semi-arid regions. These findings suggest that studies and ecosystem process based models should consider the lateral influence of local-scale ocean processes that could influence coastal ecosystem productivity. PMID:25923109

Dominant Role of Atlantic Multidecadal Oscillation in the Recent Decadal Changes in Western North Pacific Tropical Cyclone Activity

NASA Astrophysics Data System (ADS)

Zhang, Wei; Vecchi, Gabriel A.; Murakami, Hiroyuki; Villarini, Gabriele; Delworth, Thomas L.; Yang, Xiaosong; Jia, Liwei

2018-01-01

Over the 1997-2014 period, the mean frequency of western North Pacific (WNP) tropical cyclones (TCs) was markedly lower ( 18%) than the period 1980-1996. Here we show that these changes were driven by an intensification of the vertical wind shear in the southeastern/eastern WNP tied to the changes in the Walker circulation, which arose primarily in response to the enhanced sea surface temperature (SST) warming in the North Atlantic, while the SST anomalies associated with the negative phase of the Pacific Decadal Oscillation in the tropical Pacific and the anthropogenic forcing play only secondary roles. These results are based on observations and experiments using the Geophysical Fluid Dynamics Laboratory Forecast-oriented Low-ocean Resolution Coupled Climate Model coupled climate model. The present study suggests a crucial role of the North Atlantic SST in causing decadal changes to WNP TC frequency.

Solar wind: A possible factor driving the interannual sea surface temperature tripolar mode over North Atlantic

NASA Astrophysics Data System (ADS)

Xiao, Ziniu; Li, Delin

2016-06-01

The effect of solar wind (SW) on the North Atlantic sea surface temperature (SST) in boreal winter is examined through an analysis of observational data during 1964-2013. The North Atlantic SSTs show a pronounced meridional tripolar pattern in response to solar wind speed (SWS) variations. This pattern is broadly similar to the leading empirical orthogonal function (EOF) mode of interannual variations in the wintertime SSTs over North Atlantic. The time series of this leading EOF mode of SST shows a significant interannual period, which is the same as that of wintertime SWS. This response also appears as a compact north-south seesaw of sea level pressure and a vertical tripolar structure of zonal wind, which simultaneously resembles the North Atlantic Oscillation (NAO) in the overlying atmosphere. As compared with the typical low SWS winters, during the typical high SWS winters, the stratospheric polar night jet (PNJ) is evidently enhanced and extends from the stratosphere to the troposphere, even down to the North Atlantic Ocean surface. Notably, the North Atlantic Ocean is an exclusive region in which the SW signal spreads downward from the stratosphere to the troposphere. Thus, it seems that the SW is a possible factor for this North Atlantic SST tripolar mode. The dynamical process of stratosphere-troposphere coupling, together with the global atmospheric electric circuit-cloud microphysical process, probably accounts for the particular downward propagation of the SW signal.

Mid-Piacenzian Variability of Nordic Seas Surface Circulation Linked to Terrestrial Climatic Change in Norway

NASA Astrophysics Data System (ADS)

Panitz, Sina; De Schepper, Stijn; Salzmann, Ulrich; Bachem, Paul E.; Risebrobakken, Bjørg; Clotten, Caroline; Hocking, Emma P.

2017-12-01

During the mid-Piacenzian, Nordic Seas sea surface temperatures (SSTs) were higher than today. While SSTs provide crucial climatic information, on their own they do not allow a reconstruction of potential underlying changes in water masses and currents. A new dinoflagellate cyst record for Ocean Drilling Program (ODP) Site 642 is presented to evaluate changes in northward heat transport via the Norwegian Atlantic Current (NwAC) between 3.320 and 3.137 Ma. The record is compared with vegetation and SST reconstructions from Site 642 and SSTs from Iceland Sea ODP Site 907 to identify links between SSTs, ocean currents, and vegetation changes. The dinocyst record shows that strong Atlantic water influence via the NwAC corresponds to higher-than-present SSTs and cool temperate vegetation during Marine Isotope Stage (MIS) transition M2-M1 and KM5. Reduced Atlantic water inflow relative to the warm stages coincides with near-modern SSTs and boreal vegetation during MIS M2, KM6, and KM4-KM2. During most of the studied interval, a strong SST gradient between Sites 642 and 907 indicates the presence of a proto-Arctic Front (AF). An absent gradient during the first half of MIS KM6, due to reduced Atlantic water influence at Site 642 and warm, presumably Atlantic water reaching Site 907, is indicative of a weakened NwAC and East Greenland Current. We conclude that repeated changes in Atlantic water influence directly affect terrestrial climate and that an active NwAC is needed for an AF to develop. Obliquity forcing may have played a role, but the correlation is not consistent.

Investigation of the UK37' vs. SST relationship for Atlantic Ocean suspended particulate alkenones: An alternative regression model and discussion of possible sampling bias

NASA Astrophysics Data System (ADS)

Gould, Jessica; Kienast, Markus; Dowd, Michael

2017-05-01

Alkenone unsaturation, expressed as the UK37' index, is closely related to growth temperature of prymnesiophytes, thus providing a reliable proxy to infer past sea surface temperatures (SSTs). Here we address two lingering uncertainties related to this SST proxy. First, calibration models developed for core-top sediments and those developed for surface suspended particulates organic material (SPOM) show systematic offsets, raising concerns regarding the transfer of the primary signal into the sedimentary record. Second, questions remain regarding changes in slope of the UK37' vs. growth temperature relationship at the temperature extremes. Based on (re)analysis of 31 new and 394 previously published SPOM UK37' data from the Atlantic Ocean, a new regression model to relate UK37' to SST is introduced; the Richards curve (Richards, 1959). This non-linear regression model provides a robust calibration of the UK37' vs. SST relationship for Atlantic SPOM samples and uniquely accounts for both the fact that the UK37' index is a proportion, and so must lie between 0 and 1, as well as for the observed reduction in slope at the warm and cold ends of the temperature range. As with prior fits of SPOM UK37' vs. SST, the Richards model is offset from traditional regression models of sedimentary UK37' vs. SST. We posit that (some of) this offset can be attributed to the seasonally and depth biased sampling of SPOM material.

A new planktic foraminifer transfer function for estimating pliocene-Holocene paleoceanographic conditions in the North Atlantic

USGS Publications Warehouse

Dowsett, H.J.; Poore, R.Z.

1990-01-01

A new planktic foraminifer transfer function (GSF18) related 5 North Atlantic assemblages to winter and summer sea surface temperature. GSF18, based on recombined and simplified core top census data, preserves most environmental information and reproduces modern North Atlantic conditions with approximately the same accuracy as previous transfer functions, but can be more readily applied to faunal samples ranging in age from Pliocene to Holocene. Transfer function GSF18 has been applied to faunal data from Deep Sea Drilling Project Hole 552A to produce a 2.5 m.y. sea-surface temperature (SST) time series. Estimates show several periods between 2.3 and 4.6 Ma during which mean SST's were both several degrees warmer and several degrees cooler than modern conditions. Between 2.9 and 4.0 Ma SST was generally warmer than modern except for a 250 k.y. interval centered at 3.3 Ma. Maximum SST, with respect to modern conditions, occurred after the cool interval near 3.1 Ma when SST was approximately 3.6??C warmer than present conditions. Comparison of SST estimates with stable isotope data suggest that after peak warming at 3.1 Ma, there was an overall surface water cooling with concomitant build up of global ice volume, culminating in Northern Hemisphere glaciation. This event is also indicated by the presence of ice rafted detritus in 552A sediments at about 2.45 Ma. ?? 1990 Elsevier Science Publishers B.V.

Late Oligocene decoupling of temperature and pCO2: Insights from TEX86 paleothermometry

NASA Astrophysics Data System (ADS)

O'Brien, C. L.; Pagani, M.

2016-12-01

Current paleo-proxy reconstructions for the late Oligocene ( 28-23 Ma) indicate a decoupling of temperature and pCO2. Specifically, benthic oxygen isotope data suggest either stable conditions or warming/deglaciation, while alkenone-based pCO2 estimates indicate a decline from 700 to 400 ppm. Existing sea surface temperature (SST) proxy estimates for this interval are sparse and the appearance of decoupling could be fallacious. Using late Oligocene marine sediments from a range of oceanographic and latitudinal settings, in particular Atlantic Ocean sites ODP 929A (5°N), DSDP 608 (42°N) and DSDP 516F (30°S), we are applying the TEX86 paleothermometer to provide improved constraints on late Oligocene warmth. Thought to originate mainly from planktonic, ammonia-oxidizing Thaumarchaeota, the sedimentary TEX86 signal is complicated by potential influences from additional sources and non-thermal effects (e.g., water chemistry, nutrient dynamics, growth stage and ecology). Thus, we are simultaneously testing assumptions regarding the fidelity of the TEX86 paleo-SST proxy. Our new TEX86H-SST data from Atlantic site ODP 929A indicate stable SSTs in the tropics (often reflective of global conditions) during the late Oligocene, with no reduction in SST coincident with declining pCO2 during the period 28-24 Ma. Importantly, TEX86H-SST data show a lack of coherence with latitude exemplified by similar stable SSTs, 28°C, at tropical and southern mid-latitude Atlantic sites ODP 929 and DSDP 516F, respectively. This absence of a decrease in SST with increasing site latitude suggests that additional non-thermal factors may be influencing the TEX86 signal at certain locations and/or a need for regional-based TEX86-SST calibrations. Indeed, if our tropical TEX86-SST reconstructions ( 28°C) are valid then this would imply the late Oligocene tropical Atlantic was no warmer than the Pliocene, contradicting multiple lines of evidence that the world was warmer (e.g., higher pCO2, lighter benthic δ18O values and lower ice volume). We shall critically evaluate the implications of our TEX86 data with respect to both (1) late Oligocene climate dynamics and (2) the validity of the TEX86-SST proxy across multiple late Oligocene ocean sites.

The effects of SST Gradients on Tropical Convective Systems and Implications for Tropical Cyclogenesis

NASA Astrophysics Data System (ADS)

Glazer, R.; Bourassa, M. A.; Hart, R. E.

2013-12-01

It has long been known that generally the warmer the sea surface temperature (SST), the more possible tropical cyclone (TC) genesis is, assuming the atmosphere is supportive. The conventional wisdom has been that - apart from what the TC cools through upwelling -- one value of SST represents the state of the ocean surface in the region of the storm's inner circulation. With the advent of the satellite era and fine resolution SST datasets now becoming available, we know that in reality there are gradients of SST across which developing TCs move. The influence of those gradients on tropical convection and TC genesis is largely unknown at this time. Previous studies have shown that SST gradients can significantly impact the overlying ocean surface winds leading to areas of enhanced convergence/divergence and Vorticity (Chelton et al. 2004; O'Neill et al. 2005, 2010). The magnitude of this effect approximately increases as the surface wind increases. Work by Minobe et al. (2008) concluded that a sharp SST Gradient, over the Gulf Stream for instance, could produce enough surface wind convergence to maintain a band of precipitation along the ocean front. An analysis of satellite derived SST data over the Atlantic shows that it is not uncommon for SST gradients of 2 C/200km or more to exist in the immediate environment of a Tropical System. The authors seek to understand whether the conclusions made in previous works can be applied in the case of a developing Tropical System and whether SST Gradients exist in the Tropical Atlantic to a degree that would influence the cyclogenesis process. To address this, the effects of SST gradients on tropical cyclogenesis processes are investigated using model simulations of the Weather Research and Forecasting Model (WRF). WRF is run at cloud permitting scales (2km) for real cases of co-location between a tropical system and an SST gradient exceeding 2 C/200km in the environment of the system. In subsequent runs to this control run, the SSTs are modified to give a smaller or larger SST Gradient with the same atmospheric conditions. All cases are chosen from Atlantic Hurricane Seasons between 2002-2011. The results are then analyzed in the framework of previous studies that have sought to model and understand tropical cyclogenesis using WRF (Nolan 2007; Fang and Zhang 2010).

North Atlantic simulations in Coordinated Ocean-ice Reference Experiments phase II (CORE-II). Part II: Inter-annual to decadal variability

NASA Astrophysics Data System (ADS)

Danabasoglu, Gokhan; Yeager, Steve G.; Kim, Who M.; Behrens, Erik; Bentsen, Mats; Bi, Daohua; Biastoch, Arne; Bleck, Rainer; Böning, Claus; Bozec, Alexandra; Canuto, Vittorio M.; Cassou, Christophe; Chassignet, Eric; Coward, Andrew C.; Danilov, Sergey; Diansky, Nikolay; Drange, Helge; Farneti, Riccardo; Fernandez, Elodie; Fogli, Pier Giuseppe; Forget, Gael; Fujii, Yosuke; Griffies, Stephen M.; Gusev, Anatoly; Heimbach, Patrick; Howard, Armando; Ilicak, Mehmet; Jung, Thomas; Karspeck, Alicia R.; Kelley, Maxwell; Large, William G.; Leboissetier, Anthony; Lu, Jianhua; Madec, Gurvan; Marsland, Simon J.; Masina, Simona; Navarra, Antonio; Nurser, A. J. George; Pirani, Anna; Romanou, Anastasia; Salas y Mélia, David; Samuels, Bonita L.; Scheinert, Markus; Sidorenko, Dmitry; Sun, Shan; Treguier, Anne-Marie; Tsujino, Hiroyuki; Uotila, Petteri; Valcke, Sophie; Voldoire, Aurore; Wang, Qiang; Yashayaev, Igor

2016-01-01

Simulated inter-annual to decadal variability and trends in the North Atlantic for the 1958-2007 period from twenty global ocean - sea-ice coupled models are presented. These simulations are performed as contributions to the second phase of the Coordinated Ocean-ice Reference Experiments (CORE-II). The study is Part II of our companion paper (Danabasoglu et al., 2014) which documented the mean states in the North Atlantic from the same models. A major focus of the present study is the representation of Atlantic meridional overturning circulation (AMOC) variability in the participating models. Relationships between AMOC variability and those of some other related variables, such as subpolar mixed layer depths, the North Atlantic Oscillation (NAO), and the Labrador Sea upper-ocean hydrographic properties, are also investigated. In general, AMOC variability shows three distinct stages. During the first stage that lasts until the mid- to late-1970s, AMOC is relatively steady, remaining lower than its long-term (1958-2007) mean. Thereafter, AMOC intensifies with maximum transports achieved in the mid- to late-1990s. This enhancement is then followed by a weakening trend until the end of our integration period. This sequence of low frequency AMOC variability is consistent with previous studies. Regarding strengthening of AMOC between about the mid-1970s and the mid-1990s, our results support a previously identified variability mechanism where AMOC intensification is connected to increased deep water formation in the subpolar North Atlantic, driven by NAO-related surface fluxes. The simulations tend to show general agreement in their temporal representations of, for example, AMOC, sea surface temperature (SST), and subpolar mixed layer depth variabilities. In particular, the observed variability of the North Atlantic SSTs is captured well by all models. These findings indicate that simulated variability and trends are primarily dictated by the atmospheric datasets which include the influence of ocean dynamics from nature superimposed onto anthropogenic effects. Despite these general agreements, there are many differences among the model solutions, particularly in the spatial structures of variability patterns. For example, the location of the maximum AMOC variability differs among the models between Northern and Southern Hemispheres.

North Atlantic Simulations in Coordinated Ocean-Ice Reference Experiments Phase II (CORE-II) . Part II; Inter-Annual to Decadal Variability

NASA Technical Reports Server (NTRS)

Danabasoglu, Gokhan; Yeager, Steve G.; Kim, Who M.; Behrens, Erik; Bentsen, Mats; Bi, Daohua; Biastoch, Arne; Bleck, Rainer; Boening, Claus; Bozec, Alexandra;

The little ice age and medieval warm period in the Sargasso Sea

DOE Office of Scientific and Technical Information (OSTI.GOV)

Keigwin, L.D.

1996-11-29

Sea surface temperature (SST), salinity, and flux of terrigenous material oscillated on millennial time scales in the Pleistocene North Atlantic, but there are few records of Holocene variability. Because of high rates of sediment accumulation, Holocene oscillations are well documented in the northern Sargasso Sea. Results from a radiocarbondated box core show that SST was {approximately} 1{degree}C cooler than today {approximately} 400 years ago (the Little Ice Age) and 1700 years ago, and {approximately} 1{degree}C warmer than today 1000 years ago (the Medieval Warm Period). Thus, at least some of the warming since the Little Ice Age appears to bemore » part of a natural oscillation. 39 refs., 4 figs., 1 tab.« less

On the freshwater budget in the eastern tropical Atlantic during the development of the cold tongue

NASA Astrophysics Data System (ADS)

Schlundt, Michael; Krahmann, Gerd; Brandt, Peter; Karstensen, Johannes

2013-04-01

The most striking sea surface temperature (SST) phenomenon in the tropical Atlantic is the seasonal appearance of the Atlantic Cold Tongue (ACT). Onset, duration, spatial extent and strength of cooling are subject to significant interannual variability. The ACT onset is also associated with remarkable changes in upper ocean salinity. To examine the different contributions to these changes we here focus on and present a mixed layer freshwater budget in the eastern tropical Atlantic. Our investigation is based on an exceptionally large set of observations during the onset of the ACT in late boreal spring/ early boreal summer 2011: more than 5400 CTD-profiles acquired by seven gliders running simultaneously to two research cruises, 180 ship based CTD-profiles, time series data from the PIRATA buoy array as well as measurements from the Argo float program are used to derive mixed layer depth, lateral and vertical salinity gradients. To derive turbulent mixing and inferred diapycnal salt flux, microstructure observations are taken into account. Furthermore satellite measurements of sea surface salinity (SSS) by the SMOS mission and of SST by the TMI radiometer as well as atmospheric reanalysis data and the OSCAR project products are implemented. Freshwater budget terms were calculated for different sub-regions. These sub-regions are chosen using pre-defined thresholds in SSS, SST or mixed layer depth. Overall the freshwater budget is dominated by the net surface freshwater flux and horizontal advection by strong zonal currents. Other terms, like entrainment and diapycnal mixing are found to be regionally important. In particular, the observed increase in salinity in the near-equatorial region during ACT onset is found to be the result of the northward migration of the ITCZ associated with reduced net surface freshwater flux at the equator as well as mixing of salty subsurface waters into the surface mixed layer.

Suborbital timescale variability of North Atlantic Deep Water during the past 200,000 years

NASA Astrophysics Data System (ADS)

Oppo, Delia W.; Lehman, Scott J.

1995-10-01

We generated ˜200-kyr-long proxy records of surface and deepwater variability from a subpolar North Atlantic core (V29-202), enabling us to assess the linkage between surface and deepwater changes on suborbital timescales. In particular, we used a benthic δ13C record to evaluate the deep water response to Dansgaard-Oeschger temperature oscillations and to Heinrich events, times of massive iceberg delivery to the North Atlantic. We found that the reduction of North Atlantic Deep Water (NADW) production was generally associated with cold or dropping sea surface temperatures (SSTs) as indicated by planktonic foraminiferal assemblages. The NADW contribution to the site did not drop appreciably during Heinrich events H4 through H2, probably because these events followed intervals of prolonged surface cooling already characterized by low rates of NADW production. By contrast NADW reduction appears to have been synchronous with H5. SST rise associated with both Dansgaard-Oeschger oscillations and Heinrich events was usually accompanied by increasing NADW strength. In a few cases the NADW recovery appeared to lag the SST rise; however, the apparent delay is most likely an artefact of the sedimentary record (low concentrations of benthic foraminifera). As a result of low benthic foraminiferal abundances during stage 6, the stage 6 benthic foraminiferal δ13C record is of lower resolution than the younger part of the record. The stage 6 proxy records for surface hydrography nevertheless reveal millennial-scale oscillations similar to those seen in stage 3. The available δ13C data suggest that NADW weakened in association with the cold portions of stage 6 SST oscillations. We also sought to confirm a recent study which concluded that there was little NADW variability during the peak of the last interglaciation, marine oxygen isotope substage 5e (Eemian). Isotope stage 5 was marked by a trend of increasing benthic δ13C in V29-202. Rising δ13C through isotope stage 5 is also seen in tropical surface water records and at some deep Atlantic sites and may reflect the common derivation of these water masses. Variations of ≥ 0.5 ‰ superimposed on this rising δ13C trend within substage 5e in V29-202 are so far not evident in tropical feed waters and may therefore indicate that NADW production was weaker during the late than mid-Eemian. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type QUIT to leave the system.) (Paper 95PA02089, Suborbital timescale variability of North Atlantic Deep Water during the past 200,000 years, by D. W. Oppo and S. J. Lehman) Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

Positive low cloud and dust feedbacks amplify tropical North Atlantic Multidecadal Oscillation

DOE PAGES

Yuan, Tianle; Oreopoulos, Lazaros; Zelinka, Mark; ...

2016-02-04

The Atlantic Multidecadal Oscillation (AMO) is characterized by a horseshoe pattern of sea surface temperature (SST) anomalies and has a wide range of climatic impacts. While the tropical arm of AMO is responsible for many of these impacts, it is either too weak or completely absent in many climate model simulations. Here we show, using both observational and model evidence, that the radiative effect of positive low cloud and dust feedbacks is strong enough to generate the tropical arm of AMO, with the low cloud feedback more dominant. The feedbacks can be understood in a consistent dynamical framework: weakened tropicalmore » trade wind speed in response to a warm middle latitude SST anomaly reduces dust loading and low cloud fraction over the tropical Atlantic, which warms the tropical North Atlantic SST. Together they contribute to the appearance of the tropical arm of AMO. Most current climate models miss both the critical wind speed response and two positive feedbacks though realistic simulations of them may be essential for many climatic studies related to the AMO.« less

The Challenge of Simulating the Regional Climate over Florida

NASA Astrophysics Data System (ADS)

Misra, V.; Mishra, A. K.

2015-12-01

In this study we show that the unique geography of the peninsular Florida with close proximity to strong mesoscale surface ocean currents among other factors warrants the use of relatively high resolution climate models to project Florida's hydroclimate. In the absence of such high resolution climate models we highlight the deficiencies of two relatively coarse spatial resolution CMIP5 models with respect to the warm western boundary current of the Gulf Stream. As a consequence it affects the coastal SST and the land-ocean contrast, affecting the rainy summer seasonal precipitation accumulation over peninsular Florida. We also show this through two sensitivity studies conducted with a regional coupled ocean atmosphere model with different bathymetries that dislocate and modulate the strength of the Gulf Stream that locally affects the SST in the two simulations. These studies show that a stronger and more easterly displaced Gulf Stream produces warmer coastal SST's along the Atlantic coast of Florida that enhances the precipitation over peninsular Florida relative to the other regional climate model simulation. However the regional model simulations indicate that variability of wet season rainfall variability in peninsular Florida becomes less dependent on the land-ocean contrast with a stronger Gulf Stream current.

Increasing risk of Amazonian drought due to decreasing aerosol pollution.

PubMed

Cox, Peter M; Harris, Phil P; Huntingford, Chris; Betts, Richard A; Collins, Matthew; Jones, Chris D; Jupp, Tim E; Marengo, José A; Nobre, Carlos A

2008-05-08

The Amazon rainforest plays a crucial role in the climate system, helping to drive atmospheric circulations in the tropics by absorbing energy and recycling about half of the rainfall that falls on it. This region (Amazonia) is also estimated to contain about one-tenth of the total carbon stored in land ecosystems, and to account for one-tenth of global, net primary productivity. The resilience of the forest to the combined pressures of deforestation and global warming is therefore of great concern, especially as some general circulation models (GCMs) predict a severe drying of Amazonia in the twenty-first century. Here we analyse these climate projections with reference to the 2005 drought in western Amazonia, which was associated with unusually warm North Atlantic sea surface temperatures (SSTs). We show that reduction of dry-season (July-October) rainfall in western Amazonia correlates well with an index of the north-south SST gradient across the equatorial Atlantic (the 'Atlantic N-S gradient'). Our climate model is unusual among current GCMs in that it is able to reproduce this relationship and also the observed twentieth-century multidecadal variability in the Atlantic N-S gradient, provided that the effects of aerosols are included in the model. Simulations for the twenty-first century using the same model show a strong tendency for the SST conditions associated with the 2005 drought to become much more common, owing to continuing reductions in reflective aerosol pollution in the Northern Hemisphere.

Climatic teleconnections between the subtropical and polar North Atlantic during the Last Interglacial period (MIS5e)

NASA Astrophysics Data System (ADS)

Bauch, H. A.; Zhuravleva, A.

2017-12-01

Meridional gradients in sea surface temperature (SST) control ocean-atmosphere circulation patterns and, thus, regulate the global climate. Here we reconstruct variability of these gradients in the course of the Last Interglacial (MIS5e), by using sediment records from the low and high latitude North Atlantic which are linked via the Gulf Stream.In the Nordic Seas, i.e., at the northern end of the Gulf Stream extension, strong post-Saalian meltwater discharge reduced northward-directed transport of surface oceanic heat until the mid-MIS5e, resulting in a late and rather weak SST peak. To decipher the corresponding climatic changes in the area of the Gulf Stream origin, we employ stable isotopes data, planktic foraminifera assemblages as well as a new alkenone paleotemperature record from core drilled on the upper northern slope of the Little Bahama Bank. In addition, chemical composition of sediments (XRF data) was used to asses past sea level fluctuations and sedimentation regimes on this shallow-water carbonate bank. Significant variations in Sr/Ca ratios point to a two-fold structure of the Last Interglacial. Stabilized Sr/Ca values were reached only during the second phase of MIS5e, possibly representing the interval of maximum bank-top flooding after the northern hemisphere deglaciation terminated. Faunal-based proxies as well as oxygen isotopic gradients between surface and bottom-dwelling foraminifera corroborate existence of the two major climatic phases within the Last Interglacial, in agreement with the respective development in the polar region. This further suggests a strong climatic coupling between the subtropical and high-latitude North Atlantic with important implications for meridional SST gradients during the Last Interglacial.

Nonlinear responses of southern African rainfall to forcing from Atlantic SST in a high-resolution regional climate model

NASA Astrophysics Data System (ADS)

Williams, C.; Kniveton, D.; Layberry, R.

2009-04-01

It is increasingly accepted that any possible climate change will not only have an influence on mean climate but may also significantly alter climatic variability. A change in the distribution and magnitude of extreme rainfall events (associated with changing variability), such as droughts or flooding, may have a far greater impact on human and natural systems than a changing mean. This issue is of particular importance for environmentally vulnerable regions such as southern Africa. The subcontinent is considered especially vulnerable to and ill-equipped (in terms of adaptation) for extreme events, due to a number of factors including extensive poverty, famine, disease and political instability. Rainfall variability is a function of scale, so high spatial and temporal resolution data are preferred to identify extreme events and accurately predict future variability. In this research, high resolution satellite derived rainfall data from the Microwave Infra-Red Algorithm (MIRA) are used as a basis for undertaking model experiments using a state-of-the-art regional climate model. The MIRA dataset covers the period from 1993-2002 and the whole of southern Africa at a spatial resolution of 0.1 degree longitude/latitude. Once the model's ability to reproduce extremes has been assessed, idealised regions of sea surface temperature (SST) anomalies are used to force the model, with the overall aim of investigating the ways in which SST anomalies influence rainfall extremes over southern Africa. In this paper, results from sensitivity testing of the regional climate model's domain size are briefly presented, before a comparison of simulated daily rainfall from the model with the satellite-derived dataset. Secondly, simulations of current climate and rainfall extremes from the model are compared to the MIRA dataset at daily timescales. Finally, the results from the idealised SST experiments are presented, suggesting highly nonlinear associations between rainfall extremes remote SST anomalies.

The forcing of monthly precipitation variability over Southwest Asia during the Boreal cold season

USGS Publications Warehouse

Hoell, Andrew; Shukla, Shraddhanand; Barlow, Mathew; Cannon, Forest; Kelley, Colin; Funk, Christopher C.

2015-01-01

Southwest Asia, deemed as the region containing the countries of Afghanistan, Iran, Iraq and Pakistan, is water scarce and receives nearly 75% of its annual rainfall during8 the boreal cold season of November-April. The forcing of Southwest Asia precipitation has been previously examined for the entire boreal cold season from the perspective of climate variability originating over the Atlantic and tropical Indo-Pacific Oceans. Here, we examine the inter-monthly differences in precipitation variability over Southwest Asia and the atmospheric conditions directly responsible in forcing monthly November-April precipitation. Seasonally averaged November-April precipitation over Southwest Asia is significantly correlated with sea surface temperature (SST) patterns consistent with Pacific Decadal Variability (PDV), the El Nino-Southern Oscillation (ENSO) and the warming trend of SST (Trend). On the contrary, the precipitation variability during individual months of November-April are unrelated and are correlated with SST signatures that include PDV, ENSO and Trend in different combinations. Despite strong inter-monthly differences in precipitation variability during November- April over Southwest Asia, similar atmospheric circulations, highlighted by a stationary equivalent barotropic Rossby wave centered over Iraq, force the monthly spatial distributions of precipitation. Tropospheric waves on the eastern side of the equivalent barotropic Rossby wave modifies the flux of moisture and advects the mean temperature gradient, resulting in temperature advection that is balanced by vertical motions over Southwest Asia. The forcing of monthly Southwest Asia precipitation by equivalent barotropic Rossby waves is different than the forcing by baroclinic Rossby waves associated with tropically-forced-only modes of climate variability.

A Multi-Proxy Approach to Reconstruct Climate Variability in the Western Mediterranean across the Penultimate and Last Glacial Period

NASA Astrophysics Data System (ADS)

Dixit, Y.; Toucanne, S.; Bonnin, L.; Fontanier, C.; Jouet, G.; Tripati, A. K.

2016-12-01

The Mediterranean as a model miniature ocean is an ideal study area for the links between climate change and anoxia. Organic rich-sapropelic deposits punctuate Quaternary sediments series in the basin. These deposits reveal the occurrence of anoxic conditions during times when the circulation of the Mediterranean ocean was deeply perturbed. The `'Nilotic paradigm' proposes anoxia was a direct result of massive inputs of fresh water from the Nile. It is also possible that these sapropels could occur in response to periods of intense rainfall and riverine discharge on the northern Mediterranean coast. To resolve the sequence of events linked to sapropel deposition in the western Mediterranean, we use a multi-proxy (oxygen and carbon isotopes, benthic foraminifera assemblage and trace element geochemistry of foraminifera calcite) approach to examine sediments from the Tyrhennian Sea off the eastern Corsica margin in order to reconstruct climate variability during the penultimate glacial termination, and we compare results to those for the last glacial period. Our preliminary results show increased abundance of epifaunal and deep infaunal benthic species during MIS 5e ( 122-125 kyr BP), accompanied by a rise in Mg/Ca-based sea surface temperature (SST) using G. bulloides. A sharp decline in SST at 135 kyr BP coincides with Heinrich Stadial 5 in the North Atlantic. We will compare the timing of Mg/Ca-based SST minima and reconstructed water d18O variations to Heinrich Stadials in the North Atlantic in order to infer the mechanisms responsible for cooling in the Tyrrhenian Sea. This analysis should shed light on the proposed atmospheric teleconnection causing cooling of western Mediterranean waters via intensification of the Northern Hemisphere high-latitude wind systems.

Marine Climate Archives across the Medieval Climate Anomaly-Little Ice Age Transition from Viking and Medieval Age Shells, Orkney, Scotland

NASA Astrophysics Data System (ADS)

Surge, D. M.; Barrett, J. H.

2013-12-01

Proxy records reconstructing marine climatic conditions across the transition between the Medieval Climate Anomaly (MCA; ~900-1350 AD) and Little Ice Age (LIA; ~1350-1850) are strongly biased towards decadal to annual resolution and summer/growing seasons. Here we present new archives of seasonal variability in North Atlantic sea surface temperature (SST) from shells of the European limpet, Patella vulgata, which accumulated in Viking and medieval shell and fish middens at Quoygrew on Westray, Orkney. SST was reconstructed at submonthly resolution using oxygen isotope ratios preserved in shells from the 12th and mid 15th centuries (MCA and LIA, respectively). MCA shells recorded warmer summers and colder winters by ~2 degrees C relative to the late 20th Century (1961-1990). Therefore, seasonality was higher during the MCA relative to the late 20th century. Without the benefit of seasonal resolution, SST averaged from shell time series would be weighted toward the fast-growing summer season, resulting in the conclusion that the early MCA was warmer than the late 20th century by ~1°C. This conclusion is broadly true for the summer season, but not true for the winter season. Higher seasonality and cooler winters during early medieval times may result from a weakened North Atlantic Oscillation index. In contrast, the LIA shells have a more a variable inter-annual pattern. Some years record cooler summers and winters relative to the MCA shells and late 20th century, whereas other years record warmer summers and cooler winters similar to the MCA shells. Our findings provide a new test for the accuracy of seasonal amplitudes resulting from paleoclimate model experiments.

Data-Model Comparison of Pliocene Sea Surface Temperature

NASA Astrophysics Data System (ADS)

Dowsett, H. J.; Foley, K.; Robinson, M. M.; Bloemers, J. T.

2013-12-01

The mid-Piacenzian (late Pliocene) climate represents the most geologically recent interval of long-term average warmth and shares similarities with the climate projected for the end of the 21st century. As such, its fossil and sedimentary record represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. We present the first systematic comparison of Pliocene sea surface temperatures (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) and the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional (mid- to high latitude North Atlantic and tropics) and dynamic (upwelling) situations where there is discord between reconstructed SST and the PlioMIP simulations. These differences can lead to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction. Scatter plot of multi-model-mean anomalies (squares) and PRISM3 data anomalies (large blue circles) by latitude. Vertical bars on data anomalies represent the variability of warm climate phase within the time-slab at each locality. Small colored circles represent individual model anomalies and show the spread of model estimates about the multi-model-mean. While not directly comparable in terms of the development of the means nor the meaning of variability, this plot provides a first order comparison of the anomalies. Encircled areas are a, PRISM low latitude sites outside of upwelling areas; b, North Atlantic coastal sequences and Mediterranean sites; c, large anomaly PRISM sites from the northern hemisphere. Numbers identify Ocean Drilling Program sites.

Plio-Pleistocene Sea Surface Temperature Variability As Measured by Different Proxies - A Cautionary Tale

NASA Astrophysics Data System (ADS)

Lawrence, K. T.; Woodard, S. C.; Castañeda, I. S.; deMenocal, P. B.; Peterson, L.; Rosenthal, Y.; Bochner, L.; Gorbey, D. B.; Mauriello, H.

2016-12-01

Conflicting interpretations from the application of different sea surface temperature (SST) proxies seeking to characterize past climate conditions of the same region have given rise to a number of controversies about key elements of Pliocene climate. Thus, a detailed look at whether or not different temperature proxies yield consistent results is warranted. Here, we examine Pliocene climate variability at the orbital scale reporting new alkenone-derived SST estimates from ODP Site 1088 (South Atlantic) and ODP Site 846 (Eastern Equatorial Pacific). Using these novel datasets and previously published records from a variety of different sites in a variety of localities, we further examine the consistency of Plio-Pleistocene SST variability and orbital signatures from faunal, Mg/Ca, and TEX86 SST records relative to Uk'37 SST records. We find that many companion SST records produce very similar mean trends and standard deviations as well as absolute temperature estimates that are generally within error of each other. Our analysis also suggests that many companion records, with a few notable exceptions, capture the same dominant Milankovitch periodicities and produce phase estimates relative to benthic oxygen isotope estimates that are within error of each other. However, marked structural differences occur between different proxy records on glacial-interglacial timescales in Uk'37 versus Mg/Ca comparisons and some Uk'37 versus TEX86 comparisons. Therefore, the temperature estimates of individual glacial-interglacial cycles may vary significantly when a specific time slice is explored. Our preliminary investigation suggests that whether or not climate records derived from different paleothermometers yield consistent results depends on the timescale being explored and the study site, which reflects key factors such as seasonality, ecology, and diagenetic regime. Additional work that explores the underlying causes of the differences observed among proxies and uses a more systematic approach to directly compare the results from different paleothermometers is required. Until we have a better and broader sense of where/when proxies perform consistently, we recommend caution in treating SST records from different proxies as interchangeable.

An effective drift correction for dynamical downscaling of decadal global climate predictions

NASA Astrophysics Data System (ADS)

Paeth, Heiko; Li, Jingmin; Pollinger, Felix; Müller, Wolfgang A.; Pohlmann, Holger; Feldmann, Hendrik; Panitz, Hans-Jürgen

2018-04-01

Initialized decadal climate predictions with coupled climate models are often marked by substantial climate drifts that emanate from a mismatch between the climatology of the coupled model system and the data set used for initialization. While such drifts may be easily removed from the prediction system when analyzing individual variables, a major problem prevails for multivariate issues and, especially, when the output of the global prediction system shall be used for dynamical downscaling. In this study, we present a statistical approach to remove climate drifts in a multivariate context and demonstrate the effect of this drift correction on regional climate model simulations over the Euro-Atlantic sector. The statistical approach is based on an empirical orthogonal function (EOF) analysis adapted to a very large data matrix. The climate drift emerges as a dramatic cooling trend in North Atlantic sea surface temperatures (SSTs) and is captured by the leading EOF of the multivariate output from the global prediction system, accounting for 7.7% of total variability. The SST cooling pattern also imposes drifts in various atmospheric variables and levels. The removal of the first EOF effectuates the drift correction while retaining other components of intra-annual, inter-annual and decadal variability. In the regional climate model, the multivariate drift correction of the input data removes the cooling trends in most western European land regions and systematically reduces the discrepancy between the output of the regional climate model and observational data. In contrast, removing the drift only in the SST field from the global model has hardly any positive effect on the regional climate model.

Contributions of the atmosphere-land and ocean-sea ice model components to the tropical Atlantic SST bias in CESM1

NASA Astrophysics Data System (ADS)

Song, Z.; Lee, S. K.; Wang, C.; Kirtman, B. P.; Qiao, F.

2016-02-01

In order to identify and quantify intrinsic errors in the atmosphere-land and ocean-sea ice model components of the Community Earth System Model version 1 (CESM1) and their contributions to the tropical Atlantic sea surface temperature (SST) bias in CESM1, we propose a new method of diagnosis and apply it to a set of CESM1 simulations. Our analyses of the model simulations indicate that both the atmosphere-land and ocean-sea ice model components of CESM1 contain large errors in the tropical Atlantic. When the two model components are fully coupled, the intrinsic errors in the two components emerge quickly within a year with strong seasonality in their growth rates. In particular, the ocean-sea ice model contributes significantly in forcing the eastern equatorial Atlantic warm SST bias in early boreal summer. Further analysis shows that the upper thermocline water underneath the eastern equatorial Atlantic surface mixed layer is too warm in a stand-alone ocean-sea ice simulation of CESM1 forced with observed surface flux fields, suggesting that the mixed layer cooling associated with the entrainment of upper thermocline water is too weak in early boreal summer. Therefore, although we acknowledge the potential importance of the westerly wind bias in the western equatorial Atlantic and the low-level stratus cloud bias in the southeastern tropical Atlantic, both of which originate from the atmosphere-land model, we emphasize here that solving those problems in the atmosphere-land model alone does not resolve the equatorial Atlantic warm bias in CESM1.

Investigating the Uncertainty in Global SST Trends Due to Internal Variations Using an Improved Trend Estimator

NASA Astrophysics Data System (ADS)

Lian, Tao; Shen, Zheqi; Ying, Jun; Tang, Youmin; Li, Junde; Ling, Zheng

2018-03-01

A new criterion was proposed recently to measure the influence of internal variations on secular trends in a time series. When the magnitude of the trend is greater than a theoretical threshold that scales the influence from internal variations, the sign of the estimated trend can be interpreted as the underlying long-term change. Otherwise, the sign may depend on the period chosen. An improved least squares method is developed here to further reduce the theoretical threshold and is applied to eight sea surface temperature (SST) data sets covering the period 1881-2013 to investigate whether there are robust trends in global SSTs. It is found that the warming trends in the western boundary regions, the South Atlantic, and the tropical and southern-most Indian Ocean are robust. However, robust trends are not found in the North Pacific, the North Atlantic, or the South Indian Ocean. The globally averaged SST and Indian Ocean Dipole indices are found to have robustly increased, whereas trends in the zonal SST gradient across the equatorial Pacific, Niño 3.4 SST, and the Atlantic Multidecadal Oscillation indices are within the uncertainty range associated with internal variations. These results indicate that great care is required when interpreting SST trends using the available records in certain regions and indices. It is worth noting that the theoretical threshold can be strongly influenced by low-frequency oscillations, and the above conclusions are based on the assumption that trends are linear. Caution should be exercised when applying the theoretical threshold criterion to real data.

The Influence of Midlatitude Ocean-Atmosphere Coupling on the Low-Frequency Variability of a GCM. Part I: No Tropical SST Forcing*.

NASA Astrophysics Data System (ADS)

Bladé, Ileana

1997-08-01

This study examines the extent to which the thermodynamic interactions between the midlatitude atmosphere and the underlying oceanic mixed layer contribute to the low-frequency atmospheric variability. A general circulation model, run under perpetual northern winter conditions, is coupled to a motionless constant-depth mixed layer in midlatitudes, while elsewhere the sea surface temperature (SST) is kept fixed; interannual tropical SST forcing is not included. It is found that coupling does not modify the spatial organization of the variability. The influence of coupling is manifested as a slight reddening of the spectrum of 500-mb geopotential height and a significant enhancement of the lower-tropospheric thermal variance over the oceans at very low frequencies by virtue of the mixed-layer adjustment to surface air temperature variations that occurs on those timescales. This adjustment effectively reduces the thermal damping of the atmosphere associated with surface heat fluxes (or negative oceanic feedback), thus increasing the thermal variance and the persistence of circulation anomalies.In studying the covariability between ocean and atmosphere it is found that the dominant mode of natural atmospheric variability is coupled to the leading mode of SST in each ocean, with the atmosphere leading the ocean by about one month. The cross-correlation function between oceanic and atmospheric anomalies is strongly asymmetric about zero lag. The SST structures are consistent with direct forcing by the anomalous heat fluxes implied by the concurrent surface air temperature and wind fluctuations. Additionally, composites based on large amplitude SST anomaly events contain no evidence of direct driving of atmospheric perturbations by these SST anomalies. Thus, in terms of the spatial organization of the covariability and the evolution of the coupled system from one regime to another, large-scale air-sea interaction in the model is characterized by one-way atmospheric forcing of the mixed layer.These results are qualitatively consistent with those from an earlier idealized study. They imply a subtle but fundamental role for the midlatitude oceans as stabilizing rather than directly generating atmospheric anomalies. It is argued that this scenario is relevant to the dynamics of extratropical atmosphere-ocean coupling on intraseasonal timescales at least: the model is able to qualitatively reproduce the temporal and spatial characteristics of the observed dominant patterns of interaction on these timescales, particularly over the Atlantic.

The environmental context for the origins of modern human diversity: a synthesis of regional variability in African climate 150,000-30,000 years ago.

PubMed

Blome, Margaret Whiting; Cohen, Andrew S; Tryon, Christian A; Brooks, Alison S; Russell, Joellen

2012-05-01

We synthesize African paleoclimate from 150 to 30 ka (thousand years ago) using 85 diverse datasets at a regional scale, testing for coherence with North Atlantic glacial/interglacial phases and northern and southern hemisphere insolation cycles. Two major determinants of circum-African climate variability over this time period are supported by principal components analysis: North Atlantic sea surface temperature (SST) variations and local insolation maxima. North Atlantic SSTs correlated with the variability found in most circum-African SST records, whereas the variability of the majority of terrestrial temperature and precipitation records is explained by local insolation maxima, particularly at times when solar radiation was intense and highly variable (e.g., 150-75 ka). We demonstrate that climates varied with latitude, such that periods of relatively increased aridity or humidity were asynchronous across the northern, eastern, tropical and southern portions of Africa. Comparisons of the archaeological, fossil, or genetic records with generalized patterns of environmental change based solely on northern hemisphere glacial/interglacial cycles are therefore imprecise. We compare our refined climatic framework to a database of 64 radiometrically-dated paleoanthropological sites to test hypotheses of demographic response to climatic change among African hominin populations during the 150-30 ka interval. We argue that at a continental scale, population and climate changes were asynchronous and likely occurred under different regimes of climate forcing, creating alternating opportunities for migration into adjacent regions. Our results suggest little relation between large scale demographic and climate change in southern Africa during this time span, but strongly support the hypothesis of hominin occupation of the Sahara during discrete humid intervals ~135-115 ka and 105-75 ka. Hominin populations in equatorial and eastern Africa may have been buffered from the extremes of climate change by locally steep altitudinal and rainfall gradients and the complex and variable effects of increased aridity on human habitat suitability in the tropics. Our data are consistent with hominin migrations out of Africa through varying exit points from ~140-80 ka. Copyright © 2012 Elsevier Ltd. All rights reserved.

A synthesis of the environmental response of the North and South Atlantic Sub-Tropical Gyres during two decades of AMT

NASA Astrophysics Data System (ADS)

Aiken, Jim; Brewin, Robert J. W.; Dufois, Francois; Polimene, Luca; Hardman-Mountford, Nick J.; Jackson, Thomas; Loveday, Ben; Hoya, Silvana Mallor; Dall'Olmo, Giorgio; Stephens, John; Hirata, Takafumi

2017-11-01

Anthropogenically-induced global warming is expected to decrease primary productivity in the subtropical oceans by strengthening stratification of the water column and reducing the flux of nutrients from deep-waters to the sunlit surface layers. Identification of such changes is hindered by a paucity of long-term, spatially-resolved, biological time-series data at the basin scale. This paper exploits Atlantic Meridional Transect (AMT) data on physical and biogeochemical properties (1995-2014) in synergy with a wide range of remote-sensing (RS) observations from ocean colour, Sea Surface Temperature (SST), Sea Surface Salinity (SSS) and altimetry (surface currents), combined with different modelling approaches (both empirical and a coupled 1-D Ecosystem model), to produce a synthesis of the seasonal functioning of the North and South Atlantic Sub-Tropical Gyres (STGs), and assess their response to longer-term changes in climate. We explore definitive characteristics of the STGs using data of physical (SST, SSS and peripheral current systems) and biogeochemical variables (chlorophyll and nitrate), with inherent criteria (permanent thermal stratification and oligotrophy), and define the gyre boundary from a sharp gradient in these physical and biogeochemical properties. From RS data, the seasonal cycles for the period 1998-2012 show significant relationships between physical properties (SST and PAR) and gyre area. In contrast to expectations, the surface layer chlorophyll concentration from RS data (CHL) shows an upward trend for the mean values in both subtropical gyres. Furthermore, trends in physical properties (SST, PAR, gyre area) differ between the North and South STGs, suggesting the processes responsible for an upward trend in CHL may vary between gyres. There are significant anomalies in CHL and SST that are associated with El Niño events. These conclusions are drawn cautiously considering the short length of the time-series (1998-2012), emphasising the need to sustain spatially-extensive surveys such as AMT and integrate such observations with models, autonomous observations and RS data, to help address fundamental questions about how our planet is responding to climate change. A small number of dedicated AMT cruises in the keystone months of January and July would complement our understanding of seasonal cycles in the STGs.

Comparative Analysis of Upper Ocean Heat Content Variability from Ensemble Operational Ocean Analyses

NASA Technical Reports Server (NTRS)

Xue, Yan; Balmaseda, Magdalena A.; Boyer, Tim; Ferry, Nicolas; Good, Simon; Ishikawa, Ichiro; Rienecker, Michele; Rosati, Tony; Yin, Yonghong; Kumar, Arun

2012-01-01

Upper ocean heat content (HC) is one of the key indicators of climate variability on many time-scales extending from seasonal to interannual to long-term climate trends. For example, HC in the tropical Pacific provides information on thermocline anomalies that is critical for the longlead forecast skill of ENSO. Since HC variability is also associated with SST variability, a better understanding and monitoring of HC variability can help us understand and forecast SST variability associated with ENSO and other modes such as Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), Tropical Atlantic Variability (TAV) and Atlantic Multidecadal Oscillation (AMO). An accurate ocean initialization of HC anomalies in coupled climate models could also contribute to skill in decadal climate prediction. Errors, and/or uncertainties, in the estimation of HC variability can be affected by many factors including uncertainties in surface forcings, ocean model biases, and deficiencies in data assimilation schemes. Changes in observing systems can also leave an imprint on the estimated variability. The availability of multiple operational ocean analyses (ORA) that are routinely produced by operational and research centers around the world provides an opportunity to assess uncertainties in HC analyses, to help identify gaps in observing systems as they impact the quality of ORAs and therefore climate model forecasts. A comparison of ORAs also gives an opportunity to identify deficiencies in data assimilation schemes, and can be used as a basis for development of real-time multi-model ensemble HC monitoring products. The OceanObs09 Conference called for an intercomparison of ORAs and use of ORAs for global ocean monitoring. As a follow up, we intercompared HC variations from ten ORAs -- two objective analyses based on in-situ data only and eight model analyses based on ocean data assimilation systems. The mean, annual cycle, interannual variability and longterm trend of HC have been analyzed

Sea surface temperature predictions using a multi-ocean analysis ensemble scheme

NASA Astrophysics Data System (ADS)

Zhang, Ying; Zhu, Jieshun; Li, Zhongxian; Chen, Haishan; Zeng, Gang

2017-08-01

This study examined the global sea surface temperature (SST) predictions by a so-called multiple-ocean analysis ensemble (MAE) initialization method which was applied in the National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2). Different from most operational climate prediction practices which are initialized by a specific ocean analysis system, the MAE method is based on multiple ocean analyses. In the paper, the MAE method was first justified by analyzing the ocean temperature variability in four ocean analyses which all are/were applied for operational climate predictions either at the European Centre for Medium-range Weather Forecasts or at NCEP. It was found that these systems exhibit substantial uncertainties in estimating the ocean states, especially at the deep layers. Further, a set of MAE hindcasts was conducted based on the four ocean analyses with CFSv2, starting from each April during 1982-2007. The MAE hindcasts were verified against a subset of hindcasts from the NCEP CFS Reanalysis and Reforecast (CFSRR) Project. Comparisons suggested that MAE shows better SST predictions than CFSRR over most regions where ocean dynamics plays a vital role in SST evolutions, such as the El Niño and Atlantic Niño regions. Furthermore, significant improvements were also found in summer precipitation predictions over the equatorial eastern Pacific and Atlantic oceans, for which the local SST prediction improvements should be responsible. The prediction improvements by MAE imply a problem for most current climate predictions which are based on a specific ocean analysis system. That is, their predictions would drift towards states biased by errors inherent in their ocean initialization system, and thus have large prediction errors. In contrast, MAE arguably has an advantage by sampling such structural uncertainties, and could efficiently cancel these errors out in their predictions.

Global Sea Surface Temperature and Ecosystem Change Across the Mid-Miocene Climatic Optimum

NASA Astrophysics Data System (ADS)

Veenstra, T. J. T.; Bakker, V. B.; Sangiorgi, F.; Peterse, F.; Schouten, S.; Sluijs, A.

2016-12-01

Even though the term Mid-Miocene Climatic Optimum (MMCO; ca. 17 to 14 Ma) has been widely used in the literature since the early 1990's, almost no early-middle Miocene sea surface temperature (SST) proxy records have been published that support climate warming across its onset. Benthic (and diagenetically altered planktic) foram δ18O records show a decrease, suggesting (deep) ocean warming and/or Antarctic ice sheet melting. However, reliable absolute SST proxy records are absent from the tropics and very scarce in temperate and polar regions. This leaves the question if the warmth of the MMCO was truly global and how its onset relates to the widely recorded positive (Monterey) carbon isotope excursion and volcanism. Finally, it remains uncertain how marine ecosystems responded to this hypothesized warming. We present organic biomarker SST proxy records (Uk'37 and TEX86) spanning the MMCO for several locations in the Atlantic and Pacific Ocean along a pole-to-pole transect, including Ocean Drilling Program Site 959 in the eastern Tropical Atlantic, ODP Site 643 in the Norwegian Sea, ODP Site 1007 on the Great Bahama Bank and Integrated Ocean Drilling Program Site U1352 off New Zealand. Additionally, we use marine palynology (mostly dinoflagellate cysts) to assess ecosystem change at these locations. The resulting spatial reconstruction of SST change shows that Middle Miocene warming was global. Nevertheless, the records also show distinct regional variability, including relatively large warming in the Norwegian Sea and a damped signal in the southern hemisphere, suggesting pronounced changes in ocean circulation. The onset of the MMCO was marked by prominent changes in ecological and depositional setting at the studied sites, likely also related to ocean circulation changes.

Combined Effects of the North Atlantic Oscillation and the Arctic Oscillation on Sea Surface Temperature in the Alborán Sea

PubMed Central

Báez, José C.; Gimeno, Luis; Gómez-Gesteira, Moncho; Ferri-Yáñez, Francisco; Real, Raimundo

2013-01-01

We explored the possible effects of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) on interannual sea surface temperature (SST) variations in the Alborán Sea, both separately and combined. The probability of observing mean annual SST values higher than average was related to NAO and AO values of the previous year. The effect of NAO on SST was negative, while that of AO was positive. The pure effects of NAO and AO on SST are obscuring each other, due to the positive correlation between them. When decomposing SST, NAO and AO in seasonal values, we found that variation in mean annual SST and mean winter SST was significantly related to the mean autumn NAO of the previous year, while mean summer SST was related to mean autumn AO of the previous year. The one year delay in the effect of the NAO and AO on the SST could be partially related to the amount of accumulated snow, as we found a significant correlation between the total snow in the North Alborán watershed for a year with the annual average SST of the subsequent year. A positive AO implies a colder atmosphere in the Polar Regions, which could favour occasional cold waves over the Iberian Peninsula which, when coupled with precipitations favoured by a negative NAO, may result in snow precipitation. This snow may be accumulated in the high peaks and melt down in spring-summer of the following year, which consequently increases the runoff of freshwater to the sea, which in turn causes a diminution of sea surface salinity and density, and blocks the local upwelling of colder water, resulting in a higher SST. PMID:23638005

The Tropical Western Hemisphere Warm Pool

NASA Astrophysics Data System (ADS)

Wang, Chunzai; Enfield, David B.

The Western Hemisphere warm pool (WHWP) of water warmer than 28.5°C extends from the eastern North Pacific to the Gulf of Mexico and the Caribbean, and at its peak, overlaps with the tropical North Atlantic. It has a large seasonal cycle and its interannual fluctuations of area and intensity are significant. Surface heat fluxes warm the WHWP through the boreal spring to an annual maximum of SST and areal extent in the late summer/early fall, associated with eastern North Pacific and Atlantic hurricane activities and rainfall from northern South America to the southern tier of the United States. SST and area anomalies occur at high temperatures where small changes can have a large impact on tropical convection. Observations suggest that a positive ocean-atmosphere feedback operating through longwave radiation and associated cloudiness is responsible for the WHWP SST anomalies. Associated with an increase in SST anomalies is a decrease in atmospheric sea level pressure anomalies and an anomalous increase in atmospheric convection and cloudiness. The increase in convective activity and cloudiness results in less longwave radiation loss from the surface, which then reinforces SST anomalies.

Causes and Consequences of Exceptional North Atlantic Heat Loss in Recent Winters

NASA Astrophysics Data System (ADS)

Josey, Simon; Grist, Jeremy; Duchez, Aurelie; Frajka-Williams, Eleanor; Hirschi, Joel; Marsh, Robert; Sinha, Bablu

2016-04-01

The mid-high latitude North Atlantic loses large amounts of heat to the atmosphere in winter leading to dense water formation. An examination of reanalysis datasets (ERA-Interim, NCEP/NCAR) reveals that heat loss in the recent winters 2013-14 and 2014-15 was exceptionally strong. The causes and consequences of this extraordinary ocean heat loss will be discussed. In 2013-2014, the net air-sea heat flux anomaly averaged over the whole winter exceeded 100 Wm-2 in the eastern subpolar gyre (the most extreme in the period since 1979 spanned by ERA-Interim). The causes of this extreme heat loss will be shown to be severe latent and sensible heat fluxes driven primarily by anomalously strong westerly airflows from North America and northerly airflows originating in the Nordic Seas. The associated sea level pressure anomaly field reflects the dominance of the second mode of atmospheric variability, the East Atlantic Pattern (EAP) over the North Atlantic Oscillation (NAO) in this winter. The extreme winter heat loss had a significant impact on the ocean extending from the sea surface into the deeper layers and a re-emergent cold Sea Surface Temperature (SST) anomaly is evident in November 2014. The following winter 2014-15 experienced further extreme heat loss that served to amplify the strength of the re-emergent SST anomaly. By summer 2015, an unprecedented cold mid-latitude North Atlantic Ocean surface temperature anomaly is evident in observations and has been widely referred to as the 'big blue blob'. The role played by the extreme surface heat loss in the preceding winters in generating this feature and it subsequent evolution through winter 2015-16 will be explored.

Changing surface water conditions for the last 500 ka in the Southeast Atlantic: Implications for variable influences of Agulhas leakage and Benguela upwelling

NASA Astrophysics Data System (ADS)

Petrick, Benjamin F.; McClymont, Erin L.; Marret, Fabienne; van der Meer, Marcel T. J.

2015-09-01

The Southeast Atlantic Ocean is an important component of global ocean circulation, as it includes heat and salt transfer into the Atlantic through the Agulhas leakage as well as the highly productive Benguela upwelling system. Here we reconstruct sea surface temperatures (SSTs) from Ocean Drilling Program (ODP) Site 1087 in the Southeast Atlantic to investigate surface ocean circulation patterns during the late Pleistocene (0-500 ka). The UK'37 index and dinoflagellate cyst assemblages are used to reconstruct SSTs, δDalkenone is used to reconstruct changes in sea surface salinity, and mass accumulation rates of alkenones and chlorine pigments are quantified to detect changing marine export productivity. The greatest amplitude of SST warming precedes decreases in benthic δ18O and therefore occurs early in the transition from glacials to interglacials. The δDalkenone, as a salinity indicator, increases before SSTs, suggesting that the pattern of Agulhas leakage is more complex than suggested by SST proxies. Marine isotope stage (MIS) 10 shows an anomalous pattern: it is marked by a pronounced increase in chlorine concentration, which may be related to enhanced/expanded Benguela upwelling reaching the core site. We find no evidence of an absence of Agulhas leakage throughout the record, suggesting that there is no Agulhas cutoff even during MIS 10. Finally, the ODP Site 1087 record shows an increasing strength of Agulhas leakage towards the present day, which may have impacted the intensity of the Atlantic meridional overturning circulation. As a result, the new analyses from ODP Site 1087 demonstrate a complex interaction between influences of the Benguela upwelling and the Agulhas leakage through the late Pleistocene, which are inferred here to reflect changing circulation patterns in the Southern Ocean and in the atmosphere.

Interannual and intra-annual variability of rainfall in Haiti (1905-2005)

NASA Astrophysics Data System (ADS)

Moron, Vincent; Frelat, Romain; Jean-Jeune, Pierre Karly; Gaucherel, Cédric

2015-08-01

The interannual variability of annual and monthly rainfall in Haiti is examined from a database of 78 rain gauges in 1905-2005. The spatial coherence of annual rainfall is rather low, which is partly due to Haiti's rugged landscape, complex shoreline, and surrounding warm waters (mean sea surface temperatures >27 °C from May to December). The interannual variation of monthly rainfall is mostly shaped by the intensity of the low-level winds across the Caribbean Sea, leading to a drier- (or wetter-) than-average rainy season associated with easterly (or westerly) anomalies, increasing (or decreasing) winds. The varying speed of low-level easterlies across the Caribbean basin may reflect at least four different processes during the year: (1) an anomalous trough/ridge over the western edge of the Azores high from December to February, peaking in January; (2) a zonal pressure gradient between Eastern Pacific and the tropical Northern Atlantic from May/June to September, with a peak in August (i.e. lower-than-average rainfall in Haiti is associated with positive sea level pressure anomalies over the tropical North Atlantic and negative sea level pressure anomalies over the Eastern Pacific); (3) a local ocean-atmosphere coupling between the speed of the Caribbean Low Level Jet and the meridional sea surface temperature (SST) gradient across the Caribbean basin (i.e. colder-than-average SST in the southern Caribbean sea is associated with increased easterlies and below-average rainfall in Haiti). This coupling is triggered when the warmest Caribbean waters move northward toward the Gulf of Mexico; (4) in October/November, a drier- (or wetter-) than-usual rainy season is related to an almost closed anticyclonic (or cyclonic) anomaly located ENE of Haiti on the SW edge of the Azores high. This suggests a main control of the interannual variations of rainfall by intensity, track and/or recurrence of tropical depressions traveling northeast of Haiti. During this period, the teleconnection of Haitian rainfall with synchronous Atlantic and Eastern Pacific SST is at a minimum.

An Assessment of the Potential Predictability of Drought Over the United States Based on Climate Model Simulations with Specified SST

NASA Technical Reports Server (NTRS)

Schubert, Siegfried; Wang, Hailan; Suarez, Max; Koster, Randal

2010-01-01

The USCLIV AR working group on drought recently initiated a series of global climate model simulations forced with idealized SST anomaly patterns, designed to address a number of uncertainties regarding the impact of SST forcing and the role of land-atmosphere feedbacks on regional drought. The runs were done with several global atmospheric models including NASA/NSIPP-l, NCEP/GFS, GFDLlAM2, and NCAR CCM3 and CAM3.5. Specific questions that the runs are designed to address include: What are mechanisms that maintain drought across the seasonal cycle and from one year to the next. To what extent can droughts develop independently of ocean variability due to year-to-year memory that may be inherent to the land. What is the role of the different ocean basins? Here we focus on the potential predictability of drought conditions over the United States. Specific issues addressed include the seasonality and regionality of the signal-to-noise ratios associated with Pacific and Atlantic SST forcing, and the sensitivity of the results to the climatological stationary waves simulated by the different AGCMs.

1400 yr multiproxy record of climate variability from the northern Gulf of Mexico

USGS Publications Warehouse

Richey, J.N.; Poore, R.Z.; Flower, B.P.; Quinn, T.M.

2007-01-01

A continuous decadal-scale resolution record of climate variability over the past 1400 yr in the northern Gulf of Mexico was constructed from a box core recovered in the Pigmy Basin, northern Gulf of Mexico. Proxies include paired analyses of Mg/Ca and δ18O in the white variety of the planktic foraminifer Globigerinoides ruber and relative abundance variations of G. sacculifer in the foraminifer assemblages. Two multi-decadal intervals of sustained high Mg/Ca indicate that Gulf of Mexico sea surface temperatures (SSTs) were as warm or warmer than near-modern conditions between 1000 and 1400 yr B.P. Foraminiferal Mg/Ca during the coolest interval of the Little Ice Age (ca. 250 yr B.P.) indicate that SST was 2–2.5 °C below modern SST. Four minima in the Mg/Ca record between 900 and 250 yr B.P. correspond with the Maunder, Spörer, Wolf, and Oort sunspot minima, suggesting a link between changes in solar insolation and SST variability in the Gulf of Mexico. An abrupt shift recorded in both δ18Ocalcite and relative abundance of G. sacculifer occurred ca. 600 yr B.P. The shift in the Pigmy Basin record corresponds with a shift in the sea-salt-sodium (ssNa) record from the Greenland Ice Sheet Project 2 ice core, linking changes in high-latitude atmospheric circulation with the subtropical Atlantic Ocean.

Atlantic Multidecadal Oscillation footprint on global high cloud cover

NASA Astrophysics Data System (ADS)

Vaideanu, Petru; Dima, Mihai; Voiculescu, Mirela

2017-12-01

Due to the complexity of the physical processes responsible for cloud formation and to the relatively short satellite database of continuous data records, cloud behavior in a warming climate remains uncertain. Identifying physical links between climate modes and clouds would contribute not only to a better understanding of the physical processes governing their formation and dynamics, but also to an improved representation of the clouds in climate models. Here, we identify the global footprint of the Atlantic Multidecadal Oscillation (AMO) on high cloud cover, with focus on the tropical and North Atlantic, tropical Pacific and on the circum-Antarctic sector. In the tropical band, the sea surface temperature (SST) and high cloud cover (HCC) anomalies are positively correlated, indicating a dominant role played by convection in mediating the influence of the AMO-related SST anomalies on the HCC field. The negative SST-HCC correlation observed in North Atlantic could be explained by the reduced meridional temperature gradient induced by the AMO positive phase, which would be reflected in less storms and negative HCC anomalies. A similar negative SST-HCC correlation is observed around Antarctica. The corresponding negative correlation around Antarctica could be generated dynamically, as a response to the intensified upward motion in the Ferrel cell. Despite the inherent imperfection of the observed and reanalysis data sets, the AMO footprint on HCC is found to be robust to the choice of dataset, statistical method, and specific time period considered.

Ocean Spectral Data Assimilation Without Background Error Covariance Matrix

DTIC Science & Technology

2016-01-01

float data (Chu et al. 2007 ), and 97 temporal and spatial variability of the global upper ocean heat content (Chu 2011) from the data 98 of the Global...Melnichenko OV, Wells NC ( 2007 ) Long baroclinic Rossby waves in the 558 tropical North Atlantic observed from profiling floats. J Geophys Res...Hall, J, Harrison D.E. and Stammer , D., Eds., ESA Publication WPP-610 306. 611 612 Tang Y, Kleeman R (2004) SST assimilation experiments in a

Provisionally corrected surface wind data, worldwide ocean-atmosphere surface fields, and Sahelian rainfall variability

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ward, M.N.

Worldwide ship datasets of sea surface temperature (SST), sea level pressure (SLP), and surface vector wind are analyzed for a July-September composite of five Sahelian wet years (1950, 1952, 1953, 1954, 1958) minus five Sahelian dry years (1972, 1973, 1982, 1983, 1984) (W - D). The results are compared with fields for a number of individual years and for 1988 minus 1987 (88 - 87); Sahelian rainfall in 1988 was near the 1951-80 normal, whereas 1987 was very dry. An extensive study of the geostrophic consistency of trends in pressure gradients and observed wind was undertaken. The results suggest, duringmore » the period 1949-88, a mean increase in reported wind speed of about 16% that cannot be explained by trends in geostrophic winds derived from seasonal mean SLP. Estimates of the wind bias are averaged for 18 ocean regions. A map of correlations between Sahelian rainfall and SLP in all available ocean regions is shown to be field significant. Remote atmospheric associations with Sahelian rainfall are consistent with recent suggestions that SST forcing from the tropical Atlantic and the other ocean basins may contribute to variability in seasonal Sahelian rainfall. It is suggested that wetter years in the Sahel are often accompanied by a stronger surface monsoonal flow over the western Indian Ocean and low SLP in the tropical western Pacific near New Guinea, and that there is increased cyclonicity over the extratropical eastern North Atlantic and northwest Europe. In the tropical Atlantic, W - D shows many of the features identified by previous authors. However, the 88-87 fields do not reflect these large-scale tropical Atlantic changes. Instead there is only local strengthening of the pressure gradient and wind flow from Brazil to Senegal. Further individual years are presented (1958, 1972, 1975) to provide specific examples.« less

Changes in Holocene meridional circulation and poleward Atlantic flow: the Bay of Biscay as a nodal point

NASA Astrophysics Data System (ADS)

Mary, Yannick; Eynaud, Frédérique; Colin, Christophe; Rossignol, Linda; Brocheray, Sandra; Mojtahid, Meryem; Garcia, Jennifer; Peral, Marion; Howa, Hélène; Zaragosi, Sébastien; Cremer, Michel

2017-03-01

This paper documents the evolution over the last 10 kyr of one of the key parameters of climate: sea-surface temperatures (SSTs) in the North Atlantic. We focus on the southern Bay of Biscay, a highly sensitive oceanographic area regarding the dynamics of the North Atlantic subpolar and subtropical gyres (SPG and STG respectively). This site furthermore offers unique sedimentary environments characterized by exceptional accumulation rates, enabling the study of Holocene archives at (infra)centennial scales. Our results mainly derive from planktonic foraminiferal association analysis on two cores from the southern Landes Plateau. These associations are used as the basis of modern analogue technique transfer functions to track past hydrographical changes. SST reconstructions were thus obtained at an exceptional resolution and compared to a compilation of Holocene records from the northeastern North Atlantic. From this regional perspective are shown fundamental timing differences between the gyre dynamics, nuancing classical views of a simple meridional overturning cell. Our study highlights that western Europe underwent significant oscillations of (annual) SST during the last 10 kyr. During well-known intervals of mild boreal climate, warm shifts of more than 3 °C per century are accurately concomitant with positive sea-surface temperature anomalies and rise of micropalaeontological indicators of gyre dynamics in the northern North Atlantic, pointing to periods of greater intensity of the North Atlantic Current (SPG cell especially). Conversely, the SST signal records short-term cold anomalies which could be related to weaker SPG dynamics.

The Impact of the Atlantic Cold Tongue on West African Monsoon Onset in Regional Model Simulations for 1998-2002

NASA Technical Reports Server (NTRS)

Druyan, Leonard M.; Fulakeza, Matthew B.

2014-01-01

The Atlantic cold tongue (ACT) develops during spring and early summer near the Equator in the Eastern Atlantic Ocean and Gulf of Guinea. The hypothesis that the ACT accelerates the timing of West African monsoon (WAM) onset is tested by comparing two regional climate model (RM3) simulation ensembles. Observed sea surface temperatures (SST) that include the ACT are used to force a control ensemble. An idealized, warm SST perturbation is designed to represent lower boundary forcing without the ACT for the experiment ensemble. Summer simulations forced by observed SST and reanalysis boundary conditions for each of five consecutive years are compared to five parallel runs forced by SST with the warm perturbation. The article summarizes the sequence of events leading to the onset of the WAM in the Sahel region. The representation of WAM onset in RM3 simulations is examined and compared to Tropical Rainfall Measuring Mission (TRMM), Global Precipitation Climatology Project (GPCP) and reanalysis data. The study evaluates the sensitivity of WAM onset indicators to the presence of the ACT by analysing the differences between the two simulation ensembles. Results show that the timing of major rainfall events and therefore theWAM onset in the Sahel are not sensitive to the presence of the ACT. However, the warm SST perturbation does increase downstream rainfall rates over West Africa as a consequence of enhanced specific humidity and enhanced northward moisture flux in the lower troposphere.

Seasonality in the tropical Atlantic: an 800-year record of seasonally-representative Mg/Ca data from the Cariaco Basin

NASA Astrophysics Data System (ADS)

Black, D. E.; Rahman, S.; Wurtzel, J.; Thunell, R.; Mauer, B.; Tappa, E. J.

2009-12-01

The Cariaco Basin, Venezuela is well-positioned to record a detailed history of surface ocean changes along the southern margin of the Caribbean and the tropical Atlantic. Varved, high deposition rate sediments deposited under anoxic conditions and an abundance of well-preserved microfossils result in one of the few marine records capable of preserving evidence of interannual- to decadal-scale climate variability in the tropical Atlantic. Boreal winter/spring sea surface temperatures (SST) spanning the last eight centuries have previously been reconstructed using Mg/Ca measurements on the planktic foraminifer Globigerina bulloides. Here we present the complementary record using Globigerinoides ruber (pink), a summer/fall indicator. Globigerinoides ruber Mg/Ca values are generally greater than those of G. bulloides from the same sample, reflecting warmer calcification temperatures. Both species’ records display similar long-term trends, yet there are some distinctive differences. The Medieval Warm Period (MWP) and Little Ice Age (LIA) as distinctly separate climate events are more apparent in the G. ruber record than that of G. bulloides. Additionally, greater variability in the G. ruber data may indicate a stronger than expected bias from productivity during the local upwelling season. As G. bulloides and pink G. ruber are thought to be winter/spring and summer/fall SST indicators, respectively (albeit with the potential upwelling season bias), the intersample differences between the two records can potentially be interpreted as a record of seasonality. Our seasonality reconstruction shows a distinctive oscillation of 4 °C with a period of approximately 200 years. The proxy seasonality is slightly less than what has been instrumentally measured (5 to 6 °C) over the last 15 years, and does not appear related to or affected by the MWP or LIA events.

On the Role of SST Forcing in the 2011 and 2012 Extreme U.S. Heat and Drought: A Study in Contrasts

NASA Technical Reports Server (NTRS)

Wang, Hailan; Schubert, Siegfried; Koster, Randal; Ham, Yoo-Geun; Suarez, Max

2013-01-01

This study compares the extreme heat and drought that developed over the United States in 2011 and 2012 with a focus on the role of SST forcing. Experiments with the NASA GEOS-5 atmospheric general circulation model show that the winter/spring response over the U.S. to the Pacific SST is remarkably similar for the two years despite substantial differences in the tropical Pacific SST. As such, the pronounced winter and early spring temperature differences between the two years (warmth confined to the south in 2011 and covering much of the continent in 2012) primarily reflect differences in the contributions from the Atlantic and Indian Oceans, with both acting to cool the east and upper mid-west during 2011, while during 2012 the Indian Ocean reinforced the Pacific-driven continental-wide warming and the Atlantic played a less important role. During late spring and summer of 2011 the tropical Pacific SST force a continued warming and drying over the southern U.S., though considerably weaker than observed. Nevertheless, the observed anomalies fall within the models intra-ensemble spread. In contrast, the rapid development of intense heat and drying over the central U.S. during June and July of 2012 falls outside the models intra-ensemble spread. The response to the SST (a northward expansion of a modest summer warming linked to the Atlantic) gives little indication that 2012 would produce record-breaking precipitation deficits and heat in the central Great Plains. A diagnosis of the 2012 observed circulation anomalies shows that the most extreme heat and drought was tied to the development of a stationary Rossby wave and an associated anomalous upper tropospheric high maintained by weather transients.

Temperature and food-mediated variability of European Atlantic sardine recruitment

NASA Astrophysics Data System (ADS)

Garrido, Susana; Silva, Alexandra; Marques, Vitor; Figueiredo, Ivone; Bryère, Philippe; Mangin, Antoine; Santos, A. Miguel P.

2017-12-01

The influence of the environmental conditions during larval development on the resulting recruitment strength was investigated for European sardine (Sardina pilchardus) at Atlanto-Iberian waters. Satellite-derived Sea Surface Temperature (SST) and Chlorophyll-a concentration (Chla) data from the previous spawning seasons (January to March/April and October to December of the previous year) were related to recruitment success data in the main recruitment hotspots. Recruitment data was taken from yearly acoustic scientific cruises and from the ICES recruitment index estimated by an age-structured model for the entire stock. A linear discriminant analysis model using SST, Chla, and the abundance of spawners during the spawning season identified years of high and low recruitment for all the recruitment hotspots with an accuracy of ≥79%. In general, high recruitment years were associated with high Chla and low SST, although the most important variables to discriminate between the groups were area-specific. High recruitment years were mostly related to high food availability (Chla), particularly during the last quarter of the previous year. In Western Iberia and in the Gulf of Cadiz, high recruitment years were also associated to lower SST, whereas in the Bay of Biscay, where SST during the winter was generally below the optimal range ≈11-12 °C for sardine larval development, higher recruitment was associated with high SST. For ICES data of the southern European sardine stock, lower SST and higher Chla during the last quarter of the previous year were associated with high recruitment years and SST alone was able to discriminate between the two recruitment groups with 73% accuracy. Although the time-series of available data are still small, these significant relationships are consistent with field and laboratory studies relating larval growth and mortality with main environmental drivers. These relationships should be further investigated in the following years to evaluate if they can be used to construct reliable indicators to predict the level of recruitment and abundance with sufficient advance to help in the management of this important fishing resource.

A statistical model for Windstorm Variability over the British Isles based on Large-scale Atmospheric and Oceanic Mechanisms

NASA Astrophysics Data System (ADS)

Kirchner-Bossi, Nicolas; Befort, Daniel J.; Wild, Simon B.; Ulbrich, Uwe; Leckebusch, Gregor C.

2016-04-01

Time-clustered winter storms are responsible for a majority of the wind-induced losses in Europe. Over last years, different atmospheric and oceanic large-scale mechanisms as the North Atlantic Oscillation (NAO) or the Meridional Overturning Circulation (MOC) have been proven to drive some significant portion of the windstorm variability over Europe. In this work we systematically investigate the influence of different large-scale natural variability modes: more than 20 indices related to those mechanisms with proven or potential influence on the windstorm frequency variability over Europe - mostly SST- or pressure-based - are derived by means of ECMWF ERA-20C reanalysis during the last century (1902-2009), and compared to the windstorm variability for the European winter (DJF). Windstorms are defined and tracked as in Leckebusch et al. (2008). The derived indices are then employed to develop a statistical procedure including a stepwise Multiple Linear Regression (MLR) and an Artificial Neural Network (ANN), aiming to hindcast the inter-annual (DJF) regional windstorm frequency variability in a case study for the British Isles. This case study reveals 13 indices with a statistically significant coupling with seasonal windstorm counts. The Scandinavian Pattern (SCA) showed the strongest correlation (0.61), followed by the NAO (0.48) and the Polar/Eurasia Pattern (0.46). The obtained indices (standard-normalised) are selected as predictors for a windstorm variability hindcast model applied for the British Isles. First, a stepwise linear regression is performed, to identify which mechanisms can explain windstorm variability best. Finally, the indices retained by the stepwise regression are used to develop a multlayer perceptron-based ANN that hindcasted seasonal windstorm frequency and clustering. Eight indices (SCA, NAO, EA, PDO, W.NAtl.SST, AMO (unsmoothed), EA/WR and Trop.N.Atl SST) are retained by the stepwise regression. Among them, SCA showed the highest linear coefficient, followed by SST in western Atlantic, AMO and NAO. The explanatory regression model (considering all time steps) provided a Coefficient of Determination (R^2) of 0.75. A predictive version of the linear model applying a leave-one-out cross-validation (LOOCV) shows an R2 of 0.56 and a relative RMSE of 4.67 counts/season. An ANN-based nonlinear hindcast model for the seasonal windstorm frequency is developed with the aim to improve the stepwise hindcast ability and thus better predict a time-clustered season over the case study. A 7 node-hidden layer perceptron is set, and the LOOCV procedure reveals a R2 of 0.71. In comparison to the stepwise MLR the RMSE is reduced a 20%. This work shows that for the British Isles case study, most of the interannual variability can be explained by certain large-scale mechanisms, considering also nonlinear effects (ANN). This allows to discern a time-clustered season from a non-clustered one - a key issue for applications e.g., in the (re)insurance industry.

Decadal Changes in the World's Coastal Latitudinal Temperature Gradients

PubMed Central

Baumann, Hannes; Doherty, Owen

2013-01-01

Most of the world's living marine resources inhabit coastal environments, where average thermal conditions change predictably with latitude. These coastal latitudinal temperature gradients (CLTG) coincide with important ecological clines,e.g., in marine species diversity or adaptive genetic variations, but how tightly thermal and ecological gradients are linked remains unclear. A first step is to consistently characterize the world's CLTGs. We extracted coastal cells from a global 1°×1° dataset of weekly sea surface temperatures (SST, 1982–2012) to quantify spatial and temporal variability of the world's 11 major CLTGs. Gradient strength, i.e., the slope of the linear mean-SST/latitude relationship, varied 3-fold between the steepest (North-American Atlantic and Asian Pacific gradients: −0.91°C and −0.68°C lat−1, respectively) and weakest CLTGs (African Indian Ocean and the South- and North-American Pacific gradients: −0.28, −0.29, −0.32°C lat−1, respectively). Analyzing CLTG strength by year revealed that seven gradients have weakened by 3–10% over the past three decades due to increased warming at high compared to low latitudes. Almost the entire South-American Pacific gradient (6–47°S), however, has considerably cooled over the study period (−0.3 to −1.7°C, 31 years), and the substantial weakening of the North-American Atlantic gradient (−10%) was due to warming at high latitudes (42–60°N, +0.8 to +1.6°C,31 years) and significant mid-latitude cooling (Florida to Cape Hatteras 26–35°N, −0.5 to −2.2°C, 31 years). Average SST trends rarely resulted from uniform shifts throughout the year; instead individual seasonal warming or cooling patterns elicited the observed changes in annual means. This is consistent with our finding of increased seasonality (i.e., summer-winter SST amplitude) in three quarters of all coastal cells (331 of 433). Our study highlights the regionally variable footprint of global climate change, while emphasizing ecological implications of changing CLTGs, which are likely driving observed spatial and temporal clines in coastal marine life. PMID:23825672

Potential predictability of a Colombian river flow

NASA Astrophysics Data System (ADS)

Córdoba-Machado, Samir; Palomino-Lemus, Reiner; Quishpe-Vásquez, César; García-Valdecasas-Ojeda, Matilde; Raquel Gámiz-Fortis, Sonia; Castro-Díez, Yolanda; Jesús Esteban-Parra, María

2017-04-01

In this study the predictability of an important Colombian river (Cauca) has been analysed based on the use of climatic variables as potential predictors. Cauca River is considered one of the most important rivers of Colombia because its basin supports important productive activities related with the agriculture, such as the production of coffee or sugar. Potential relationships between the Cauca River seasonal streamflow anomalies and different climatic variables such as sea surface temperature (SST), precipitation (Pt), temperature over land (Tm) and soil water (Sw) have been analysed for the period 1949-2009. For this end, moving correlation analysis of 30 years have been carried out for lags from one to four seasons for the global SST, and from one to two seasons for South America Pt, Tm and Sw. Also, the stability of the significant correlations have been also studied, identifying the regions used as potential predictors of streamflow. Finally, in order to establish a prediction scheme based on the previous stable correlations, a Principal Component Analysis (PCA) applied on the potential predictor regions has been carried out in order to obtain a representative time series for each predictor field. Significant and stable correlations between the seasonal streamflow and the tropical Pacific SST (El Niño region) are found for lags from one to four (one-year) season. Additionally, some regions in the Indian and Atlantic Oceans also show significant and stable correlations at different lags, highlighting the importance that exerts the Atlantic SST on the hydrology of Colombia. Also significant and stable correlations are found with the Pt, Tm and Sw for some regions over South America, at lags of one and two seasons. The prediction of Cauca seasonal streamflow based on this scheme shows an acceptable skill and represents a relative improvement compared with the predictability obtained using the teleconnection indices associated with El Niño. Keywords: Streamflow, predictability, Cauca, Colombia. Acknowledgements: This work has been financed by the projects P11-RNM-7941 (Junta de Andalucía-Spain) and CGL2013-48539-R (MINECO-Spain, FEDER).

SST Variation Due to Interactive Convective-Radiative Processes

NASA Technical Reports Server (NTRS)

Tao, W.-K.; Shie, C.-L.; Johnson, D.; Simpson, J.; Li, X.; Sui, C.-H.

2000-01-01

The recent linking of Cloud-Resolving Models (CRMs) to Ocean-Mixed Layer (OML) models has provided a powerful new means of quantifying the role of cloud systems in ocean-atmosphere coupling. This is due to the fact that the CRM can better resolve clouds and cloud systems and allow for explicit cloud-radiation interaction. For example, Anderson (1997) applied an atmospheric forcing associated with a CRM simulated squall line to a 3-D OML model (one way or passive interaction). His results suggested that the spatial variability resulting from the squall forcing can last at least 24 hours when forced with otherwise spatially uniform fluxes. In addition, the sea surface salinity (SSS) variability continuously decreased following the forcing, while some of the SST variability remained when a diurnal mixed layer capped off the surface structure. The forcing used in the OML model, however, focused on shorter time (8 h) and smaller spatial scales (100-120 km). In this study, the 3-D Goddard Cumulus Ensemble Model (GCE; 512 x 512 x 23 cu km, 2-km horizontal resolution) is used to simulate convective active episodes occurring in the Western Pacific warm pool and Eastern Atlantic regions. The model is integrated for seven days, and the simulated results are coupled to an OML model to better understand the impact of precipitation and changes in the planetary boundary layer upon SST variation. We will specifically examine and compare the results of linking the OML model with various spatially-averaged outputs from GCE simulations (i.e., 2 km vs. 10-50 km horizontal resolutions), in order to help understand the SST sensitivity to multi-scale influences. This will allow us to assess the importance of explicitly simulated deep and shallow clouds, as well as the subgrid-scale effects (in coarse-model runs) upon SST variation. Results using both 1-D and 2-D OML models will be evaluated to assess the effects of horizontal advection.

Variability of CO2 fugacity at the western edge of the tropical Atlantic Ocean from the 8°N to 38°W PIRATA buoy

NASA Astrophysics Data System (ADS)

Bruto, Leonardo; Araujo, Moacyr; Noriega, Carlos; Veleda, Dóris; Lefèvre, Nathalie

2017-06-01

Hourly data of CO2 fugacity (fCO2) at 8°N-38°W were analyzed from 2008 to 2011. Analyses of wind, rainfall, temperature and salinity data from the buoy indicated two distinct seasonal periods. The first period (January to July) had a mean fCO2 of 378.9 μatm (n = 7512). During this period, in which the study area was characterized by small salinity variations, the fCO2 is mainly controlled by sea surface temperature (SST) variations (fCO2 = 24.4*SST-281.1, r2 = 0.8). During the second period (August-December), the mean fCO2 was 421.9 μatm (n = 11571). During these months, the region is subjected to the simultaneous action of (a) rainfall induced by the presence of the Intertropical Convergence Zone (ITCZ); (b) arrival of fresh water from the Amazon River plume that is transported to the east by the North Equatorial Countercurrent (NECC) after the retroflection of the North Brazil Current (NBC); and (c) vertical input of CO2-rich water due to Ekman pumping. The data indicated the existence of high-frequency fCO2 variability (periods less than 24 h). This high variability is related to two different mechanisms. In the first mechanism, fCO2 increases are associated to rapid increases in SST and are attributed to the diurnal cycle of solar radiation. In addition, low wind speed contributes to SST rising by inhibiting vertical mixing. In the second mechanism, fCO2 decreases are associated to SSS decreases caused by heavy rainfall.

Variability in Bias of Gridded Sea Surface Temperature Data Products: Implications for Seasonally Resolved Marine Proxy Reconstructions

NASA Astrophysics Data System (ADS)

Ouellette, G., Jr.; DeLong, K. L.

2016-12-01

Seasonally resolved reconstructions of sea surface temperature (SST) are commonly produced using isotopic ratios and trace elemental ratios within the skeletal material of marine organisms such as corals, coralline algae, and mollusks. Using these geochemical proxies to produce paleoclimate reconstructions requires using regression methods to calibrate the proxy to observed SST, ideally with in situ SST records that span many years. Unfortunately, the few locations with in situ SST records rarely coincide with the time span of the marine proxy archive. Therefore, SST data products are often used for calibration and they are based on MOHSST or ICOADS SST observations as their main SST source but use different algorithms to produce globally gridded data products. These products include the Hadley Center's HADSST (5º) and interpolated HADISST (1º), NOAA's extended reconstructed SST (ERSST; 2º), optimum interpolation SST (OISST; 1º), and the Kaplan SST (5º). This study assessed the potential bias in these data products at marine archive sites throughout the tropical Atlantic using in situ SST where it was available, and a high-resolution (4 km) satellite-based SST data product from NOAA Pathfinder that has been shown to closely reflect in situ SST for our locations. Bias was assessed at each site, and then within each data product across the region for spatial homogeneity. Our results reveal seasonal biases in all data products, but not for all locations and not of a uniform magnitude or season among products. We found the largest differences in mean SST on the order of 1-3°C for single sites in the Gulf of Mexico, and differences for regional mean SST bias were 0.5-1°C when sites in the Gulf of Mexico were compared to sites in the Caribbean Sea within the same data product. No one SST data product outperformed the others and no systematic bias was found. This analysis illustrates regional strengths and weaknesses of these data products, and serves as a cautionary note against the wholesale use of a particular gridded data product for marine proxy calibration, whether for a single site or larger regional reconstruction, without considering the inherent heterogeneous bias present in each data product that we show varies among locations. Furthermore, this study has implications for comparing climate models and these SST data products.

Bay of Bengal Exhibits Warming Trend During the Younger Dryas: Implications of AMOC

NASA Astrophysics Data System (ADS)

Panmei, Champoungam; Divakar Naidu, Pothuri; Mohtadi, Mahyar

2017-12-01

A sharp decline in temperature during the Younger Dryas (YD) preceding the current warmer Holocene is well documented in climate archives from the Northern Hemisphere high latitudes. Although the magnitude of YD cooling varied spatially, the response of YD cooling was well documented in the Atlantic and Pacific Oceans but not in the Indian Ocean. Here we investigate whether the modern remote forcing of tropical Indian Ocean sea surface temperature (SST) by Northern Hemisphere climate changes holds true for events such as the YD. Our SST reconstruction from the western Bay of Bengal exhibits an overall warming of ˜1.8°C during the YD. We further compared our data with other existing Mg/Ca-based SST records from the Northern Indian Ocean and found no significant negative SST anomalies in both the Arabian Sea and the Bay of Bengal compared to pre- and post-YD, suggesting that no apparent cooling occurred during the YD in the Northern Indian Ocean. In contrast, most part of the YD exhibits positive SST anomalies in the Northern Indian Ocean that coincide with the slowdown of the Atlantic Meridional Overturning Circulation during this period.

Equatorial Pacific forcing of western Amazonian precipitation during Heinrich Stadial 1.

PubMed

Zhang, Yancheng; Zhang, Xu; Chiessi, Cristiano M; Mulitza, Stefan; Zhang, Xiao; Lohmann, Gerrit; Prange, Matthias; Behling, Hermann; Zabel, Matthias; Govin, Aline; Sawakuchi, André O; Cruz, Francisco W; Wefer, Gerold

2016-10-25

Abundant hydroclimatic evidence from western Amazonia and the adjacent Andes documents wet conditions during Heinrich Stadial 1 (HS1, 18-15 ka), a cold period in the high latitudes of the North Atlantic. This precipitation anomaly was attributed to a strengthening of the South American summer monsoon due to a change in the Atlantic interhemispheric sea surface temperature (SST) gradient. However, the physical viability of this mechanism has never been rigorously tested. We address this issue by combining a thorough compilation of tropical South American paleorecords and a set of atmosphere model sensitivity experiments. Our results show that the Atlantic SST variations alone, although leading to dry conditions in northern South America and wet conditions in northeastern Brazil, cannot produce increased precipitation over western Amazonia and the adjacent Andes during HS1. Instead, an eastern equatorial Pacific SST increase (i.e., 0.5-1.5 °C), in response to the slowdown of the Atlantic Meridional Overturning Circulation during HS1, is crucial to generate the wet conditions in these regions. The mechanism works via anomalous low sea level pressure over the eastern equatorial Pacific, which promotes a regional easterly low-level wind anomaly and moisture recycling from central Amazonia towards the Andes.

Amazon Basin climate under global warming: the role of the sea surface temperature.

PubMed

Harris, Phil P; Huntingford, Chris; Cox, Peter M

2008-05-27

The Hadley Centre coupled climate-carbon cycle model (HadCM3LC) predicts loss of the Amazon rainforest in response to future anthropogenic greenhouse gas emissions. In this study, the atmospheric component of HadCM3LC is used to assess the role of simulated changes in mid-twenty-first century sea surface temperature (SST) in Amazon Basin climate change. When the full HadCM3LC SST anomalies (SSTAs) are used, the atmosphere model reproduces the Amazon Basin climate change exhibited by HadCM3LC, including much of the reduction in Amazon Basin rainfall. This rainfall change is shown to be the combined effect of SSTAs in both the tropical Atlantic and the Pacific, with roughly equal contributions from each basin. The greatest rainfall reduction occurs from May to October, outside of the mature South American monsoon (SAM) season. This dry season response is the combined effect of a more rapid warming of the tropical North Atlantic relative to the south, and warm SSTAs in the tropical east Pacific. Conversely, a weak enhancement of mature SAM season rainfall in response to Atlantic SST change is suppressed by the atmospheric response to Pacific SST. This net wet season response is sufficient to prevent dry season soil moisture deficits from being recharged through the SAM season, leading to a perennial soil moisture reduction and an associated 30% reduction in annual Amazon Basin net primary productivity (NPP). A further 23% NPP reduction occurs in response to a 3.5 degrees C warmer air temperature associated with a global mean SST warming.

Modeling SST gradient changes, the hydrological cycle response, and deep water formation in the North Pacific

NASA Astrophysics Data System (ADS)

Burls, N.; Ford, H. L.; Fedorov, A. V.; Jahn, A.; Jacobs, P.

2017-12-01

The absence of deep-water formation and a deep meridional overturning cell in the modern North Pacific has been attributed to the relatively fresh surface conditions in the subarctic. These conditions are, in turn, best explained by the local excess of precipitation over evaporation in the northern Pacific due to net moisture transport from the Atlantic to the Pacific and/or moisture transport associated with the Asian monsoon. Some studies link the lack of deep-water formation in the Pacific directly to its occurrence in the Atlantic via the Atlantic-Pacific seesaw effect and idealized experiments indicate that the smaller width of the Atlantic predisposes it to higher salinity and deep-water formation. We have conducted a series of coupled model experiments across which global mean temperatures and large-scale meridional SST gradients are varied. We perturb either atmospheric CO2 concentrations or the meridional gradient in cloud radiative forcing and run each experiment out to 3000 years so that the deep ocean has equilibrated. As the strength of the meridional temperature gradient decreases across our experiments, a Pacific Meridional Overturning Circulation develops. The strength of this Pacific Meridional Overturning Circulation generally increases as the gradient weakens. In one of these experiments where the meridional SST gradient most closely resembles Pliocene reconstructions, a PMOC exists of comparable in strength to the modern AMOC. We will describe how the hydrological cycle response to reduced meridional SST gradients acts to increase the strength of the PMOC across our sensitivity experiments. Additionally, we will discuss our effort to include carbon isotopes in our Pliocene-like simulation for data-model comparisons. Calcium carbonate accumulation data from Subarctic North Pacific Site 882 and new and previously published carbon isotope records from the Pacific appear to support our modelling results suggesting that weaker meridonal SST gradients during the Pliocene could have supported deep water formation in the subarctic Pacific and a strong PMOC.

Regionally coherent Little Ice Age cooling in the Atlantic Warm Pool

USGS Publications Warehouse

Richey, J.N.; Poore, R.Z.; Flower, B.P.; Quinn, T.M.; Hollander, D.J.

2009-01-01

We present 2 new decadal-resolution foraminiferal Mg/Ca-SST records covering the past 6-8 centuries from the northern Gulf of Mexico (GOM). These records provide evidence for a Little Ice Age (LIA) cooling of 2??C, consistent with a published Mg/Ca record from Pigmy Basin. Comparison of these 3 records with existing SST proxy records from the GOM-Caribbean region show that the magnitude of LIA cooling in the Atlantic Warm Pool (AWP) was significantly larger than the mean hemispheric cooling of <1??C. We propose that a reduction in the intensity and spatial extent of the AWP during the LIA, combined with associated changes in atmospheric circulation may account for the regional SST patterns observed in the GOM-Caribbean region during the LIA. Copyright 2009 by the American Geophysical Union.

Simulations of Western North American Hydroclimate during the Little Ice Age and Medieval Climate Anomaly

NASA Astrophysics Data System (ADS)

Simon, S. M.; Mann, M. E.; Steinman, B. A.; Feng, S.; Zhang, Y.; Miller, S. K.

2013-12-01

Despite the immense impact that large, modern North American droughts, such as those of the 1930s and 1950s, have had on economic, social, aquacultural, and agricultural systems, they are smaller in duration and magnitude than the multidecadal megadroughts that affected North America, in particular the western United States, during the Medieval Climate Anomaly (MCA, ~ 900-1300 AD) and the Little Age (LIA, ~1450-1850 AD). Although various proxy records have been used to reconstruct the timing of these MCA and LIA megadroughts in the western United States, there still exists great uncertainty in the magnitude and spatial coherence of such droughts in the Pacific Northwest region, especially on decadal to centennial timescales. This uncertainty motivates the following study to establish a causal link between the climate forcing that induced these megadroughts and the spatiotemporal response of regional North American hydroclimates to this forcing. This study seeks to establish a better understanding of the influence of tropical Pacific and North Atlantic SSTs on North American drought during the MCA and LIA. We force NCAR's Community Atmospheric Model version 5.1.1 (CAM 5) with prescribed proxy-reconstructed tropical Pacific and North Atlantic SST anomalies from the MCA and LIA, in order to investigate the influence that these SST anomalies had on the spatiotemporal patterns of drought in North America. To isolate the effects of individual ocean basin SSTs on the North American climate system, the model experiments use a variety of SST permutations in the tropical Pacific and North Atlantic basin as external forcing. In order to quantify the spatiotemporal response of the North American climate system to these SST forcing permutations, temperature and precipitation data derived from the MCA and LIA model experiments are compared to lake sediment isotope and tree ring-based hydroclimate reconstructions from the Pacific Northwest. The spatiotemporal temperature and precipitation patterns from the model experiments indicate that in the Pacific Northwest, the MCA and LIA were anomalously wet and dry periods, respectively, a finding that is largely supported by the lake sediment records. This pattern contrasts with the dry MCA/wet LIA pattern diagnosed in model experiments for the U.S Southwest and indicated by tree ring-based proxy data. Thus, the CAM 5 model experiments confirm the wet/dry dipole pattern suggested by proxy data for the western U.S. during the MCA and LIA and highlights the role that the natural variability of tropical Pacific and North Atlantic SSTs played in driving this spatiotemporal climate pattern and its related teleconnections.

A sensitivity study of the coupled simulation of the Northeast Brazil rainfall variability

NASA Astrophysics Data System (ADS)

Misra, Vasubandhu

2007-06-01

Two long-term coupled ocean-land-atmosphere simulations with slightly different parameterization of the diagnostic shallow inversion clouds in the atmospheric general circulation model (AGCM) of the Center for Ocean-Land-Atmosphere Studies (COLA) coupled climate model are compared for their annual cycle and interannual variability of the northeast Brazil (NEB) rainfall variability. It is seen that the solar insolation affected by the changes to the shallow inversion clouds results in large scale changes to the gradients of the SST and the surface pressure. The latter in turn modulates the surface convergence and the associated Atlantic ITCZ precipitation and the NEB annual rainfall variability. In contrast, the differences in the NEB interannual rainfall variability between the two coupled simulations is attributed to their different remote ENSO forcing.

Influence of SST from Pacific and Atlantic Ocean and atmospheric circulation in the precipitation regime of basin from Brazilian SIN

NASA Astrophysics Data System (ADS)

Custodio, M. D.; Ramos, C. G.; Madeira, P.; de Macedo, A. L.

2013-12-01

The South American climate presents tropical, subtropical and extratropical features because of its territorial extension, being influenced by a variety of dynamical systems with different spatial and temporal scales which result in different climatic regimes in their subregions. Furthermore, the precipitation regime in South America is influenced by low-frequency phenomena as El Niño-Southern Oscillation (ENSO), the Atlantic dipole and the Madden Julian Oscilation (MJO), in other words, is directly influenced by variations of the Sea Surface Temperature (SST). Due to the importance of the precipitation for many sectors including the planning of productive activities, such as agriculture, livestock and hydropower energy, many studies about climate variations in Brazil have tried to determine and explain the mechanisms that affect the precipitation regime. However, because of complexity of the climate system, and consequently of their impacts on the global precipitation regime, its interactions are not totally understood and therefore misrepresented in numerical models used to forecast climate. The precipitation pattern over hydrographic basin which form the Brasilian National Interconnected System (Sistema Interligado Nacional-SIN) are not yet known and therefore the climate forecast of these regions still presents considerable failure that need to be corrected due to its economic importance. In this context, the purpose here is to determine the precipitation patterns on the Brazilian SIN, based on SST and circulation observed data. In a second phase a forecast climate model for these regions will be produced. In this first moment 30 years (1983 to 2012) of SST over Pacific and Atlantic Ocean were analyzed, along with wind in 850 and 200 hPa and precipitation observed data. The precipitation patterns were analyzed through statistical analyses for interannual (ENSO) and intraseasonal (MJO) anomalies for these variables over the SIN basin. Subsequently, these precipitation patterns will be used for the development of a statistical model for climate prediction for each of these regions, with which it is expected an improvement of up to 20% of climate prediction in these basins. In this first stage was evident a high correlation between precipitation in the basins of SIN and SST Pacific anomalies over the region of Niños, as well as on the coast of Chile and Peru. The effect of SST anomalies in the Niños region on precipitation in the South America is already known, however its quantification was not yet well understood. The coast of Chile determines the positioning and movement of cold fronts directly affecting rainfall in southern and southeastern of Brazil, then the correlation and rain pattern indicate the parameters for the climate prediction model. The anomalies over the Atlantic ocean present high correlation with the precipitation in North and Northeast of Brazil, as well as its connection with the Pacific anomalies. This quantification generated climatic parameters for predictions for these regions. The relationship between the canonical ENSO events and precipitation regime on the basins were also quantified which represents a high degree of assertiveness in predicting climate of these regions.

An Assessment of the SST Simulation Using the Climate Forecast System Coupled to the SSiB Surface Model

NASA Astrophysics Data System (ADS)

Wang, Y.; Xue, Y.; Huang, B.; Lee, J.; De Sales, F.

2016-12-01

A long term simulation has been conducted using the Climate Forecast System (CFSv2) coupled to the SSiB-2 land model, which consists of the Global Forecast System atmospheric model (GFS) and the Modular Ocean model - version 4 (MOM4) as the ocean component. This study evaluates the model's performance in simulating sea surface temperature (SST) mean state, trend, and inter-annual and decadal variabilities. The model is able to produce the reasonable spatial distribution of the SST climatology; however, it has prominent large scale biases. In the middle latitude of the Northern Hemisphere, major cold biases is close to the warm side of the large SST gradients, which may be associated with the weaker Kuroshio and Gulf Stream extensions that diffuse the SST gradient. IN addition, warm biases extend along the west coast of the North America continent to the high latitude, which may be related with excessive Ekman down-welling and solar radiation fluxes reaching to the surface due to the lack of cloud there. Warm biases also exist over the tropical cold tough areas in the Pacific and Atlantic. The global SST trend and interannual variations are well captured except for that in the south Hemisphere after year 2000, which is mainly contributed by the bias from the southern Pacific Ocean. Although the model fails to accurately produce ENSO events in proper years, it does reproduce the ENSO frequency well; they are skewed toward more warm events after 1990. The model also shows ability in SST decadal variation, such as the so-called inter-decadal Pacific oscillation (IPO); however, its phases seem to go reversely compared with the observation.

A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather

NASA Astrophysics Data System (ADS)

Otomi, Yuriko; Tachibana, Yoshihiro; Nakamura, Tetsu

2013-04-01

In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the "memory" of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.

A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme hot summer

NASA Astrophysics Data System (ADS)

Tachibana, Yoshihiro; Otomi, Yuriko; Nakamura, Tetsu

2013-04-01

In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the "memory" of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.

Investigating the use of paired Uk'37 and TEX86 measurements to reconstruct past sea surface and subsurface (thermocline) temperatures (Invited)

NASA Astrophysics Data System (ADS)

Castaneda, I. S.; Urann, B.; Phu, V.

2013-12-01

Two organic geochemical temperature proxies widely applied to marine sediments are the Uk'37 Index, based on long-chain alkenones produced by haptophyte algae, and TEX86, based on isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), produced by Thaumarchaeota. At some locations, temperature reconstructions based on Uk'37 and TEX86 are in agreement within the calibration errors of each proxy, while at other sites absolute Uk'37 and TEX86 reconstructed temperatures differ but both proxies reveal similar overall trends (e.g. Caley et al., 2011). In contrast, at other locations Uk'37 and TEX86 temperature reconstructions from the same samples yield dramatically different overall trends. Differences observed between Uk'37 and TEX86 temperature reconstructions have been attributed to a variety of factors including seasonal production biases, differences in preservation and lateral transport, and differences related to the depth habitat of the source organisms. An increasing number of studies have provided evidence that TEX86 likely reflects a subsurface water temperature in certain areas of the world's oceans and have used paired Uk'37 and TEX86 measurements to simultaneously examine sea surface and subsurface (in some cases thermocline) temperature variability (e.g. Lopes dos Santos et al., 2010; Rommerskirchen et al., 2011; Li et al., 2013). In the tropical N Atlantic, a distinctive signature of Atlantic Meridional Overturning Circulation (AMOC) slowdown is anticorrelated variation between surface and subsurface water temperatures (e.g. Chang et al., 2008; Zhang et al., 2007) where sea surface temperature (SST) cooling is accompanied by shallow subsurface warming (e.g. Chang et al., 2008). Lopes dos Santos et al. (2010) examined a site in the tropical NE Atlantic where they showed that in the modern Uk'37 reflects SST while TEXH86 likely reflects a thermocline temperature. The authors noted several periods during the past 200 kyr when surface cooling and subsurface warming occurred, which they attributed to AMOC slowdown. In this study, we examine sediments from ODP site 660 (NE Atlantic), located near the site studied by Lopes dos Santos et al. (2010), and use paired Uk'37-TEXH86 temperature measurements to investigate changes in sea surface and thermocline temperature variability over the past 4 Ma. We find that following Pliocene warmth, the Uk'37 record indicates an overall cooling trend since ~2.2 Ma, superimposed on glacial-interglacial temperature fluctuations. In contrast, the TEX86 record, which yields consistently cooler temperatures in comparison to Uk'37, does not exhibit an overall cooling trend during the Pleistocene nor elevated warmth during the Pliocene. In portions of the record, anticorrelated variability between Uk'37 and TEX86 temperatures is observed, likely reflecting differences in SST and thermocline temperatures related to AMOC variability. In addition, we examine the carbon (δ13C) and deuterium (δD) isotopic composition of plant leaf waxes, proxies for vegetation type (C3 vs. C4) and precipitation amount, respectively, as several studies have demonstrated close ties between AMOC variability and hydrological conditions in N Africa during the late Pleistocene and Holocene.

Sclerochronological study of a dog cockle (Glycymeris glycymeris L.) population from the Madeira Islands

NASA Astrophysics Data System (ADS)

Németh, Alexandra; Kern, Zoltán

2017-04-01

The use of the annual increments in the shell of the abundant dog cockle (Glycymeris glycymeris L.) in sclerochronological studies has been demonstrated to be a valid method to reconstruct oceanographic variability in the North Atlantic region [1,2]. However, the sclerochronological potential of the species has not been investigated at the southern limit of its distribution, at the southern North East Atlantic. Therefore the first aim of our study was to analyse growth patterns of G. glycymeris specimens collected around the Madeira Islands to understand which environmental variable has the strongest limiting effect on their growth. The second aim was to find out whether G. glycymeris could be used to build chronologies for sea surface temperature (SST) reconstruction for the region. In 2013 a group of dead shells were collected near the Desertas Islands, Madeira, at 80-300 m water depth range, together with two living specimens. The live collected shells were relatively young (<37 years) while the dead shells proved to have much higher ontogenetic ages (up to 164 years). All groups showed similar ontogenetic trends, however, the average growth rate in the shallower environment was higher in the first four years of their life compared to the deeper collection sites. Subsets of the Madeira (n=18) samples could be collected into a robust chronology: between 1950 and 2000 the value of the expressed population signal EPS was found to be higher than 0.8. In this time interval their composite chronology exhibited negative correlation (r=-0.6, p<0.1.) with the averaged February-May SST fields around Madeira. At the same time, it correlated positively (r>0.8) with the averaged February satellite derived Chlorophyll (Chl) concentrations in the region, although this data was available only between 1998 and 2012. Our results suggest that the relationship between shell growth of G. glycymeris and SST is complex and not direct. In contrast with the northern populations the low water temperature was not a limiting environmental factor for the shells. Chl concentration of the surface water around the Madeira Islands usually exhibits a maximum in February or March (spring phytoplankton bloom) when vertical mixing is the most intensive and SST is the lowest during the year. This could explain the negative correlation between the shell growth and SST. The vertical mixing, timing and the intensity of the blooms are controlled by regional atmospheric patterns, such as the North Atlantic Oscillation (NAO) [3]. [1] Brocas, W.M., Reynolds, D.J., Butler, P.G., Richardson, C.A., Scourse, J.D., Ridgway, I.D., Ramsay, K., 2013. The dog cockle, Glycymeris glycymeris (L.), a new annually resolved sclerochronological archive for the Irish Sea. Paleogeogr. Paleoclimatol. Palaeoecol. 373, 133-140. [2] Reynolds, D.J., Butler, P.G., Williams, S.M., Scourse, J.D., Richardson, C.A., Wanamaker, A.D., Austin, W.E.N., Cage, A.G., 2013. A multiproxy reconstruction of Hebridean (NW Scotland) spring sea surface temperatures between AD 1805 and 2010. Paleogeogr. Paleoclimatol. Paleoecol. 386, 275-285. [3] Bashmachnikov, I., Belonenko, T.V., Koldunov, A.V., 2013 Intra-annual and interannual non-stationary cycles of chlorophyll concentration in the Northeast Atlantic. Remote Sens. of Environ. 137, 55-68.

Seasonality in the Western Mediterranean During the Last Glacial From Paired Oxygen Isotopes and Mg/Ca in Limpet Shells

NASA Astrophysics Data System (ADS)

Ferguson, J. E.; Henderson, G. M.; Fa, D.; Finlayson, C.

2008-12-01

Molluscs have shown great potential to act as seasonal-resolution archives of sea-surface temperatures (SST) at mid to high latitudes, outside the range of tropical surface corals. Seasonal resolution climate records from higher latitudes are important to allow investigation of the role of seasonality in controlling mean climate on diverse timescales, and of the evolution of climate systems such as the North Atlantic Oscillation. Long sequences of intertidal mollusc shells are difficult to find due to sea level fluctuations over glacial- interglacial periods. This study makes use of Patella shells collected by Neanderthals and humans and transported inland to caves on Gibraltar over at least the last 120 kyrs. Some 30 fossil Patella shells were selected from several hundred excavated from Gorham's and Vanguard Caves at Gibraltar. Oxygen isotope analysis of micromilled samples of modern Patella shells from the Gibraltar coastline demonstrate that the shells accurately record absolute SSTs and capture more than 80% of the full seasonal range. Analysis of fossil Patella shells, dated using 14C, provides records of the change in absolute SST and seasonality during the last glacial. Paired Mg/Ca ratios of micromilled samples in modern Patella shells follow a consistent positive relationship with SST providing an independent paleothermometer, analogous with coral Sr/Ca. Applying this Mg/Ca-SST relationship to fossil Patella shells allows the independent reconstruction of the absolute values and range of SSTs and the reconstruction of seawater δ18O for the western Mediterranean. Results show a cooling of glacial summer SSTs from 36 kyr BP to the LGM with maximum cooling of glacial summer SSTs of 7.5 °C relative to modern. In contrast, winter SSTs show greater variability on millennial timescales with a maximum cooling of up to 10 °C. SST seasonality is therefore extended due to greater winter cooling but SST seasonality is highly variable as a result of large fluctuation in the extent of winter cooling. These results contrast with GCM model estimates of SST values and seasonality during the glacial.

Coherent response of the Indo-African boreal summer monsoon to Pacific SST captured in Ethiopian rain δ18O

NASA Astrophysics Data System (ADS)

Madhavan, M.; Palliyil, L. R.; Ramesh, R.

2017-12-01

Pacific Sea Surface Temperature (SST) plays an important role in the inter-annual to inter-decadal variability of boreal monsoons. We identified a common mode of inter annual variability in the Indian and African boreal summer monsoon (June to September) rainfalls, which is linked to Pacific SSTs, using Empirical Orthogonal Function (EOF) analysis. Temporal coefficients (Principle component: PC1) of the leading mode of variability (EOF-1) is well correlated with the Indian summer monsoon rainfall and Sahel rainfall. About forty year long monthly observations of δ18O (and δD) at Addis Ababa, Ethiopia show a strong association with PC1 (r=0.69 for δ18O and r=0.75 for δD). Analysis of SST, sea level pressure and lower tropospheric winds suggest that 18O depletion in Ethiopian rainfall (and wet phases of PC1) is associated with cooler eastern tropical Pacific and warmer western Pacific and strengthening of Pacific subtropical high in both the hemispheres. Associated changes in the trade winds cause enhanced westerly moisture transport into the Indian subcontinent and northern Africa and cause enhanced rainfall. The intrusion of Atlantic westerly component of moisture transport at Addis Ababa during wet phases of PC1 is clearly recorded in δ18O of rain. We also observe the same common mode of variability (EOF1) of Indo-African boreal summer monsoon rain on decadal time scales. A 100 year long δ18O record of actively growing speleothem from the Mechara cave, Ethiopia, matches very well with the PC1 on the decadal time scale. This highlights the potential of speleothem δ18O and leaf wax δD from Ethiopia to investigate the natural variability and teleconnections of Indo-African boreal monsoon.

A century of Amazon burning driven by Atlantic climate

NASA Astrophysics Data System (ADS)

Makou, M.; Thompson, L. G.; Davis, M. E.; Eglinton, T. I.

2011-12-01

Very little is known about annual burning trends in the Amazon Basin prior to remote sensing of fires beginning in the late 1970's. Fires reduce Amazon forest biomass and species richness, release pollutant aerosols, and impact the carbon cycle, compelling further investigation of fire-climate dynamics. We measured organic compounds derived from vegetation burning in ice core samples from the Quelccaya Ice Cap in Peru at better than annual resolution to reconstruct wet and dry season burning throughout the Twentieth Century. Variations in the abundance of methyl hexadecanoate, which is produced by thermal alteration of vascular plant alkanoic acids, were used as a proxy for past fire activity. Concentrations of this compound in Quelccaya ice varied strongly on seasonal, interannual, and decadal time scales over the last 100 years, with high-amplitude dry season variability and muted, decadal-scale changes in wet season fire activity. Decade-long periods of repeatedly enhanced burning occurred during the 1930's and 1960's when dry season precipitation was perpetually reduced, as evidenced by low stages of the Rio Negro. These decadal trends suggest that changes in dry season precipitation drive fire activity in the western Amazon and highlight the potential of Amazon forests to undergo repeated strong burning. Fires occurred during years when sea surface temperatures (SSTs) in the north tropical Atlantic were elevated and the north-south tropical Atlantic SST gradient was enhanced; this SST pattern likely displaced the intertropical convergence zone northward, driving subsidence and drought in the western and southern Amazon basin. Thus, our novel ice core record suggests that Amazon forest fire activity during the Twentieth Century was driven primarily by Atlantic climate processes, and future forest health will depend heavily on the evolution of tropical climate.

Seasonal prediction of extreme precipitation events and frequency of rainy days over Costa Rica, Central America, using Canonical Correlation Analysis

NASA Astrophysics Data System (ADS)

Maldonado, T.; Alfaro, E.; Fallas-López, B.; Alvarado, L.

2013-04-01

High mountains divide Costa Rica, Central America, into two main climate regions, the Pacific and Caribbean slopes, which are lee and windward, respectively, according to the North Atlantic trade winds - the dominant wind regime. The rain over the Pacific slope has a bimodal annual cycle, having two maxima, one in May-June and the other in August-September-October (ASO), separated by the mid-summer drought in July. A first maximum of deep convection activity, and hence a first maximum of precipitation, is reached when sea surface temperature (SST) exceeds 29 °C (around May). Then, the SST decreases to around 1 °C due to diminished downwelling solar radiation and stronger easterly winds (during July and August), resulting in a decrease in deep convection activity. Such a reduction in deep convection activity allows an increase in down welling solar radiation and a slight increase in SST (about 28.5 °C) by the end of August and early September, resulting once again in an enhanced deep convection activity, and, consequently, in a second maximum of precipitation. Most of the extreme events are found during ASO. Central American National Meteorological and Hydrological Services (NMHS) have periodic Regional Climate Outlook Fora (RCOF) to elaborate seasonal predictions. Recently, meetings after RCOF with different socioeconomic stakeholders took place to translate the probable climate impacts from predictions. From the feedback processes of these meetings has emerged that extreme event and rainy days seasonal predictions are necessary for different sectors. As is shown in this work, these predictions can be tailored using Canonical Correlation Analysis for rain during ASO, showing that extreme events and rainy days in Central America are influenced by interannual variability related to El Niño-Southern Oscillation and decadal variability associated mainly with Atlantic Multidecadal Oscillation. Analyzing the geographical distribution of the ASO-2010 disaster reports, we noticed that they did not necessarily agree with the geographical extreme precipitation event distribution, meaning that social variables, like population vulnerability, should be included in the extreme events impact analysis.

Recent SST trends and Flood Disasters in Brazil

NASA Astrophysics Data System (ADS)

Yamashiki, Y.; Behera, S. K.; Inoue, S.; Netrananda, S.; Silva, R. D.; Takara, K. T.; Yamagata, T.

2010-12-01

We analyzed recent variations in the sea surface temperature (SST) anomalies of Pacific and Atlantic Oceans to understand their roles in extreme discharge of Amazon River Basin. In general, higher than monthly average discharge appears when La Niña condition forms and lower than monthly average discharge appears when El Niño condition forms. We also investigated the relationship between SST anomalies and recent floods in Brazil during the period of 1980-2010. Most severe floods (e.g. 2003 and 2010 Rio de Janeiro-São Paulo Flood) in austral summer occurred when El Niño Modoki appears in the Pacific Ocean. In addition, warm waters in tropical South Atlantic Ocean between American and African Coast also helped the moisture convergence to the affected region. Floods in some other locations (for example, Itaipava flood occurred in Maranhao State in 2008) occurred when a La Niña Modoki appeared in Pacific Ocean. These flood disasters in Brazil associated with climate phenomena may increase due to warmer SST trend under the global warming stress.

Towards the prediction of the East Africa short rains based on sea-surface temperature-atmosphere coupling

NASA Astrophysics Data System (ADS)

Mutai, C. C.; Ward, M. N.; Colman, A. W.

1998-07-01

It is shown that the July-September sea-surface temperature (SST) pattern contains moderately strong relationships with the October-December (OND) seasonal rainfall total averaged across East Africa 15°S-5°N, 30°-41.25°E. The relations can be described by using three rotated global SST empirical orthogonal functions (EOFs), mainly measuring aspects of SST patterns in the tropical Pacific (related to El Niño/Southern Oscillation), tropical Indian and, to a lesser extent, tropical Atlantic. Confidence in the relationships is raised because the three EOFs correlate significantly with OND near-surface divergence over the tropical Pacific, Indian and Atlantic Oceans (extending into Northern mid-latitudes), as well as with the rainfall in East Africa and also with rainfall across southern and western tropical Africa.For the East African region, multiple linear regression (MLR) and linear discriminant analysis prediction models are tested. The predictors are pre-rainfall season values of the three rotated SST EOFs. The predictors use information through September. Validating MLR hindcasts using a 1945-1966 (1967-1988) training period and a 1967-1988 (1945-1966) testing period between 30 to 60% of the area-averaged rainfall variance is explained. To achieve unbiased estimates of the expected skill of a forecast system, it is safest to keep model training and testing periods completely separate. The above strategy achieves this in the most important step of ensuring that the models fit the SST predictors to the rainfall predictand using years independent of the testing period. However, the EOFs were calculated over 1901-1980, so for hindcasts prior to 1981, the EOFs describe the SST variability a little better than could be achieved in real-time, which could inflate skill estimates. Tests in the years 1981-1994, independent of the 1901-1980 eigenvector analysis period, do produce similar levels of skill, but a few more forecast years are needed to confirm this result. It is shown that the mean verification at each individual location within East Africa is somewhat lower, which is important to consider for some applications. The need to monitor the prediction relationships and update the models is emphasised. Furthermore, these forecasts only become available as the OND season is underway, though some evidence is found for one of the EOF predictors having skill as early as June.

Diagnosing Mechanisms of Oceanic Influence on Sahel Precipitation Variability

NASA Astrophysics Data System (ADS)

Pomposi, Catherine A.

The West African Monsoon (WAM) is a significant component of the global monsoon system and plays a key role in the annual cycle of precipitation in the Sahel region of Africa (10°N to 20°N) during the summer months (July to September). Rainfall in the Sahel varies on timescales ranging from seasons to millennia as a result of changes in the WAM. In the last century, the Sahel experienced a relatively wet period (prior to the 1960s) followed by a period of severe drought (1970s-1980s) with higher-frequency variability superimposed on this low-frequency background signal. Understanding precipitation variability like that which occurred over the 20th Century and its impact on Sahel precipitation is critically important for skillful hydroclimate predictions and disaster preparedness in the region. Previous work has shown that the WAM responds to both internal atmospheric variability and external oceanic forcing. A large fraction of 20th Century Sahel rainfall variability has been linked to nearby and remote oceanic forcing from the Atlantic, Pacific, and Indian Oceans, suggesting that the ocean is the primary driver of variability. However, the mechanisms underlying the influence of sea surface temperature (SST) forcing to land based precipitation and the relative importance of the roles of different basins are not as well understood. To this end, the work completed in this thesis examines the physical mechanisms linking oceanic forcing to recent precipitation variability in the Sahel and identifies them alongside large-scale environmental conditions. A series of moisture budget decomposition studies are performed for the Sahel in order to understand the processes that govern regional hydroclimate variability on decadal and interannual time scales. The results show that the oceanic forcing of atmospheric mass convergence and divergence explains the moisture balance patterns in the region to first order on the timescales considered. On decadal timescales, forcing by the Indian and Atlantic Oceans correlate strongly with precipitation variability. The combination of a warm Indian Ocean and negative gradient across the Atlantic forces anomalous circulation patterns that result in net moisture divergence by the mean and transient flow. Together with negative moisture advection, these processes result in a strong drying of the Sahel during the later part of the 20th Century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation. On interannual timescales, results show that warm conditions in the Eastern Tropical Pacific remotely force anomalously dry conditions primarily through affecting the low-troposphere mass divergence field. This behavior is related to increased subsidence over the tropical Atlantic and into the Sahel and an anomalous westward flow of moisture from the continent, both resulting in a coherent drying pattern. The interannual signal is then further explored, particularly in light of the expected link between the El Nino Southern Oscillation and dry conditions in the Sahel, notably unseen during the historic El Nino event of 2015. Motivated by this, recent El Nino years and their precipitation signature in the Sahel along with the associated large-scale environmental conditions are examined. Two different outcomes for Sahel summer season are defined; an anomalously wet or an anomalously dry season coincident with El Nino conditions. The different precipitation patterns are distinguished by increased moisture supply for the wet years, which can be driven by both regional oceanic conditions that favor increased moisture convergence over the continent as well as weaker El Nino forcing. Finally, a series of new idealized SST-forced experiments that explore the causal link between oceanic forcing and the response of convection in the region on daily time resolution are discussed and preliminary results shown. These experiments aim to understand how convection in the Sahel responds to SST forcing using transient model simulations that track the evolving response of the WAM through time, day-by-day, under different oceanic conditions. Preliminary results show the stark differences in seasonal precipitation that occur when anomalies of opposite sign are applied in parts of the Atlantic and Pacific basin. There is also a suggestion of a difference in the timing of the rainy season when the model is run with different SST configurations.

The Effect of Ocean Currents on Sea Surface Temperature Anomalies

NASA Technical Reports Server (NTRS)

Stammer, Detlef; Leeuwenburgh, Olwijn

2000-01-01

We investigate regional and global-scale correlations between observed anomalies in sea surface temperature and height. A strong agreement between the two fields is found over a broad range of latitudes for different ocean basins. Both time-longitude plots and wavenumber-frequency spectra suggest an advective forcing of SST anomalies by a first-mode baroclinic wave field on spatial scales down to 400 km and time scales as short as 1 month. Even though the magnitude of the mean background temperature gradient is determining for the effectiveness of the forcing, there is no obvious seasonality that can be detected in the amplitudes of SST anomalies. Instead, individual wave signatures in the SST can in some cases be followed over periods of two years. The phase relationship between SST and SSH anomalies is dependent upon frequency and wavenumber and displays a clear decrease of the phase lag toward higher latitudes where the two fields come into phase at low frequencies. Estimates of the damping coefficient are larger than generally obtained for a purely atmospheric feedback. From a global frequency spectrum a damping time scale of 2-3 month was found. Regionally results are very variable and range from 1 month near strong currents to 10 month at low latitudes and in the sub-polar North Atlantic. Strong agreement is found between the first global EOF modes of 10 day averaged and spatially smoothed SST and SSH grids. The accompanying time series display low frequency oscillations in both fields.

Impact of global SST gradients on the Mediterranean runoff changes across the Plio-Pleistocene transition

NASA Astrophysics Data System (ADS)

Colleoni, Florence; Cherchi, Annalisa; Masina, Simona; Brierley, Christopher M.

2015-06-01

This work explores the impact of the development of global meridional and zonal sea surface temperature (SST) gradients on the Mediterranean runoff variability during the Plio-Pleistocene transition, about 3 Ma. Results show that total annual mean Pliocene Mediterranean runoff is about 40% larger than during the preindustrial period due to more increased extratropical specific humidity. As a consequence of a weakened and extended Hadley cell, the Pliocene northwest Africa hydrological network produces a discharge 30 times larger than today. Our results support the conclusion that during the Pliocene, the Mediterranean water deficit was reduced relative to today due to a larger river discharge. By means of a stand-alone atmospheric general circulation model, we simulate the separate impact of extratropical and equatorial SST cooling on the Mediterranean runoff. While cooling the equatorial SST does not imply significant changes to the Pliocene Mediterranean hydrological budget, the extratropical SST cooling increases the water deficit due to a decrease in precipitation and runoff. Consequently, river discharge from this area reduces to preindustrial levels. The main teleconnections acting upon the Mediterranean area today, i.e., the North Atlantic Oscillation during winter and the "monsoon-desert" mechanism during summer already have a large influence on the climate of our Pliocene simulations. Finally, our results also suggest that in a climate state significantly warmer than today, changes of the Hadley circulation could potentially lead to increased water resources in northwest Africa.

Equilibrium Atmospheric Response to North Atlantic SST Anomalies.

NASA Astrophysics Data System (ADS)

Kushnir, Yochanan; Held, Isaac M.

1996-06-01

The equilibrium general circulation model (GCM) response to sea surface temperature (SST) anomalies in the western North Atlantic region is studied. A coarse resolution GCM, with realistic lower boundary conditions including topography and climatological SST distribution, is integrated in perpetual January and perpetual October modes, distinguished from one another by the strength of the midlatitude westerlies. An SST anomaly with a maximum of 4°C is added to the climatological SST distribution of the model with both positive and negative polarity. These anomaly runs are compared to one another, and to a control integration, to determine the atmospheric response. In all cases warming (cooling) of the midlatitude ocean surface yields a warming (cooling) of the atmosphere over and to the east of the SST anomaly center. The atmospheric temperature change is largest near the surface and decreases upward. Consistent with this simple thermal response, the geopotential height field displays a baroclinic response with a shallow anomalous low somewhat downstream from the warm SST anomaly. The equivalent barotropic, downstream response is weak and not robust. To help interpret the results, the realistic GCM integrations are compared with parallel idealized model runs. The idealized model has full physics and a similar horizontal and vertical resolution, but an all-ocean surface with a single, permanent zonal asymmetry. The idealized and realistic versions of the GCM display compatible response patterns that are qualitatively consistent with stationary, linear, quasigeostrophic theory. However, the idealized model response is stronger and more coherent. The differences between the two model response patterns can be reconciled based on the size of the anomaly, the model treatment of cloud-radiation interaction, and the static stability of the model atmosphere in the vicinity of the SST anomaly. Model results are contrasted with other GCM studies and observations.

Computer-Assisted Interactive Documentary and Performance Arts in Illimitable Space

NASA Astrophysics Data System (ADS)

Sheridan, William Michael

Winter can bring significant snow storm systems or nor'easters to New England. Understanding each factor which can affect nor'easters will allow forecasters to better predict the subsequent weather conditions. One important parameter is the sea surface temperature (SST) of the Atlantic Ocean, where many of these systems strengthen and gain much of their structure. The Weather Research and Forecasting (WRF) model was used to simulate four different nor'easters (Mar 2007, Dec 2007, Jan 2008, Dec 2010) using both observed and warmed SSTs. For the wanner SST simulations, the SSTs over the model domain were increased by 1°C. This change increased the total surface heat fluxes in all of the storms, and the resulting simulated storms were all more intense. The influence on the amount of snowfall over land was highly variable, depending on how close to the coastline the storms were and temperatures across the region.

The extreme 2014 flood in south-western Amazon basin: the role of tropical-subtropical South Atlantic SST gradient

NASA Astrophysics Data System (ADS)

Carlo Espinoza, Jhan; Marengo, José Antonio; Ronchail, Josyane; Molina Carpio, Jorge; Noriega Flores, Luís; Loup Guyot, Jean

2014-12-01

Unprecedented wet conditions are reported in the 2014 summer (December-March) in South-western Amazon, with rainfall about 100% above normal. Discharge in the Madeira River (the main southern Amazon tributary) has been 74% higher than normal (58 000 m3 s-1) at Porto Velho and 380% (25 000 m3 s-1) at Rurrenabaque, at the exit of the Andes in summer, while levels of the Rio Negro at Manaus were 29.47 m in June 2014, corresponding to the fifth highest record during the 113 years record of the Rio Negro. While previous floods in Amazonia have been related to La Niña and/or warmer than normal tropical South Atlantic, the 2014 rainfall and flood anomalies are associated with warm condition in the western Pacific-Indian Ocean and with an exceptionally warm Subtropical South Atlantic. Our results suggest that the tropical and subtropical South Atlantic SST gradient is a main driver for moisture transport from the Atlantic toward south-western Amazon, and this became exceptionally intense during summer of 2014.

Paleohydrology of tropical South America and paleoceanography of the tropical Atlantic as deduced from two new sediment cores on the Brazilian continental slope

NASA Astrophysics Data System (ADS)

Nace, T.; Baker, P. A.; Dwyer, G. S.; Silva, C. G.; Hollander, D. J.; Rigsby, C. A.; Giosan, L.; Burns, S. J.

2011-12-01

Paleoclimate/paleoceanographic reconstructions of the Amazon Basin, Brazilian Nordeste, and western equatorial Atlantic have been undertaken on two new sediment cores located on the Brazilian continental slope (Core CDH-5 at 1708 mbsl, 4N, 48W, 32m long, ~30 ka record; Core CDH-86 at 3708 mbsl, 0N/S, 44W, 30m long, ~100ka record). High-resolution XRF analyses of Fe, Ti, and Ca are used to define the paleohydrologic history of the adjacent continent at both sites. Large and abrupt excursions of Ti/Ca ratios are observed in both cores, but are significantly better defined in the southern core, representative of Nordeste conditions. In this core there are a total of 9 Ti/Ca excursions, the oldest recovered dating to ~98ka. These excursions correlate well with Heinrich events from the North Atlantic. High-resolution stable oxygen isotopic analysis and Mg/Ca paleothermometry undertaken on the near-surface-dwelling planktic foraminiferal species Globierinoides ruber provide a picture of paleoceanographic forcings in the western equatorial Atlantic. The northern and southern cores respectively exhibit rapid warming of ~3C and ~3.5C between the last glacial maximum and the early Holocene. Furthermore, in almost all cases, during the last glacial stage, there was a 0.5C to 2C warming of the western equatorial Atlantic during the periods of high Ti/Ca ratios that correlate with Heinrich events. Thus, as observed in some previous studies, the western equatorial Atlantic was warm and the adjacent southern tropical continent was wet at the same time that the high-latitude North Atlantic was cold. The largely accepted paradigm is that Northern hemisphere cold events result in a southward migration of the Intertropical Convergence Zone (ITCZ), contributing to drier conditions at the northern extent of the ITCZ annual range (Cariaco Basin) and increased precipitation in the southern tropics of South America. The ITCZ appears to have been influenced by millennial variability of North-South Atlantic sea surface temperature (SST) gradients, with a southward migration in the zone of maximum tropical SSTs leading to a southward migration of the ITCZ. It is commonly modeled that Northern Hemisphere stadial events are associated with a SST increase in the equatorial Atlantic and a cooling of the North Atlantic. Our records of increased runoff synchronous with warmer SSTs in the western equatorial Atlantic coincident with cold Heinrich events of the north Atlantic strongly support this interpretation.

The coastal ocean response to the global warming acceleration and hiatus

PubMed Central

Liao, Enhui; Lu, Wenfang; Yan, Xiao-Hai; Jiang, Yuwu; Kidwell, Autumn

2015-01-01

Coastlines are fundamental to humans for habitation, commerce, and natural resources. Many coastal ecosystem disasters, caused by extreme sea surface temperature (SST), were reported when the global climate shifted from global warming to global surface warming hiatus after 1998. The task of understanding the coastal SST variations within the global context is an urgent matter. Our study on the global coastal SST from 1982 to 2013 revealed a significant cooling trend in the low and mid latitudes (31.4% of the global coastlines) after 1998, while 17.9% of the global coastlines changed from a cooling trend to a warming trend concurrently. The trend reversals in the Northern Pacific and Atlantic coincided with the phase shift of Pacific Decadal Oscillation and North Atlantic Oscillation, respectively. These coastal SST changes are larger than the changes of the global mean and open ocean, resulting in a fast increase of extremely hot/cold days, and thus extremely hot/cold events. Meanwhile, a continuous increase of SST was detected for a considerable portion of coastlines (46.7%) with a strengthened warming along the coastlines in the high northern latitudes. This suggests the warming still continued and strengthened in some regions after 1998, but with a weaker pattern in the low and mid latitudes. PMID:26568024

The coastal ocean response to the global warming acceleration and hiatus.

PubMed

Liao, Enhui; Lu, Wenfang; Yan, Xiao-Hai; Jiang, Yuwu; Kidwell, Autumn

2015-11-16

Coastlines are fundamental to humans for habitation, commerce, and natural resources. Many coastal ecosystem disasters, caused by extreme sea surface temperature (SST), were reported when the global climate shifted from global warming to global surface warming hiatus after 1998. The task of understanding the coastal SST variations within the global context is an urgent matter. Our study on the global coastal SST from 1982 to 2013 revealed a significant cooling trend in the low and mid latitudes (31.4% of the global coastlines) after 1998, while 17.9% of the global coastlines changed from a cooling trend to a warming trend concurrently. The trend reversals in the Northern Pacific and Atlantic coincided with the phase shift of Pacific Decadal Oscillation and North Atlantic Oscillation, respectively. These coastal SST changes are larger than the changes of the global mean and open ocean, resulting in a fast increase of extremely hot/cold days, and thus extremely hot/cold events. Meanwhile, a continuous increase of SST was detected for a considerable portion of coastlines (46.7%) with a strengthened warming along the coastlines in the high northern latitudes. This suggests the warming still continued and strengthened in some regions after 1998, but with a weaker pattern in the low and mid latitudes.

The impact of sea surface temperature on winter wheat in Iberian Peninsula

NASA Astrophysics Data System (ADS)

Capa-Morocho, Mirian; Rodríguez-Fonseca, Belen; Ruiz-Ramos, Margarita

2016-04-01

Climate variability is the main driver of changes in crops yield, especially for rainfed production systems. This is also the case of Iberian Peninsula (IP) (Capa-Morocho et al., 2014), where wheat yields are strongly dependent on seasonal rainfall amount and temporal distribution of rainfall during the growing season. Previous works have shown that large-scale oceanic patterns have a significant impact on precipitation over IP (Rodriguez-Fonseca and de Castro, 2002; Rodríguez-Fonseca et al., 2006). The existence of some predictability of precipitation has encouraged us to analyze the possible predictability of the wheat yield in the IP using sea surface temperature (SST) anomalies as predictor. For this purpose, a crop model site specific calibrated for the Northeast of IP and several reanalysis climate datasets have been used to obtain long time series of attainable wheat yield and relate their variability with SST anomalies. The results show that wheat yield anomalies are associated with changes in the Tropical Pacific (El Niño) and Atlantic (TNA) SST. For these events, the regional associated atmospheric pattern resembles the NAO, which also influences directly on the maximum temperatures and precipitation experienced by the crop during flowering and grain filling. Results from this study could have important implications for predictability issues in agricultural planning and management, such as insurance coverage, changes in sowing dates and choice of species and varieties.

Pliocene Seasonality along the US Atlantic Coastal Plain Inferred from Growth Increment Analysis of Mercenaria carolinensis

NASA Astrophysics Data System (ADS)

Winkelstern, I. Z.; Surge, D. M.

2010-12-01

Pliocene sea surface temperature (SST) data from the US Atlantic coastal plain is currently insufficient for a detailed understanding of the climatic shifts that occurred during the period. Previous studies, based on oxygen isotope proxy data from marine shells and bryozoan zooid size analysis, have provided constraints on possible annual-scale SST ranges for the region. However, more data are required to fully understand the forcing mechanisms affecting regional Pliocene climate and evaluate modeled temperature projections. Bivalve sclerochronology (growth increment analysis) is an alternative proxy for SST that can provide annually resolved multi-year time series. The method has been validated in previous studies using modern Arctica, Chione, and Mercenaria. We analyzed Pliocene Mercenaria carolinensis shells using sclerochronologic methods and tested the hypothesis that higher SST ranges are reflected in shells selected from the warmest climate interval (3.5-3.3 Ma, upper Yorktown Formation, Virginia) and lower SST ranges are observable in shells selected from the subsequent cooling interval (2.4-1.8 Ma, Chowan River Formation, North Carolina). These results further establish the validity of growth increment analysis using fossil shells and provide the first large dataset (from the region) of reconstructed annual SST from floating time series during these intervals. These data will enhance our knowledge about a warm climate state that has been identified in the 2007 IPCC report as an analogue for expected global warming. Future work will expand this study to include sampling in Florida to gain detailed information about Pliocene SST along a latitudinal gradient.

Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula

NASA Astrophysics Data System (ADS)

Garaboa-Paz, Daniel; Lorenzo, Nieves; Pérez-Muñuzuri, Vicente

2017-05-01

Seasonal forecasts have improved during the last decades, mostly due to an increase in understanding of the coupled ocean-atmosphere dynamics, and the development of models able to predict the atmosphere variability. Correlations between different teleconnection patterns and severe weather in different parts of the world are constantly evolving and changing. This paper evaluates the connection between winter precipitation over the Iberian Peninsula and the large-scale tropospheric mixing over the eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that significant negative correlations exist between summer FTLE anomalies and winter precipitation over Portugal and Spain. To understand the mechanisms behind this correlation, summer anomalies of the FTLE have also been correlated with other climatic variables such as the sea surface temperature (SST), the sea level pressure (SLP) or the geopotential. The East Atlantic (EA) teleconnection index correlates with the summer FTLE anomalies, confirming their role as a seasonal predictor for winter precipitation over the Iberian Peninsula.

The relationship between the ratio of strontium to calcium and sea-surface temperature in a modern Porites astreoides coral: Implications for using P. astreoides as a paleoclimate archive

USGS Publications Warehouse

Tess E. Busch,; Flannery, Jennifer A.; Richey, Julie N.; Stathakopoulos, Anastasios

2015-11-13

An inverse relationship has been demonstrated between water temperature and the ratio of strontium to calcium (Sr/Ca) in coral aragonite for a number of Pacific species of the genus Porites. This empirically determined relationship has been used to reconstruct past sea-surface temperature (SST) from modern and Holocene age coral archives. A study was conducted to investigate this relationship for Porites astreoides to determine the potential for using these corals as a paleotemperature archive in the Caribbean and western tropical Atlantic Ocean. Skeletal aragonite from a P. astreoides colony growing offshore of the southeast coast of Florida was subsampled with a mean temporal resolution of 14 samples per year and analyzed for Sr/Ca. The resulting Sr/Ca time series yielded well-defined annual cycles that correspond to annual growth bands in the coral. Sr/Ca was regressed against a monthly SST record from C-MAN buoy station FWYF1 (located at Fowey Rocks, Florida), resulting in the following Sr/Ca-SST relationship: Sr/Ca = –0.040*SST + 10.128 (R = –0.77). A 10-year time series of Sr/Ca-derived SST yields annual cycles with a 10–12 degree Celsius seasonal amplitude, consistent with available local instrumental records. We conclude that Sr/Ca in Porites astreoides from the Caribbean/Atlantic region has high potential for developing subannually resolved modern and recent Holocene SST records.

Trend and variability in western and central Africa streamflow, and major drivers of variability between 1950 and 2005

NASA Astrophysics Data System (ADS)

Dieppois, B.; Sidibe, M.; Mahe, G. M.; Paturel, J. E.; Anifowose, B. A.; Lawler, D.; Amoussou, E.

2017-12-01

Unprecedented drought episodes that struck western and central Africa between the late 1960s and 1980s, triggered many studies investigating rainfall variability and its impacts on water resources and food production systems. However, most studies were focused at the catchment scale. In this study, we aim at investigating the key large-scale controls determining and modulating climate-river flows relationships at the subcontinental scale between 1950 and 2005. Using the first complete monthly streamflow data set (1950-2005) over western and central Africa, streamflow trend and variability are seasonally assessed at this subcontinental scale and compared to those observed in other hydroclimatic variables (precipitation, temperature and potential evapotranspiration). Long-term trends and variability in streamflow are mainly consistent with trends in rainfall. In particular, the recent post-1990s partial recovery in Sahel rainfall could have, at least partially, positively impacted river flows (e.g. the Senegal and Niger rivers). However, these relationships may have been moderated by: i) changes in land use; and ii) contributions from groundwater resources. In addition, the time-evolution of river flows is shown to be primarily driven by very strong decadal fluctuations, which can be interpreted as modulations in the baseflow, as determined using multi-temporal trend and continuous wavelet analysis. These decadal fluctuations, which are also significantly detected in rainfall, are likely related to large-scale sea-surface temperature (SST) anomaly patterns (such as the tropical Atlantic SST variability, the Atlantic Multidecadal Oscillation, the Interdecadal Pacific Oscillation and the Pacific Decadal Oscillation), which are together modulating the West African monsoon. Furthermore, influences of the catchment properties (e.g. size, vegetation and land use cover, soil properties, direction of stream flow across climate zones) on these decadal fluctuations in river flows have been examined. This study therefore aims to improve the ability of current global to regional climate models to simulate such ranges of variability and understand regional hydroclimate, as a means for improving the development of future scenarios for water resources in western and central Africa.

Tropical Atlantic climate response to different freshwater input in high latitudes with an ocean-only general circulation model

NASA Astrophysics Data System (ADS)

Men, Guang; Wan, Xiuquan; Liu, Zedong

2016-10-01

Tropical Atlantic climate change is relevant to the variation of Atlantic meridional overturning circulation (AMOC) through different physical processes. Previous coupled climate model simulation suggested a dipole-like SST structure cooling over the North Atlantic and warming over the South Tropical Atlantic in response to the slowdown of the AMOC. Using an ocean-only global ocean model here, an attempt was made to separate the total influence of various AMOC change scenarios into an oceanic-induced component and an atmospheric-induced component. In contrast with previous freshwater-hosing experiments with coupled climate models, the ocean-only modeling presented here shows a surface warming in the whole tropical Atlantic region and the oceanic-induced processes may play an important role in the SST change in the equatorial south Atlantic. Our result shows that the warming is partly governed by oceanic process through the mechanism of oceanic gateway change, which operates in the regime where freshwater forcing is strong, exceeding 0.3 Sv. Strong AMOC change is required for the gateway mechanism to work in our model because only when the AMOC is sufficiently weak, the North Brazil Undercurrent can flow equatorward, carrying warm and salty north Atlantic subtropical gyre water into the equatorial zone. This threshold is likely to be model-dependent. An improved understanding of these issues may have help with abrupt climate change prediction later.

Testing the fidelity of the Sr/Ca proxy in recording ocean temperature in a western Atlantic coral

NASA Astrophysics Data System (ADS)

Kuffner, I. B.; Roberts, K.; Flannery, J. A.; Richey, J. N.; Morrison, J. M.

2017-12-01

Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a field-based coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates less than 1.7 mg cm-2 d-1 or LE rates less than 1.7 mm yr-1 returned spuriously high Sr/Ca values, leading to a cold bias in Sr/Ca-based SST estimates. The threshold-type response curves suggest that LE rate can be used as a quality-control indicator during sample and microdrill-path selection when using long cores for SST paleoreconstruction. For our corals that passed this quality control step, the Sr/Ca-SST proxy performed well in estimating mean annual SST across three sites spanning 350 km of the Florida reef tract. However, there was some evidence that extreme temperature stress in 2010 (cold snap) and 2011 (SST above coral-bleaching threshold) may have caused the corals not to record the temperature extremes. Known stress events could be avoided during modern calibrations of paleoproxies.

Is the Aquarius sea surface salinity variability representative?

NASA Astrophysics Data System (ADS)

Carton, J.; Grodsky, S.

2016-12-01

The leading mode of the Aquarius monthly anomalous sea surface salinity (SSS) is evaluated within the 50S-50N belt, where SSS retrieval accuracy is higher. This mode accounts for about 18% of the variance and resembles a pattern of the ENSO-induced anomalous rainfall. The leading mode of SSS variability deducted from a longer JAMSTEC analysis also accounts for about 17% of the variance and has very similar spatial pattern and almost a perfect correspondence of its temporal principal component to the SOI index. In that sense, the Aquarius SSS variability at low and middle latitudes is representative of SSS variability that may be obtained from longer records. This is explained by the fact that during the Aquarius period (2011-2015), the SOI index changed significantly from La Nina toward El Nino state, thus spanning a significant range of its characteristic variations. Multivariate EOF analysis of anomalous SSS and SST suggests that ENSO-induced shift in the tropical Pacific rainfall produces negatively correlated variability of temperature and salinity, which are expected if the anomalous surface flux (stronger rainfall coincident with less downward radiation) drives the system. But, anomalous SSS and SST are positively correlated in some areas including the northwestern Atlantic shelf (north of the Gulfstream) and the Pacific sector adjusting to the California peninsula. This positive correlation is indicative of an advection driven regime that is analyzed separately.

Impact of climate variability on various Rabi crops over Northwest India

NASA Astrophysics Data System (ADS)

Nageswararao, M. M.; Dhekale, B. S.; Mohanty, U. C.

2018-01-01

The Indian agriculture with its two prominent cropping seasons [summer ( Kharif) and winter ( Rabi)] is the mainstay of the rural economy. Northwest India (NWI) is an important region for the cultivation of Rabi crops grown during the period from October to April. In the present study, state wise impact analysis is carried out to ascertain the influence of climate indices Nino3.4 region Sea Surface Temperature (SST), Southern Oscillation Index (SOI), Arctic Oscillation (AO), North Atlantic Oscillation (NAO) and local precipitation, soil moisture, minimum ( T min), maximum ( T max) and mean ( T mean) temperatures on different Rabi crops (wheat, gram, rapeseed-mustard, oilseeds, and total Rabi food grains) over NWI during the years 1966-2011. To study the impact of climate variability on different Rabi crops, firstly, the influence of technology on the productivity of these crops has been removed by using linear function, as linear trend has noticed in all the time series. Correlation analysis provides an indication of the influence of local precipitation, soil moisture, T min, T max and T mean and some of its potential predictors (Nino3.4 region SST, SOI, AO, and NAO) on the productivity of different Rabi crops. Overall impact analysis indicates that the productivity of different Rabi crops in most of the places of NWI is most likely influenced by variability in local temperatures. Moreover, Nino3.4 region SST (SOI) positively (negatively) affects the productivity of gram, rapeseed-mustard, and total Rabi oilseeds in most of the states. The results of this study are useful in determining the strategies for increasing sustainable production through better agronomic practices.

A Modeling Study of the Causes and Predictability of the Spring 2011 Extreme U.S. Weather Activity

NASA Technical Reports Server (NTRS)

Schubert, Siegfried D.; Chang, Yehui; Wang, Hailan; Koster, Randal; Suarez, Max

2016-01-01

This study examines the causes and predictability of the spring 2011 U.S. extreme weather using the Modern-Era Retrospective Analysis for Research and Applications (MERRA) and Goddard Earth Observing System Model, version 5, (GEOS-5) atmospheric general circulation model simulations. The focus is on assessing the impact on precipitation of sea surface temperature (SST) anomalies, land conditions, and large-scale atmospheric modes of variability. A key result is that the April record-breaking precipitation in the Ohio River valley was primarily the result of the unforced development of a positive North Atlantic Oscillation (NAO)-like mode of variability with unusually large amplitude, limiting the predictability of the precipitation in that region at 1-month leads. SST forcing (La Nia conditions) contributed to the broader continental-scale pattern of precipitation anomalies, producing drying in the southern plains and weak wet anomalies in the northeast, while the impact of realistic initial North American land conditions was to enhance precipitation in the upper Midwest and produce deficits in the Southeast. It was further found that 1) the 1 March atmospheric initial condition was the primary source of the ensemble mean precipitation response over the eastern United States in April (well beyond the limit of weather predictability), suggesting an influence on the initial state of the previous SST forcing and/or tropospheric/stratospheric coupling linked to an unusually persistent and cold polar vortex; and 2) stationary wave model experiments suggest that the SST-forced base state for April enhanced the amplitude of the NAO response compared to that of the climatological state, though the impact is modest and can be of either sign.

Interannual Variation in Offshore Advection of Amazon-Orinoco Plume Waters: Observations, Forcing Mechanisms, and Impacts

NASA Astrophysics Data System (ADS)

Fournier, S.; Vandemark, D.; Gaultier, L.; Lee, T.; Jonsson, B.; Gierach, M. M.

2017-11-01

This study investigates sea surface salinity (SSS) and sea surface temperature (SST) variations in the tropical Atlantic east of the Lesser Antilles, a region where freshwater advection from the Amazon and Orinoco Rivers, may potentially impact air-sea interaction. Observations are used to document later-summer variability and evaluate offshore riverine transport from 2010 to 2014. During this period, the largest difference in plume-affected areas, defined as the extent covered by SSS lower than 35.5 pss, is found between 2011 and 2014. Plume waters covered 92% of the study region in 2011 and 60% in 2014, with the average SSS in the study region being 2 pss lower in 2011. Lagrangian particle tracking based on satellite-derived ocean currents is used to diagnose the impact of the river plumes on SSS and SST from 2010 to 2014. Northward freshwater flux in summer 2014 was significantly weaker than fluxes in 2010-2013. This difference is not due to interannual discharge variability, but to significant changes in eddy-driven transport and cross-shore winds. In particular, the stronger cross-shore wind in May 2014 restricted offshore freshwater flow and lead to a smaller plume-affected area. Persistent SST gradients are often found near the plume edge, which may have implications for ocean-atmosphere coupling associated with atmospheric convection. SST in the study region was 1°C higher in 2010 compared to other years, and is related to basin-scale ocean-atmosphere processes. Interannual variation in Amazon advective pathways and the associated SSS changes are also influenced by changes in the ITCZ position between 2011 and 2014.

Variability of sea-surface temperature in the South Atlantic bight as observed from satellite: Implications for offshore-spawning fish

NASA Astrophysics Data System (ADS)

Stegmann, P. M.; Yoder, J. A.

1996-06-01

We examined full-resolution (1 × 1 km) satellite images of sea-surface temperature (SST) over five consecutive years (1981-1986) covering the Atlantic menhaden ( Brevoortia tyrannus) recruitment period (November-April) in the SABRE (South Atlantic Bight recruitment experiment) study site. The results of our image time series indicated two processes which could be possible mechanisms for the onshore transport of fish larvae into coastal regions. One is the influx of warm Gulf Stream water that oscillates in and out of the Carolina Bays. These oscillations occurred throughout the study period over distances of 20-40 km and on time-scales as short as two days. The other is a tongue of relatively cold water located adjacent to the Virginia coast that moved southward and penetrated into Onslow Bay between January and March. Previous studies showed that Atlantic menhaden preferentially spawn in 18-22°C waters on the outer shelf. On the assumption that the 18°C isotherm (18DI) indicates where high larval abundance may occur, we used AVHRR-SST imagery to track the onshore-offshore movement of the 18DI along a transect extending onshore-offshore in Onslow Bay. Owing to seasonal warming and cooling, this isotherm was always found closest to the coast in early November, reached maximum offshore displacement by January/March, and then moved onshore again in April/May. Our results also showed that the position of this isotherm can move offshore or onshore in a matter of a few days. An important influence and possibly the major cause of the higher frequency displacements of the 18DI are Gulf Stream meanders or filaments moving through Onslow Bay. Our estimates of onshore isotherm speeds as determined from satellite SST ranged from 2 to 25 cm s -1 and are within the same order as those calculated by physical models or larval age determinations. If the onshore pulses of warm Gulf Stream water are indeed a mode by which menhaden larvae are transported cross-shelf, then the use of satellite-based observations to determine their frequency and onshore extent, as done in the present study, is a useful tool to study variations in fish recruitment.

Linear and nonlinear winter atmospheric responses to extreme phases of low frequency Pacific sea surface temperature variability

NASA Astrophysics Data System (ADS)

Cao, Dandan; Wu, Qigang; Hu, Aixue; Yao, Yonghong; Liu, Shizuo; Schroeder, Steven R.; Yang, Fucheng

2018-02-01

This study examines Northern Hemisphere winter (DJFM) atmospheric responses to opposite strong phases of interdecadal (low frequency, LF) Pacific sea surface temperature (SST) forcing, which resembles El Niño-Southern Oscillation (ENSO) on a longer time scale, in observations and GFDL and CAM4 model simulations. Over the Pacific-North America (PNA) sector, linear observed responses of 500-hPa height (Z500) anomalies resemble the PNA teleconnection pattern, but show a PNA-like nonlinear response because of a westward Z500 shift in the negative (LF-) relative to the positive LF (LF+) phase. Significant extratropical linear responses include a North Atlantic Oscillation (NAO)-like Z500 anomaly, a dipole-like Z500 anomaly over northern Eurasia associated with warming over mid-high latitude Eurasia, and a Southern Annular anomaly pattern associated with warming in southern land areas. Significant nonlinear Z500 responses also include a NAO-like anomaly pattern. Models forced by LF+ and LF- SST anomalies reproduce many aspects of observed linear and nonlinear responses over the Pacific-North America sector, and linear responses over southern land, but not in the North Atlantic-European sector and Eurasia. Both models simulate PNA-like linear responses in the North Pacific-North America region similar to observed, but show larger PNA-like LF+ responses, resulting in a PNA nonlinear response. The nonlinear PNA responses result from both nonlinear western tropical Pacific rainfall changes and extratropical transient eddy feedbacks. With LF tropical Pacific forcing only (LFTP+ and LFTP-, climatological SST elsewhere), CAM4 simulates a significant NAO response to LFTP-, including a linear negative and nonlinear positive NAO response.

Air-sea heat fluxes associated to mesoscale eddies in the Southwestern Atlantic Ocean and their dependence on different regional conditions

NASA Astrophysics Data System (ADS)

Leyba, Inés M.; Saraceno, Martín; Solman, Silvina A.

2017-10-01

Heat fluxes between the ocean and the atmosphere largely represent the link between the two media. A possible mechanism of interaction is generated by mesoscale ocean eddies. In this work we evaluate if eddies in Southwestern Atlantic (SWA) Ocean may significantly affect flows between the ocean and the atmosphere. Atmospherics conditions associated with eddies were examined using data of sea surface temperature (SST), sensible (SHF) and latent heat flux (LHF) from NCEP-CFSR reanalysis. On average, we found that NCEP-CFSR reanalysis adequately reflects the variability expected from eddies in the SWA, considering the classical eddy-pumping theory: anticyclonic (cyclonic) eddies cause maximum positive (negative) anomalies with maximum mean anomalies of 0.5 °C (-0.5 °C) in SST, 6 W/m2 (-4 W/m2) in SHF and 12 W/m2 (-9 W/m2) in LHF. However, a regional dependence of heat fluxes associated to mesoscale cyclonic eddies was found: in the turbulent Brazil-Malvinas Confluence (BMC) region they are related with positive heat flux anomaly (ocean heat loss), while in the rest of the SWA they behave as expected (ocean heat gain). We argue that eddy-pumping do not cool enough the center of the cyclonic eddies in the BMC region simply because most of them trapped very warm waters when they originate in the subtropics. The article therefore concludes that in the SWA: (1) a robust link exists between the SST anomalies generated by eddies and the local anomalous heat flow between the ocean and the atmosphere; (2) in the BMC region cyclonic eddies are related with positive heat anomalies, contrary to what is expected.

A Millennial-Scale Sea Surface Temperature Record From the North Atlantic Based on Diatoms

NASA Astrophysics Data System (ADS)

Miettinen, A.; Koc, N.

2008-12-01

Sea surfaces temperatures (SSTs) are generated from a 1000-year-long sediment core from the eastern flank of Reykjanes Ridge in the subpolar North Atlantic with a time resolution of 2-10 years. 54.3 cm long box core (Rapid 21-12B) and 370 cm long gravity core (RAPID 21-3K) were recovered from deep-sea sediments (2630 m water depth) during the RRS Charles Darwin cruise 159 in 2004. The box core is dated using the 210Pb method and it is continuously subsampled and investigated at 0.5 cm intervals for the last 230 years with a two years average time resolution. The gravity core is dated 14C AMS method and it is investigated continuously at 1.0 cm intervals with a ten years average resolution for the interval representing 230-1000 cal. years BP. August SSTs are reconstructed using marine planktonic diatom species with the Weighted Averages - Partial Least Squares (WA-PLS) method. Results achieved from the box core indicate August SST warming of c. 1 °C from 1773 AD to the present. The interval 1773-1830 represents the cold period at the investigated site. It is followed by warm period between 1830 and 1885. After this the temperature frequency is more stable with short cool events around 1890 and 1930. The last 60 years represent the warm period with a slow warming trend, especially during the past 25 years. However, results do not indicate distinct SST warming since 1870s. The most high-frequency SST variability with amplitude of c. 1 °C appears after 1970 indicating several very warm years, but also coldest years since 1820s.

Impact of Gulf Stream SST biases on the global atmospheric circulation

NASA Astrophysics Data System (ADS)

Lee, Robert W.; Woollings, Tim J.; Hoskins, Brian J.; Williams, Keith D.; O'Reilly, Christopher H.; Masato, Giacomo

2018-02-01

The UK Met Office Unified Model in the Global Coupled 2 (GC2) configuration has a warm bias of up to almost 7 K in the Gulf Stream SSTs in the winter season, which is associated with surface heat flux biases and potentially related to biases in the atmospheric circulation. The role of this SST bias is examined with a focus on the tropospheric response by performing three sensitivity experiments. The SST biases are imposed on the atmosphere-only configuration of the model over a small and medium section of the Gulf Stream, and also the wider North Atlantic. Here we show that the dynamical response to this anomalous Gulf Stream heating (and associated shifting and changing SST gradients) is to enhance vertical motion in the transient eddies over the Gulf Stream, rather than balance the heating with a linear dynamical meridional wind or meridional eddy heat transport. Together with the imposed Gulf Stream heating bias, the response affects the troposphere not only locally but also in remote regions of the Northern Hemisphere via a planetary Rossby wave response. The sensitivity experiments partially reproduce some of the differences in the coupled configuration of the model relative to the atmosphere-only configuration and to the ERA-Interim reanalysis. These biases may have implications for the ability of the model to respond correctly to variability or changes in the Gulf Stream. Better global prediction therefore requires particular focus on reducing any large western boundary current SST biases in these regions of high ocean-atmosphere interaction.

Global Warming Attenuates the Tropical Atlantic-Pacific Teleconnection

PubMed Central

Jia, Fan; Wu, Lixin; Gan, Bolan; Cai, Wenju

2016-01-01

Changes in global sea surface temperature (SST) since the end of last century display a pattern of widespread warming intercepted by cooling in the eastern equatorial Pacific and western coasts of the American continent. Studies have suggested that the cooling in the eastern equatorial Pacific may be partly induced by warming in the North Atlantic. However, it remains unknown how stable this inter-tropical teleconnection will be under global warming. Here we show that the inter-tropical teleconnection from the tropical Atlantic to Pacific weakens substantially as the CO2 concentration increases. This reduced impact is related to the El Niño-like warming of the tropical Pacific mean state, which leads to limited seasonal migration of the Pacific inter-tropical convergence zone (ITCZ) and weakened ocean heat transport. A fast decay of the tropical Atlantic SST anomalies in a warmer climate also contributes to the weakened teleconnection. Our study suggests that as greenhouse warming continues, the trend in the tropical Pacific as well as the development of ENSO will be less frequently interrupted by the Atlantic because of this attenuation. The weakened teleconnection is also supported by CMIP5 models, although only a few of these models can capture this inter-tropical teleconnection. PMID:26838053

Global Warming Attenuates the Tropical Atlantic-Pacific Teleconnection.

PubMed

Jia, Fan; Wu, Lixin; Gan, Bolan; Cai, Wenju

2016-02-03

Changes in global sea surface temperature (SST) since the end of last century display a pattern of widespread warming intercepted by cooling in the eastern equatorial Pacific and western coasts of the American continent. Studies have suggested that the cooling in the eastern equatorial Pacific may be partly induced by warming in the North Atlantic. However, it remains unknown how stable this inter-tropical teleconnection will be under global warming. Here we show that the inter-tropical teleconnection from the tropical Atlantic to Pacific weakens substantially as the CO2 concentration increases. This reduced impact is related to the El Niño-like warming of the tropical Pacific mean state, which leads to limited seasonal migration of the Pacific inter-tropical convergence zone (ITCZ) and weakened ocean heat transport. A fast decay of the tropical Atlantic SST anomalies in a warmer climate also contributes to the weakened teleconnection. Our study suggests that as greenhouse warming continues, the trend in the tropical Pacific as well as the development of ENSO will be less frequently interrupted by the Atlantic because of this attenuation. The weakened teleconnection is also supported by CMIP5 models, although only a few of these models can capture this inter-tropical teleconnection.

Southern Oscillation in surface circulation and climate over the tropical Atlantic, eastern Pacific, and Indian Oceans as captured by cluster analysis

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wolter, K.

Clusters of sea level pressure (SLP), surface wind, cloudiness, and sea surface temperature (SST) in the domain of the tropical Atlantic, eastern Pacific, and Indian Oceans are introduced and discussed in terms of general circulation and climate. They appear to capture well the large-scale degrees of freedom of the seasonal fields. In the Atlantic, and, to a lesser extent, in the eastern Pacific, most analyzed fields group into zonally oriented trade wind clusters. These are separated distinctly by the near-equatorial trough axis. By contrast, the Indian Ocean features strong interhemispheric connections associations with the monsoon systems of boreal summer and,more » to a lesser degree, of boreal winter. The usefulness of clusters thus established is elucidated with respect to the Southern Oscillation (SO). General circulation changes associated with this planetary pressure seesaw are deduced from the correlation maps of surface field clusters for January/February and July/August. During the positive SO phase (i.e., anomalously high pressure over the eastern Pacific and anomalously low pressure over Indonesia), both the Atlantic and eastern Pacific near-equatorial troughs are inferred to be shifted towards the north from July/August SLP, wind, and cloudiness fields. While eastern Pacific trade winds are weakened in both seasons in the positive PO phase, the Atlantic trades appear strengthened at the same time in the winter hemisphere only. Over the Indian Ocean, the monsoon circulation seems to be strengthened during the positive SO phase, with the summer monsoon displaying a more complex picture. Its SLP, cloudiness, and SST fields support an enhanced southwest monsoon, while its surface winds appear largely inconclusive. SST is lowered during the positive SO phase in all three tropical oceans.« less

Joint variability of global runoff and global sea surface temperatures

USGS Publications Warehouse

McCabe, G.J.; Wolock, D.M.

2008-01-01

Global land surface runoff and sea surface temperatures (SST) are analyzed to identify the primary modes of variability of these hydroclimatic data for the period 1905-2002. A monthly water-balance model first is used with global monthly temperature and precipitation data to compute time series of annual gridded runoff for the analysis period. The annual runoff time series data are combined with gridded annual sea surface temperature data, and the combined dataset is subjected to a principal components analysis (PCA) to identify the primary modes of variability. The first three components from the PCA explain 29% of the total variability in the combined runoff/SST dataset. The first component explains 15% of the total variance and primarily represents long-term trends in the data. The long-term trends in SSTs are evident as warming in all of the oceans. The associated long-term trends in runoff suggest increasing flows for parts of North America, South America, Eurasia, and Australia; decreasing runoff is most notable in western Africa. The second principal component explains 9% of the total variance and reflects variability of the El Ni??o-Southern Oscillation (ENSO) and its associated influence on global annual runoff patterns. The third component explains 5% of the total variance and indicates a response of global annual runoff to variability in North Aflantic SSTs. The association between runoff and North Atlantic SSTs may explain an apparent steplike change in runoff that occurred around 1970 for a number of continental regions.

Predictable and unpredictable modes of seasonal mean precipitation over Northeast China

NASA Astrophysics Data System (ADS)

Ying, Kairan; Frederiksen, Carsten S.; Zhao, Tianbao; Zheng, Xiaogu; Xiong, Zhe; Yi, Xue; Li, Chunxiang

2018-04-01

This study investigates the patterns of interannual variability that arise from the potentially predictable (slow) and unpredictable (intraseasonal) components of seasonal mean precipitation over Northeast (NE) China, using observations from a network of 162 meteorological stations for the period 1961-2014. A variance decomposition method is applied to identify the sources of predictability, as well as the sources of prediction uncertainty, for January-February-March (JFM), April-May-June (AMJ), July-August-September (JAS) and October-November-December (OND). The averaged potential predictability (ratio of slow to total variance) of NE China precipitation has the highest value of 0.32 during JAS and lowest value of 0.1 in AMJ. Possible sources of seasonal prediction for the leading predictable precipitation EOF modes come from the SST anomalies in the Japan Sea, as well as the North Atlantic during JFM, the Indian Ocean SST in AMJ, and the eastern tropical Pacific SST in JAS and OND. The prolonged linear trend, which is seen in the principal component time series of the leading predictable mode in JFM and OND, may also serve as a source of predictability. The Polar-Eurasia and Northern Annular Mode atmospheric teleconnection patterns are closely connected with the leading and the second predictable mode of JAS, respectively. The Hadley cell circulation is closely related to the leading predictable mode of OND. The leading/second unpredictable precipitation modes for all these four seasons show a similar monopole/dipole structure, and can be largely attributed to the intraseasonal variabilities of the atmosphere.

High Resolution Upwelling Cycles in Guaymas and Cariaco Basins over the late Holocene: Coupling Between the Western Atlantic and Eastern Pacific?

NASA Astrophysics Data System (ADS)

Goni, M. A.

2005-12-01

The recent past history of sea surface temperature (SST) conditions in two wind-dominated upwelling systems, Guaymas Basin (Gulf of California) and Cariaco Basin (Venezuela) was investigated using the alkenone-based UK'37 index. Both of these systems undergo marked seasonal SST changes of 4-10 degrees C, which are associated with wind-driven upwelling and thermal stratification cycles. Both Guaymas and Cariaco Basins are also characterized by suboxic to anoxic bottom waters that result in undisturbed, varved sediments. Confirmation that the seasonal SST trends are accurately incorporated into the UK'37 ratios of sinking particles was achieved using sediment trap samples. Analyses of sediment cores from Guaymas and Cariaco Basins yielded high-resolution (decadal) records of SST conditions in the overlying water column from 1700 to 2000 AD. The trends in the UK'37 index revealed general increases in the SST at both sites over that last 300 years associated with the end of the little ice age. However, in addition to this long-term trend, higher-frequency (~ 50 years) changes in SST that ranged from 1-3 degrees C were observed. We speculate that these decadal trends in SST reflect variations in the intensity of wind-driven upwelling at these sites. Most interestingly, there is a marked contrast in the timing of the SST values between Guaymas and Cariaco so that periods of enhanced upwelling in Guyamas Basin are characterized by decreased upwelling in Cariaco Basin (and vice versa). We propose that these contrasting records reflect differences in the response of wind-driven upwelling to changes in the position of the Intertropical Convergence Zone over the western Atlantic and the subtropical High over the eastern Pacific. The connection between these two upwelling systems and its significance for paleoreconstruction studies will be explored further.

Remote sensing of SST in the coastal ocean and inland seas

NASA Astrophysics Data System (ADS)

Kostianoy, Andrey

Sea Surface Temperature (SST) is the main oceanographic parameter widely used in oceanogra-phy that can be easily obtained from satellite measurements. Oceanic infrared remote sensing, based on the measurement of the thermal radiance emitted by the ocean, allows retrieving the SST corresponding to the temperature of the uppermost thin layer of the ocean. Theoretically the infrared signal only comes from the upper few microns "skin layer", therefore the thermal signatures cannot represent the dynamics of the mixed layer. But wind mixing during the daytime and nighttime convection mix the upper layer, so that SST usually is representative of that of the mixed layer. This is why nighttime passes of satellites are preferred for SST analysis. Since 1978 the Advanced Very High Resolution Radiometer (AVHRR), onboard the meteorolog-ical satellites of the NOAA series are widely used to derive SST maps. The temporal coverage is ensured by two-three NOAA satellites which provide 4-6 images/day over the globe with a swath of about 2800 km, the spatial resolution by a pixel of about 1.1 km, and thermal resolu-tion of about 0.1 deg. C. The typical data processing includes the retrieval of the SST from the combination of NN 3, 4, and 5 infrared channels of AVHRR, the geographical correction and localisation, with a generation of cloud and land masks. SST data can be then composed into daily to monthly (as well as season to yearly) maps/products. Moderate Resolution Imaging Spectroradiometer (MODIS)-Terra (since 2000) and -Aqua (since 2002), among the others, are the most known satellite instruments which increase the flow of the remote sensing SST data. In the regions with almost permanent cloudy conditions passive microwave radiometers are of vital importance for SST measurements, but they have significantly low spatial (25 km) and thermal (0.8 deg. C) resolution. Today, SST images/data are routinely acquired by satellite receiving stations worldwide including research vessels, as well as generated and made available via Internet by numerous world data centers for free. Examples of SST application for analy-sis/study/research/monitoring of SST fields, SST fronts, large-and meso-scale water dynamics and structure (currents, eddies, dipoles, jets, etc.), upwellings, SST seasonal and interannual variability, etc. will be shown. Combined analysis of SST data with optical (ocean color), SAR, altimetry, in-situ oceanographic, drifter and meteorological data was shown to be very successful for many purposes in physical oceanography, environment research and operational monitoring, regional and global climate change study, marine chemistry, marine biology and fishery. The presentation will include examples for different case studies in the Arctic Ocean (the Barents and Kara seas), the Atlantic Ocean (the Canary and Benguela upwellings), the Southern Indian Ocean, the Mediterranean, Black, Caspian, Aral, and Baltic seas.

Increasing magnitude of Hurricane Rapid Intensification in the central-eastern Atlantic over the past 30 years

NASA Astrophysics Data System (ADS)

Leung, L. R.; Balaguru, K.; Foltz, G. R.

2017-12-01

During the 2017 Atlantic hurricane season, several hurricanes underwent rapid intensification (RI) in the central-eastern Atlantic. This motivates an analysis of trends in the strength of hurricane RI during the 30-year post-satellite period of 1986-2015. Our results show that in the eastern tropical Atlantic, to the east of 60W, the mean RI magnitude averaged during 2001-2015 was 3.8 kt per 24 hr higher than during 1986-2000. However, in the western tropical Atlantic, to the west of 60W, changes in RI magnitude over the same period were not statistically significant. We examined the large-scale environment to understand the causes behind these changes in RI magnitude and found that various oceanic and atmospheric parameters that play an important role in RI changed favorably in the eastern tropical Atlantic. More specifically, changes in SST, Potential Intensity, upper-ocean heat content, wind shear, relative humidity and upper-level divergence enhanced the ability for hurricanes to undergo RI in the eastern tropical Atlantic. In contrast, changes in the same factors are inconsistent in the western tropical Atlantic. While changes in SST and Potential Intensity were positive, changes in upper-ocean heat content, wind shear and upper-level divergence were either insignificant or unfavorable for RI. Finally, we examined the potential role of various climate phenomena, which are well-known to impact Atlantic hurricane activity, in causing the changes in the large-scale environment. Our analysis reveals that changes in the Atlantic Multidecadal Oscillation over the 30-year period are predominantly responsible. These results provide important aspects of the large-scale context to understand the Atlantic hurricane season of 2017.

Eocene climate and Arctic paleobathymetry: A tectonic sensitivity study using GISS ModelE-R

NASA Astrophysics Data System (ADS)

Roberts, C. D.; Legrande, A. N.; Tripati, A. K.

2009-12-01

The early Paleogene (65-45 million years ago, Ma) was a ‘greenhouse’ interval with global temperatures warmer than any other time in the last 65 Ma. This period was characterized by high levels of CO2, warm high-latitudes, warm surface-and-deep oceans, and an intensified hydrological cycle. Sediments from the Arctic suggest that the Eocene surface Arctic Ocean was warm, brackish, and episodically enabled the freshwater fern Azolla to bloom. The precise mechanisms responsible for the development of these conditions remain uncertain. We present equilibrium climate conditions derived from a fully-coupled, water-isotope enabled, general circulation model (GISS ModelE-R) configured for the early Eocene. We also present model-data comparison plots for key climatic variables (SST and δ18O) and analyses of the leading modes of variability in the tropical Pacific and North Atlantic regions. Our tectonic sensitivity study indicates that Northern Hemisphere climate would have been very sensitive to the degree of oceanic exchange through the seaways connecting the Arctic to the Atlantic and Tethys. By restricting these seaways, we simulate freshening of the surface Arctic Ocean to ~6 psu and warming of sea-surface temperatures by 2°C in the North Atlantic and 5-10°C in the Labrador Sea. Our results may help explain the occurrence of low-salinity tolerant taxa in the Arctic Ocean during the Eocene and provide a mechanism for enhanced warmth in the north western Atlantic. We also suggest that the formation of a volcanic land-bridge between Greenland and Europe could have caused increased ocean convection and warming of intermediate waters in the Atlantic. If true, this result is consistent with the theory that bathymetry changes may have caused thermal destabilisation of methane clathrates in the Atlantic.

Climate variability and Dinophysis acuta blooms in an upwelling system.

PubMed

Díaz, Patricio A; Ruiz-Villarreal, Manuel; Pazos, Yolanda; Moita, Teresa; Reguera, Beatriz

2016-03-01

Dinophysis acuta is a frequent seasonal lipophilic toxin producer in European Atlantic coastal waters associated with thermal stratification. In the Galician Rías, populations of D. acuta with their epicentre located off Aveiro (northern Portugal), typically co-occur with and follow those of Dinophysis acuminata during the upwelling transition (early autumn) as a result of longshore transport. During hotter than average summers, D. acuta blooms also occur in August in the Rías, when they replace D. acuminata. Here we examined a 30-year (1985-2014) time series of D. acuta from samples collected by the same method in the Galician Rías. Our main objective was to identify patterns of distribution and their relation with climate variability, and to explain the exceptional summer blooms of D. acuta in 1989-1990. A dome-shaped relationship was found between summer upwelling intensity and D. acuta blooms; cell maxima were associated with conditions where the balance between upwelling intensity and heating, leading to deepened thermoclines, combined with tidal phase (3 days after neap tides) created windows of opportunity for this species. The application of a generalized additive model based on biological (D. acuta inoculum) and environmental predictors (Cumulative June-August upwelling CUI JJA , average June-August SST JJA and tidal range) explained more than 70% of the deviance for the exceptional summer blooms of D. acuta, through a combination of moderate (35,000-50,000m 3 s -1 km -1 ) summer upwelling (CUI JJA ), thermal stratification (SST JJA >17°C) and moderate tidal range (∼2.5m), provided D. acuta cells (inoculum) were present in July. There was no evidence of increasing trends in D. acuta bloom frequency/intensity nor a clear relationship with NAO or other long-term climatic cycles. Instead, the exceptional summer blooms of 1989-1990 appeared linked to extreme hydroclimatic anomalies (high positive anomalies in SST and NAO index), which affected most of the European Atlantic coast. Copyright © 2015 The Authors. Published by Elsevier B.V. All rights reserved.

Suitability of satellite derived and gridded sea surface temperature data sets for calibrating high-resolution marine proxy records

NASA Astrophysics Data System (ADS)

Ouellette, G., Jr.; DeLong, K. L.

2016-02-01

High-resolution proxy records of sea surface temperature (SST) are increasingly being produced using trace element and isotope variability within the skeletal materials of marine organisms such as corals, mollusks, sclerosponges, and coralline algae. Translating the geochemical variations within these organisms into records of SST requires calibration with SST observations using linear regression methods, preferably with in situ SST records that span several years. However, locations with such records are sparse; therefore, calibration is often accomplished using gridded SST data products such as the Hadley Center's HADSST (5º) and interpolated HADISST (1º) data sets, NOAA's extended reconstructed SST data set (ERSST; 2º), optimum interpolation SST (OISST; 1º), and Kaplan SST data sets (5º). From these data products, the SST used for proxy calibration is obtained for a single grid cell that includes the proxy's study site. The gridded data sets are based on the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) and each uses different methods of interpolation to produce the globally and temporally complete data products except for HadSST, which is not interpolated but quality controlled. This study compares SST for a single site from these gridded data products with a high-resolution satellite-based SST data set from NOAA (Pathfinder; 4 km) with in situ SST data and coral Sr/Ca variability for our study site in Haiti to assess differences between these SST records with a focus on seasonal variability. Our results indicate substantial differences in the seasonal variability captured for the same site among these data sets on the order of 1-3°C. This analysis suggests that of the data products, high-resolution satellite SST best captured seasonal variability at the study site. Unfortunately, satellite SST records are limited to the past few decades. If satellite SST are to be used to calibrate proxy records, collecting modern, living samples is desirable.

Latitudinal and Longitudinal Basin-scale Surface Salinity Contrasts and Freshwater Transport by Ocean Thermohaline Circulation

NASA Astrophysics Data System (ADS)

Seidov, D.; Haupt, B. J.

2003-12-01

The role of sea surface salinity (SSS) contrasts in maintaining vigorous global ocean thermohaline circulation (THC) is revisited. Relative importance of different generalizations of sea surface conditions in climate studies is explored. In numerical experiments using an ocean general circulation model, we have aggregated the observed sea surface temperature (SST) and SSS in several different ways: we used observed unchanged SST with SSS taken as constant (34.25 psu) everywhere; SST unchanged, and SSS zonally averaged globally, i.e., in the whole World Ocean; SST averaged globally, and SSS unchanged; SST zonally averaged globally and SSS zonally averaged basin-wide in individual basins, i.e., in the Atlantic, Indian, Pacific, and Southern Oceans separately; and, finally, both SST and SSS zonally averaged in individual basins. Global zonal averaging removes all longitudinal differences in sea surface climatology among ocean basins. However, latitudinal profiles of zonally averaged parameters preserve the main character of large-scale equator-to-pole sea surface variability. Basin-wide zonal averaging does an even better job of preserving latitudinal distributions within each basin. The results of the experiments could hardly be anticipated a priory. Surprisingly, SST could be used as a 2-D field, or as a zonally-averaged field without much difference in the THC dynamics. Moreover, SST could be averaged either globally, or basin-wide, and it also did not change the overall character of THC. At the same time, THC responded vigorously to how the SSS has been changed. It appeared that the THC structure with the globally averaged SST and basin-wide averaged SSS was very close to the one obtained in the control run (control run operates with 2-D observed SST and SSS). Our main conclusion is that ocean-wide inter-basin sea surface salinity contrasts serve as the major controlling element in global thermohaline circulation. Thermal inter-basin contrasts, as well as longitudinal variation in SSS, are less important than latitudinal thermal gradients and inter-basin salinity contrasts. Details of SSS also decrease in importance as soon as its inter-basin contrasts are retained. This is especially important for paleoclimate and future climate simulations, as only the large-scale inter-basin contrasts of the sea surface conditions really matter.

Satellite Analysis of Ocean Biogeochemistry and Mesoscale Variability in the Sargasso Sea

NASA Technical Reports Server (NTRS)

Siegel, D. A.; Micheals, A. F.; Nelson, N. B.

1997-01-01

The objective of this study was to analyze the impact of spatial variability on the time-series of biogeochemical measurements made at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site. Originally the study was planned to use SeaWiFS as well as AVHRR high-resolution data. Despite the SeaWiFS delays we were able to make progress on the following fronts: (1) Operational acquisition, processing, and archive of HRPT data from a ground station located in Bermuda; (2) Validation of AVHRR SST data using BATS time-series and spatial validation cruise CTD data; (3) Use of AVHRR sea surface temperature imagery and ancillary data to assess the impact of mesoscale spatial variability on P(CO2) and carbon flux in the Sargasso Sea; (4) Spatial and temporal extent of tropical cyclone induced surface modifications; and (5) Assessment of eddy variability using TOPEX/Poseidon data.

Relationship Between South Atlantic Subtropical High and South Atlantic SST Anomalies during Extreme Precipitation Events on Southeast Brazil

NASA Astrophysics Data System (ADS)

Pampuch, L.; Ambrizzi, T.

2012-12-01

The Southeast region of Brazil comprises the states of Sao Paulo, Minas Gerais, Rio de Janeiro and Espirito Santo. It occupies 10.85% of Brazilian territory and is highly urbanized. The Southeast Brazil is the biggest geoeconomic region of the country having a strong and diverse economy. Agriculture dominates in all states of the region. The main agricultural products are sugar cane, coffee, cotton, maize, cassava, rice, beans and fruits. Livestock farming is also practiced in the region. The largest herd of cattle is found in the state of Minas Gerais. These activities are highly dependent on the amount and distribution of rainfall. Studies of extreme precipitation events over Brazil have been well emphasized in the literature over the years and their relationship with anomalies of sea surface temperature (SST) in both the Pacific and the Atlantic Ocean have been analyzed. This paper investigates the extreme events occurring in southeastern Brazil from 1982 to 2004 using the technique of quantiles. The composite technique was applied to precipitation, sea level pressure anomaly (SLP) and sea surface temperature anomaly (SST) data in order to investigate the characteristics of rainfall patterns, the position and intensity of South Atlantic subtropical high (SASH) and SST anomalies in the Southern Atlantic Ocean (SAO) in the occurrence of these events and to make a distinction between dry and wet extremes. Analyzing the precipitation patterns, it was noticed that the composition of dry events throughout the Southeast Brazil has negative precipitation anomalies. Particularly, in the southern part of the region there is a large precipitation deficit, having an average of 50mm in the winter months. The composition for the wet events shows that, on average, positive precipitation anomalies with the southern region containing the highest cumulative average, reaching a positive anomaly of 100mm. The composition of SLP in the case of dry events indicates a positive anomaly of pressure on SAO close to the South America continent and a negative anomaly far from the continent. This configuration might represent a southwest movement of the SASH. For the wet events composition is possible to note an opposite configuration: an negative anomaly is seen near the South American continent and a positive one is away of it. Such a configuration may represent a weakening of SASH and a shift to northeast part of the SAO. In the composition of the SST anomalies is possible to note a different pattern for both cases with regard to the tropical Pacific, indicating that in dry years an El Niño pattern is evident and during the wet years a La Niña pattern prevails. On the other hand, for the SAO, colder SST anomalies in the dry years was observed next to the coast of South America, and during the rainy years a positive anomaly was observed away from the continent.

ENSO modulation of tropical Indian Ocean subseasonal variability

NASA Astrophysics Data System (ADS)

Jung, Eunsil; Kirtman, Ben P.

2016-12-01

In this study, we use 30 years of retrospective climate model forecasts and observational estimates to show that El Niño/Southern Oscillation (ENSO) affects the amplitude of subseasonal variability of sea surface temperature (SST) in the southwest Indian Ocean, an important Tropical Intraseasonal Oscillation (TISO) onset region. The analysis shows that deeper background mixed-layer depths and warmer upper ocean conditions during El Niño reduce the amplitude of the subseasonal SST variability over Seychelles-Chagos Thermocline Ridge (SCTR), which may reduce SST-wind coupling and the amplitude of TISO variability. The opposite holds for La Niña where the shallower mixed-layer depth enhances SST variability over SCTR, which may increase SST-wind coupling and the amplitude of TISO variability.

Interdecadal change of the controlling mechanisms for East Asian early summer rainfall variation around the mid-1990s

NASA Astrophysics Data System (ADS)

Yim, So-Young; Wang, Bin; Kwon, MinHo

2014-03-01

East Asian (EA) summer monsoon shows considerable differences in the mean state and principal modes of interannual variation between early summer (May-June, MJ) and late summer (July-August, JA). The present study focuses on the early summer (MJ) precipitation variability. We find that the interannual variation of the MJ precipitation and the processes controlling the variation have been changed abruptly around the mid-1990s. The rainfall anomaly represented by the leading empirical orthogonal function has changed from a dipole-like pattern in pre-95 epoch (1979-1994) to a tripole-like pattern in post-95 epoch (1995-2010); the prevailing period of the corresponding principal component has also changed from 3-5 to 2-3 years. These changes are concurrent with the changes of the corresponding El Nino-Southern Oscillation (ENSO) evolutions. During the pre-95 epoch, the MJ EA rainfall anomaly is coupled to a slow decay of canonical ENSO events signified by an eastern Pacific warming, which induces a dipole rainfall feature over EA. On the other hand, during the post-95 epoch the anomalous MJ EA rainfall is significantly linked to a rapid decay of a central Pacific warming and a distinct tripolar sea surface temperature (SST) in North Atlantic. The central Pacific warming-induced Philippine Sea anticyclone induces an increased rainfall in southern China and decreased rainfall in central eastern China. The North Atlantic Oscillation-related tripolar North Atlantic SST anomaly induces a wave train that is responsible for the increase northern EA rainfall. Those two impacts form the tripole-like rainfall pattern over EA. Understanding such changes is important for improving seasonal to decadal predictions and long-term climate change in EA.

Circulation and teleconnection mechanisms of Northeast Brazil droughts

NASA Astrophysics Data System (ADS)

Hastenrath, Stefan

2006-08-01

The Northern Nordeste of Brazil has its short rainy season narrowly concentrated around March-April, when the interhemispheric southward gradient of sea surface temperature (SST) is weakest and the Intertropical Convergence Zone (ITCZ), which is the main rainbearing system for the Nordeste, reaches its southernmost position in the course of the year. The recurrent Secas (droughts) have a severe socio-economic impact in this semi-arid region. In drought years, the pre-season (October-January) rainfall is scarce, the interhemispheric SST gradient weakened and the basin-wide southerly (northerly) wind component enhanced (reduced), all manifestations of an anomalously far northward ITCZ position. Apart from this ensemble of Atlantic indicators, the Secas also tend to be preceded by anomalously warm equatorial Pacific waters in January. During El Niño years, an upper-tropospheric wave train extends from the equatorial eastern Pacific to the northern tropical Atlantic, affecting the patterns of upper-tropospheric topography and divergence, and hence of vertical motion over the Atlantic. The altered vertical motion leads to a weaker meridional pressure gradient on the equatorward flank of the North Atlantic subtropical high, and thus weaker North Atlantic tradewinds. The concomitant reduction of evaporation and wind stirring allows for warmer surface waters in the tropical North Atlantic and thus steeper interhemispheric meridional thermal gradient. Consequently, the ITCZ stays anomalously far North and the Nordeste rainy season becomes deficient.

Prediction of Meiyu rainfall in Taiwan by multi-lead physical-empirical models

NASA Astrophysics Data System (ADS)

Yim, So-Young; Wang, Bin; Xing, Wen; Lu, Mong-Ming

2015-06-01

Taiwan is located at the dividing point of the tropical and subtropical monsoons over East Asia. Taiwan has double rainy seasons, the Meiyu in May-June and the Typhoon rains in August-September. To predict the amount of Meiyu rainfall is of profound importance to disaster preparedness and water resource management. The seasonal forecast of May-June Meiyu rainfall has been a challenge to current dynamical models and the factors controlling Taiwan Meiyu variability has eluded climate scientists for decades. Here we investigate the physical processes that are possibly important for leading to significant fluctuation of the Taiwan Meiyu rainfall. Based on this understanding, we develop a physical-empirical model to predict Taiwan Meiyu rainfall at a lead time of 0- (end of April), 1-, and 2-month, respectively. Three physically consequential and complementary predictors are used: (1) a contrasting sea surface temperature (SST) tendency in the Indo-Pacific warm pool, (2) the tripolar SST tendency in North Atlantic that is associated with North Atlantic Oscillation, and (3) a surface warming tendency in northeast Asia. These precursors foreshadow an enhanced Philippine Sea anticyclonic anomalies and the anomalous cyclone near the southeastern China in the ensuing summer, which together favor increasing Taiwan Meiyu rainfall. Note that the identified precursors at various lead-times represent essentially the same physical processes, suggesting the robustness of the predictors. The physical empirical model made by these predictors is capable of capturing the Taiwan rainfall variability with a significant cross-validated temporal correlation coefficient skill of 0.75, 0.64, and 0.61 for 1979-2012 at the 0-, 1-, and 2-month lead time, respectively. The physical-empirical model concept used here can be extended to summer monsoon rainfall prediction over the Southeast Asia and other regions.

Abrupt aridities in the Levant-Sahel linked with solar activities

NASA Astrophysics Data System (ADS)

Stein, M.; Kushnir, Y.

2012-04-01

Observations of 19th and 20th century precipitation in the Dead Sea watershed region display a multidecadal, anti-phase relationship to North Atlantic (NAtl) sea surface temperature (SST) variability, such that when the NAtl is relatively cold, Jerusalem experiences higher than normal precipitation and vice versa. This association is underlined by a negative correlation to precipitation in the sub-Saharan Sahel and a positive correlation to precipitation in western North America, areas that are also affected by multidecadal NAtl SST variability. These observations are consistent with broad range of Holocene hydroclimatic fluctuations from the epochal, to the millennial and centennial time scales, as displayed by the Dead Sea and Sahelian lake levels and by direct and indirect proxy indicators of NAtl SSTs. On the epochal time scale, the gradual cooling of NAtl SSTs throughout the Holocene in response to precession-driven reduction of summer insolation is associated with previously well-studied wet-to-dry transition in the Sahel and with a general increase in Dead Sea lake levels from low stands after the Younger Dryas to higher stands in the mid- to late-Holocene. On the millennial and centennial time scales there is also evidence for an antiphase relationship between Holocene variations in the Dead Sea and Sahelian lake levels and with proxy indicators of NAtl SSTs. However, the records are punctuated by abrupt lake-level drops and extensive expansion of the desert belt at ~8.1, 5.7, 3.3 and 1.4 ka cal BP, which appear to be in-phase and which occur during previously documented abrupt major cooling events in the Northern Hemisphere. We link these cooling to solar activity variations that were identified in the North Atlantic IRD and cosmogenic isotopes records.

Improving seasonal forecasts of hydroclimatic variables through the state of multiple large-scale climate signals

NASA Astrophysics Data System (ADS)

Castelletti, A.; Giuliani, M.; Block, P. J.

2017-12-01

Increasingly uncertain hydrologic regimes combined with more frequent and intense extreme events are challenging water systems management worldwide, emphasizing the need of accurate medium- to long-term predictions to timely prompt anticipatory operations. Despite modern forecasts are skillful over short lead time (from hours to days), predictability generally tends to decrease on longer lead times. Global climate teleconnection, such as El Niño Southern Oscillation (ENSO), may contribute in extending forecast lead times. However, ENSO teleconnection is well defined in some locations, such as Western USA and Australia, while there is no consensus on how it can be detected and used in other regions, particularly in Europe, Africa, and Asia. In this work, we generalize the Niño Index Phase Analysis (NIPA) framework by contributing the Multi Variate Niño Index Phase Analysis (MV-NIPA), which allows capturing the state of multiple large-scale climate signals (i.e. ENSO, North Atlantic Oscillation, Pacific Decadal Oscillation, Atlantic Multi-decadal Oscillation, Indian Ocean Dipole) to forecast hydroclimatic variables on a seasonal time scale. Specifically, our approach distinguishes the different phases of the considered climate signals and, for each phase, identifies relevant anomalies in Sea Surface Temperature (SST) that influence the local hydrologic conditions. The potential of the MV-NIPA framework is demonstrated through an application to the Lake Como system, a regulated lake in northern Italy which is mainly operated for flood control and irrigation supply. Numerical results show high correlations between seasonal SST values and one season-ahead precipitation in the Lake Como basin. The skill of the resulting MV-NIPA forecast outperforms the one of ECMWF products. This information represents a valuable contribution to partially anticipate the summer water availability, especially during drought events, ultimately supporting the improvement of the Lake Como operations.

Covariability of Central America/Mexico winter precipitation and tropical sea surface temperatures

NASA Astrophysics Data System (ADS)

Pan, Yutong; Zeng, Ning; Mariotti, Annarita; Wang, Hui; Kumar, Arun; Sánchez, René Lobato; Jha, Bhaskar

2018-06-01

In this study, the relationships between Central America/Mexico (CAM) winter precipitation and tropical Pacific/Atlantic sea surface temperatures (SSTs) are examined based on 68-year (1948-2015) observations and 59-year (1957-2015) atmospheric model simulations forced by observed SSTs. The covariability of the winter precipitation and SSTs is quantified using the singular value decomposition (SVD) method with observational data. The first SVD mode relates out-of-phase precipitation anomalies in northern Mexico and Central America to the tropical Pacific El Niño/La Niña SST variation. The second mode links a decreasing trend in the precipitation over Central America to the warming of SSTs in the tropical Atlantic, as well as in the tropical western Pacific and the tropical Indian Ocean. The first mode represents 67% of the covariance between the two fields, indicating a strong association between CAM winter precipitation and El Niño/La Niña, whereas the second mode represents 20% of the covariance. The two modes account for 32% of CAM winter precipitation variance, of which, 17% is related to the El Niño/La Niña SST and 15% is related to the SST warming trend. The atmospheric circulation patterns, including 500-hPa height and low-level winds obtained by linear regressions against the SVD SST time series, are dynamically consistent with the precipitation anomaly patterns. The model simulations driven by the observed SSTs suggest that these precipitation anomalies are likely a response to tropical SST forcing. It is also shown that there is significant potential predictability of CAM winter precipitation given tropical SST information.

Stratigraphic framework for Pliocene paleoclimate reconstruction: The correlation conundrum

USGS Publications Warehouse

Dowsett, H.J.; Robinson, M.M.

2006-01-01

Pre-Holocene paleoclimate reconstructions face a correlation conundrum because complications inherent in the stratigraphic record impede the development of synchronous reconstruction. The Pliocene Research, Interpretation and Synoptic Mapping (PRISM) paleoenvironmental reconstructions have carefully balanced temporal resolution and paleoclimate proxy data to achieve a useful and reliable product and are the most comprehensive pre-Pleistocene data sets available for analysis of warmer-than-present climate and for climate modeling experiments. This paper documents the stratigraphic framework for the mid-Pliocene sea surface temperature (SST) reconstruction of the North Atlantic and explores the relationship between stratigraphic/temporal resolution and various paleoceanographic estimates of SST. The magnetobiostratigraphic framework for the PRISM North Atlantic region is constructed from planktic foraminifer, calcareous nannofossil and paleomagnetic reversal events recorded in deep-sea cores and calibrated to age. Planktic foraminifer census data from multiple samples within the mid-Pliocene yield multiple SST estimates for each site. Extracting a single SST value at each site from multiple estimates, given the limitations of the material and stratigraphic resolution, is problematic but necessary for climate model experiments. The PRISM reconstruction, unprecedented in its integration of many different types of data at a focused stratigraphic interval, utilizes a time slab approach and is based on warm peak average temperatures. A greater understanding of the dynamics of the climate system and significant advances in models now mandate more precise, globally distributed yet temporally synchronous SST estimates than are available through averaging techniques. Regardless of the precision used to correlate between sequences within the midd-Pliocene, a truly synoptic reconstruction in the temporal sense is unlikely. SST estimates from multiple proxies promise to further refine paleoclimate reconstructions but must consider the complications associated with each method, what each proxy actually records, and how these different proxies compare in time-averaged samples.

The 30-60-day Intraseasonal Variability of Sea Surface Temperature in the South China Sea dur1ing May-September

NASA Astrophysics Data System (ADS)

Mao, Jiangyu; Wang, Ming

2018-05-01

This study investigates the structure and propagation of intraseasonal sea surface temperature (SST) variability in the South China Sea (SCS) on the 30-60-day timescale during boreal summer (May-September). TRMM-based SST, GODAS oceanic reanalysis and ERA-Interim atmospheric reanalysis datasets from 1998 to 2013 are used to examine quantitatively the atmospheric thermodynamic and oceanic dynamic mechanisms responsible for its formation. Power spectra show that the 30-60-day SST variability is predominant, accounting for 60% of the variance of the 10-90-day variability over most of the SCS. Composite analyses demonstrate that the 30-60-day SST variability is characterized by the alternate occurrence of basin-wide positive and negative SST anomalies in the SCS, with positive (negative) SST anomalies accompanied by anomalous northeasterlies (southwesterlies). The transition and expansion of SST anomalies are driven by the monsoonal trough-ridge seesaw pattern that migrates northward from the equator to the northern SCS. Quantitative diagnosis of the composite mixed-layer heat budgets shows that, within a strong 30-60-day cycle, the atmospheric thermal forcing is indeed a dominant factor, with the mixed-layer net heat flux (MNHF) contributing around 60% of the total SST tendency, while vertical entrainment contributes more than 30%. However, the entrainment-induced SST tendency is sometimes as large as the MNHF-induced component, implying that ocean processes are sometimes as important as surface fluxes in generating the 30-60-day SST variability in the SCS.

Modern average global sea-surface temperature

USGS Publications Warehouse

Schweitzer, Peter N.

1993-01-01

The data contained in this data set are derived from the NOAA Advanced Very High Resolution Radiometer Multichannel Sea Surface Temperature data (AVHRR MCSST), which are obtainable from the Distributed Active Archive Center at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. The JPL tapes contain weekly images of SST from October 1981 through December 1990 in nine regions of the world ocean: North Atlantic, Eastern North Atlantic, South Atlantic, Agulhas, Indian, Southeast Pacific, Southwest Pacific, Northeast Pacific, and Northwest Pacific. This data set represents the results of calculations carried out on the NOAA data and also contains the source code of the programs that made the calculations. The objective was to derive the average sea-surface temperature of each month and week throughout the whole 10-year series, meaning, for example, that data from January of each year would be averaged together. The result is 12 monthly and 52 weekly images for each of the oceanic regions. Averaging the images in this way tends to reduce the number of grid cells that lack valid data and to suppress interannual variability.

Modulation of sea surface temperature warming in the Bay of Biscay by Loire and Gironde Rivers

NASA Astrophysics Data System (ADS)

Costoya, X.; Fernández-Nóvoa, D.; deCastro, M.; Santos, F.; Lazure, P.; Gómez-Gesteira, M.

2016-01-01

The influence of Loire and Gironde River discharges over the sea surface temperature (SST) in the eastern Bay of Biscay (0.6º-36.6ºW, 44.2º-47.8ºW) was analyzed by means of two complementary databases (MODIS and OISST1/4). The area influenced by river plume showed a different SST when compared with the adjacent oceanic area for the months when the plume attains its highest extension (December, January, and February). Ocean was observed to warm at a rate of approximately 0.3ºC dec-1 while temperature at the area influenced by the rivers cooled at a rate of -0.15ºC dec-1 over the period 1982-2014. The mere presence of a freshwater layer is able to modulate the warming observed at adjacent ocean locations since the coastal area is isolated from the rest of the Bay. This nearshore strip is the only part of the Bay where changes in SST depend on North Atlantic Oscillation (NAO) but not on North Atlantic SST represented by the Atlantic Multidecadal Oscillation (AMO). These different cooling-warming trends are even more patent over the last years (2002-2014) under atmospheric favorable conditions for plume enhancement. River runoff increased at a rate on the order of 120 m3s-1dec-1 over that period and southwesterly winds, which favor the confinement of the plume, showed a positive and significant trend both in duration and intensity. Thus, the coastal strip has been observed to cool at a rate of -0.5°C dec-1.

Fidelity of the Sr/Ca proxy in recording ocean temperature in the western Atlantic coral Siderastrea siderea

USGS Publications Warehouse

Kuffner, Ilsa B.; Roberts, Kelsey E.; Flannery, Jennifer A.; Morrison, Jennifer M.; Richey, Julie

2017-01-01

Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates < 1.7 mg cm−2 d−1 or < 1.7 mm yr−1 LE returned spuriously high Sr/Ca values, leading to a cold-bias in Sr/Ca-based SST estimates. The threshold-type response curves suggest that extension rate can be used as a quality-control indicator during sample and drill-path selection when using long cores for SST paleoreconstruction. For our corals that passed this quality control step, the Sr/Ca-SST proxy performed well in estimating mean annual temperature across three sites spanning 350 km of the Florida reef tract. However, there was some evidence that extreme temperature stress in 2010 (cold snap) and 2011 (SST above coral-bleaching threshold) may have caused the corals not to record the temperature extremes. Known stress events could be avoided during modern calibrations of paleoproxies.

An evaluation of the effect of recent temperature variability on the prediction of coral bleaching events.

PubMed

Donner, Simon D

2011-07-01

Over the past 30 years, warm thermal disturbances have become commonplace on coral reefs worldwide. These periods of anomalous sea surface temperature (SST) can lead to coral bleaching, a breakdown of the symbiosis between the host coral and symbiotic dinoflagellates which reside in coral tissue. The onset of bleaching is typically predicted to occur when the SST exceeds a local climatological maximum by 1 degrees C for a month or more. However, recent evidence suggests that the threshold at which bleaching occurs may depend on thermal history. This study uses global SST data sets (HadISST and NOAA AVHRR) and mass coral bleaching reports (from Reefbase) to examine the effect of historical SST variability on the accuracy of bleaching prediction. Two variability-based bleaching prediction methods are developed from global analysis of seasonal and interannual SST variability. The first method employs a local bleaching threshold derived from the historical variability in maximum annual SST to account for spatial variability in past thermal disturbance frequency. The second method uses a different formula to estimate the local climatological maximum to account for the low seasonality of SST in the tropics. The new prediction methods are tested against the common globally fixed threshold method using the observed bleaching reports. The results find that estimating the bleaching threshold from local historical SST variability delivers the highest predictive power, but also a higher rate of Type I errors. The second method has the lowest predictive power globally, though regional analysis suggests that it may be applicable in equatorial regions. The historical data analysis suggests that the bleaching threshold may have appeared to be constant globally because the magnitude of interannual variability in maximum SST is similar for many of the world's coral reef ecosystems. For example, the results show that a SST anomaly of 1 degrees C is equivalent to 1.73-2.94 standard deviations of the maximum monthly SST for two-thirds of the world's coral reefs. Coral reefs in the few regions that experience anomalously high interannual SST variability like the equatorial Pacific could prove critical to understanding how coral communities acclimate or adapt to frequent and/or severe thermal disturbances.

Influence of the Summer NAO on the Spring-NAO-Based Predictability of the East Asian Summer Monsoon

NASA Astrophysics Data System (ADS)

Zheng, Fei

2017-04-01

The dominant mode of atmospheric circulation over the North Atlantic region is the North Atlantic Oscillation (NAO). The boreal spring NAO may imprint its signal on contemporaneous sea surface temperature (SST), leading to a North Atlantic SST tripolar pattern (NAST). This pattern persists into the following summer and modulates the East Asian summer monsoon (EASM). Previous studies have shown that the summer NAST is caused mainly by the preceding spring NAO, whereas the contemporaneous summer NAO plays a secondary role. The results of this study illustrate that, even if the summer NAO plays a secondary role, it may also perturb summer SST anomalies caused by the spring NAO. There are two types of perturbation caused by the summer NAO. If the spring and summer NAO patterns have the same (opposite) polarities, the summer NAST tends to be enhanced (reduced) by the summer NAO, and the correlation between the spring NAO and EASM is usually stronger (weaker). In the former (latter) case, the spring-NAO-based prediction of the EASM tends to have better (limited) skill. These results indicate that it is important to consider the evolution of the NAO when forecasting the EASM, particular when there is a clear reversal in the polarity of the NAO, because it may impair the spring-NAO-based EASM prediction.

Sea surface temperature: Observations from geostationary satellites

NASA Astrophysics Data System (ADS)

Bates, John J.; Smith, William L.

1985-11-01

A procedure is developed for estimating sea surface temperatures (SST) from multispectral image data acquired from the VISSR atmospheric sounder (VAS) on the geostationary GOES satellites. Theoretical regression equations for two and three infrared window channels are empirically tuned by using clear field of view satellite radiances matched with reports of SST from NOAA fixed environmental buoys from 1982. The empirical regression equations are then used to produce daily regional analyses of SST. The daily analyses are used to study the response of SST's to the passage of Hurricane Alicia (1983) and Hurricane Debbie (1982) and are also used as a first guess surface temperature in the retrieval of atmospheric temperature and moisture profiles over the oceanic regions. Monthly mean SST's for the western North Atlantic and the eastern equatorial Pacific during March and July 1982 were produced for use in the NASA/JPL SST intercomparison workshop series. Workshop results showed VAS SST's have a scatter of 0.8°-1.0°C and a slight warm bias with respect to the other measurements of SST. Subsequently, a second set of VAS/ buoy matches collected during 1983 and 1984 was used to produce a set of bias corrected regression relations for VAS.

The interest of combining micropaleontological and geochemical data for understanding the climate system: the example of the last interglacial-glacial transition

NASA Astrophysics Data System (ADS)

Fernanda Sanchez Goñi, Maria; Bard, Edouard; Landais, Amaelle; Rossignol, Linda

2014-05-01

Theoretical and numerical models predict that rapid ice sheet growth in the North Atlantic high latitudes was the consequence of a) a decrease in summer insolation, b) a strong thermal gradient between ocean and landmasses, and c) moisture generated by persisting warmth and salinity in the subpolar and northern subtropical Atlantic. So far, however, no data have demonstrated the strong land-sea thermal gradient, and how this process was affected by the sub-orbital climatic variability. To fine tune our understanding of this process we examined the MIS 5a/4 transition, between ~80 and 70 thousand years before present (ka), a period marked by decrease in summer insolation and a succession of cooling events, C20 to C19, affecting large parts of the subpolar and central North Atlantic, and Greenland (GS21 to 19). We combined high resolution pollen-based vegetation and foraminifera-based sea surface temperature (SST) data for the interval 85-50 ka, MIS5a-MIS3, from core MD04-2845 located in the Bay of Biscay (northern subtropical gyre, 45°21'N, 5°13'W, 4100 m water depth) with Ice Rafted Debris (IRD), N. pachyderma (s) and benthic foraminifera δ18O records from the same core. This approach allows the identification, without chronological ambiguity, of offsets between eastern North Atlantic Ocean surface hydrology (temperatures and iceberg melting) and atmospherically-driven changes in western European vegetation. The Bay of Biscay palaeoclimatic records were compared with foraminifera and Uk'37-based SST and pollen-based vegetation records from another core, MD99-2331, located in the northwestern Iberian margin. Data from these two cores located in the northern subtropical gyre reveal for the first time a decoupling between atmospheric and oceanic responses to orbital and sub-orbital climatic variability during the last interglacial-glacial transition. We have identified a long-term increase in the thermal gradient (cold land-warm sea) along the western European margin punctuated by three phases of highly pronounced land-sea thermal gradients. We argue that this composite trend was responsible for the production of moisture that continued to feed, via northward tracking storms, northern European, Greenland and Arctic ice sheets during the C20, onset C19 and C18' cold events.

On the Predictability of Northeast Monsoon Rainfall over South Peninsular India in General Circulation Models

NASA Astrophysics Data System (ADS)

Nair, Archana; Acharya, Nachiketa; Singh, Ankita; Mohanty, U. C.; Panda, T. C.

2013-11-01

In this study the predictability of northeast monsoon (Oct-Nov-Dec) rainfall over peninsular India by eight general circulation model (GCM) outputs was analyzed. These GCM outputs (forecasts for the whole season issued in September) were compared with high-resolution observed gridded rainfall data obtained from the India Meteorological Department for the period 1982-2010. Rainfall, interannual variability (IAV), correlation coefficients, and index of agreement were examined for the outputs of eight GCMs and compared with observation. It was found that the models are able to reproduce rainfall and IAV to different extents. The predictive power of GCMs was also judged by determining the signal-to-noise ratio and the external error variance; it was noted that the predictive power of the models was usually very low. To examine dominant modes of interannual variability, empirical orthogonal function (EOF) analysis was also conducted. EOF analysis of the models revealed they were capable of representing the observed precipitation variability to some extent. The teleconnection between the sea surface temperature (SST) and northeast monsoon rainfall was also investigated and results suggest that during OND the SST over the equatorial Indian Ocean, the Bay of Bengal, the central Pacific Ocean (over Nino3 region), and the north and south Atlantic Ocean enhances northeast monsoon rainfall. This observed phenomenon is only predicted by the CCM3v6 model.

Decadal prediction skill in the ocean with surface nudging in the IPSL-CM5A-LR climate model

NASA Astrophysics Data System (ADS)

Mignot, Juliette; García-Serrano, Javier; Swingedouw, Didier; Germe, Agathe; Nguyen, Sébastien; Ortega, Pablo; Guilyardi, Eric; Ray, Sulagna

2016-08-01

Two decadal prediction ensembles, based on the same climate model (IPSL-CM5A-LR) and the same surface nudging initialization strategy are analyzed and compared with a focus on upper-ocean variables in different regions of the globe. One ensemble consists of 3-member hindcasts launched every year since 1961 while the other ensemble benefits from 9 members but with start dates only every 5 years. Analysis includes anomaly correlation coefficients and root mean square errors computed against several reanalysis and gridded observational fields, as well as against the nudged simulation used to produce the hindcasts initial conditions. The last skill measure gives an upper limit of the predictability horizon one can expect in the forecast system, while the comparison with different datasets highlights uncertainty when assessing the actual skill. Results provide a potential prediction skill (verification against the nudged simulation) beyond the linear trend of the order of 10 years ahead at the global scale, but essentially associated with non-linear radiative forcings, in particular from volcanoes. At regional scale, we obtain 1 year in the tropical band, 10 years at midlatitudes in the North Atlantic and North Pacific, and 5 years at tropical latitudes in the North Atlantic, for both sea surface temperature (SST) and upper-ocean heat content. Actual prediction skill (verified against observational or reanalysis data) is overall more limited and less robust. Even so, large actual skill is found in the extratropical North Atlantic for SST and in the tropical to subtropical North Pacific for upper-ocean heat content. Results are analyzed with respect to the specific dynamics of the model and the way it is influenced by the nudging. The interplay between initialization and internal modes of variability is also analyzed for sea surface salinity. The study illustrates the importance of two key ingredients both necessary for the success of future coordinated decadal prediction exercises, a high frequency of start dates is needed to achieve robust statistical significance, and a large ensemble size is required to increase the signal to noise ratio.

Climatic variability of river outflow in the Pantanal region and the influence of sea surface temperature

NASA Astrophysics Data System (ADS)

Silva, Carlos Batista; Silva, Maria Elisa Siqueira; Ambrizzi, Tércio

2017-07-01

This paper investigates possible linear relationships between climate, hydrology, and oceanic surface variability in the Pantanal region (in South America's central area), over interannual and interdecadal time ranges. In order to verify the mentioned relations, lagged correlation analysis and linear adjustment between river discharge at the Pantanal region and sea surface temperature were used. Composite analysis for atmospheric fields, air humidity flux divergence, and atmospheric circulation at low and high levels, for the period between 1970 and 2003, was analyzed. Results suggest that the river discharge in the Pantanal region is linearly associated with interdecadal and interannual oscillations in the Pacific and Atlantic oceans, making them good predictors to continental hydrological variables. Considering oceanic areas, 51 % of the annual discharge in the Pantanal region can be linearly explained by mean sea surface temperature (SST) in the Subtropical North Pacific, Tropical North Pacific, Extratropical South Pacific, and Extratropical North Atlantic over the period. Considering a forecast approach in seasonal scale, 66 % of the monthly discharge variance in Pantanal, 3 months ahead of SST, is explained by the oceanic variables, providing accuracy around 65 %. Annual discharge values in the Pantanal region are strongly related to the Pacific Decadal Oscillation (PDO) variability (with 52 % of linear correlation), making it possible to consider an interdecadal variability and a consequent subdivision of the whole period in three parts: 1st (1970-1977), 2nd (1978-1996), and 3rd (1997-2003) subperiods. The three subperiods coincide with distinct PDO phases: negative, positive, and negative, respectively. Convergence of humidity flux at low levels and the circulation pattern at high levels help to explain the drier and wetter subperiods. During the wetter 2nd subperiod, the air humidity convergence at low levels is much more evident than during the other two drier subperiods, which mostly show air humidity divergence. While the drier periods are particularly characterized by the strengthening of northerly wind over the center of South America, including the Pantanal region, the wetter period is characterized by its weakening. The circulation pattern at 850 hPa levels during the drier subperiods shows anticyclonic anomalies centered over east central South America. Also, the drier subperiods (1st and 3rd) are characterized by negative stream function anomalies over southeastern South America and adjacent South Atlantic, and the wetter subperiod is characterized by positive stream function anomalies. In the three subperiods, one can see mean atmospheric patterns associated with Rossby wave propagation coming from the South Pacific basin—similar to the Pacific South America pattern, but with reverse signals between the wetter and the drier periods. This result suggests a possible relationship between climatic patterns over southeastern South America regions and the Pacific conditions in a decadal scale.

Improved Decadal Climate Prediction in the North Atlantic using EnOI-Assimilated Initial Condition

NASA Astrophysics Data System (ADS)

Li, Q.; Xin, X.; Wei, M.; Zhou, W.

2017-12-01

Decadal prediction experiments of Beijing Climate Center climate system model version 1.1(BCC-CSM1.1) participated in Coupled Model Intercomparison Project Phase 5 (CMIP5) had poor skill in extratropics of the North Atlantic, the initialization of which was done by relaxing modeled ocean temperature to the Simple Ocean Data Assimilation (SODA) reanalysis data. This study aims to improve the prediction skill of this model by using the assimilation technique in the initialization. New ocean data are firstly generated by assimilating the sea surface temperature (SST) of the Hadley Centre Sea Ice and Sea Surface Temperature (HadISST) dataset to the ocean model of BCC-CSM1.1 via Ensemble Optimum Interpolation (EnOI). Then a suite of decadal re-forecasts launched annually over the period 1961-2005 is carried out with simulated ocean temperature restored to the assimilated ocean data. Comparisons between the re-forecasts and previous CMIP5 forecasts show that the re-forecasts are more skillful in mid-to-high latitude SST of the North Atlantic. Improved prediction skill is also found for the Atlantic multi-decadal Oscillation (AMO), which is consistent with the better skill of Atlantic meridional overturning circulation (AMOC) predicted by the re-forecasts. We conclude that the EnOI assimilation generates better ocean data than the SODA reanalysis for initializing decadal climate prediction of BCC-CSM1.1 model.

Precipitation interannual variability and predictions for West Africa from the National Multi-Model Ensemble dataset

NASA Astrophysics Data System (ADS)

Thiaw, W. M.

2013-12-01

The ability of coupled climate models from the national multi-model ensemble (NMME) dataset to reproduce the basic state and interannual variability of precipitation in West Africa and associated teleconnections is examined. The analysis is for the period 1982-2010 for most of the models, which corresponds to the NMME hindcast period, except for the CFS version 1 (CFSv1) which covers the period 1981-2009. The satellite based CPC African Rainfall Climatology (ARC2) data is used as proxy for observed rainfall and to validate the models. We examine rainfall patterns throughout the year. Models are able to reproduce the north-south migration of precipitation from winter and spring when the area of maximum precipitation is located in Central Africa and the Gulf of Guinea region to the summer when it is in northern Sub-Saharan Africa, and the later return to the south. Models also appropriately place precipitation over the Gulf of Guinea region during the equinoxes in MAM and OND. However, there are considerable differences in the representation of the intensities and locations of the rainfall. Three of the models including the two versions of the NCEP CFS and the NASA models also have a systematic dry (wet) bias over the Sahel (Gulf of Guinea region) during the summer rainfall season, while the others show alternating wet and dry biases across West Africa. All models have spatially averaged values of standard deviation lower than that observed. Models are also able to reproduce to some extent the main features of the precipitation variability maximum, but again with deficiencies in the amplitudes and locations. The areas of highest variability are generally depicted, but there are significant differences among the models, and even between the two versions of the CFS. Teleconnections in the models are investigated by first conducting an EOF in the precipitation anomaly fields and then perform a regression of the first or second EOF time series onto the global SST. Focusing on JAS rainfall season, only the CFSv1 and the NASA models were able to depict the dipole pattern between the Sahel and the Gulf of Guinea rainfall. However, none of the models was able to reproduce the observed upward trend of Sahel rainfall in the last decade. The relationship to SST is also examined. The observed influence of tropical north Atlantic SST on the Sahel rainfall is only partially represented even in the CFSv1, while the NASA model inconsistently emphasizes the role of the tropical South Atlantic. A majority of the models show a partial ENSO teleconnection combined with the tropical south Atlantic mode. However, observations indicate that the influence of ENSO on northern Sub-Saharan summer rainfall has been very weak over the past 30 years. Results for MAM, and OND are also presented. The influence of model errors on the predictions of African rainfall is presented. Canonical correlation analysis (CCA) is employed to correct the model simulations. A new ensemble based on models corrected forecasts is then formed and the results are presented.

Characterizing and understanding the climatic determinism of high- to low-frequency variations in precipitation in northwestern France using a coupled wavelet multiresolution/statistical downscaling approach

NASA Astrophysics Data System (ADS)

Massei, Nicolas; Dieppois, Bastien; Hannah, David; Lavers, David; Fossa, Manuel; Laignel, Benoit; Debret, Maxime

2017-04-01

Geophysical signals oscillate over several time-scales that explain different amount of their overall variability and may be related to different physical processes. Characterizing and understanding such variabilities in hydrological variations and investigating their determinism is one important issue in a context of climate change, as these variabilities can be occasionally superimposed to long-term trend possibly due to climate change. It is also important to refine our understanding of time-scale dependent linkages between large-scale climatic variations and hydrological responses on the regional or local-scale. Here we investigate such links by conducting a wavelet multiresolution statistical dowscaling approach of precipitation in northwestern France (Seine river catchment) over 1950-2016 using sea level pressure (SLP) and sea surface temperature (SST) as indicators of atmospheric and oceanic circulations, respectively. Previous results demonstrated that including multiresolution decomposition in a statistical downscaling model (within a so-called multiresolution ESD model) using SLP as large-scale predictor greatly improved simulation of low-frequency, i.e. interannual to interdecadal, fluctuations observed in precipitation. Building on these results, continuous wavelet transform of simulated precipiation using multiresolution ESD confirmed the good performance of the model to better explain variability at all time-scales. A sensitivity analysis of the model to the choice of the scale and wavelet function used was also tested. It appeared that whatever the wavelet used, the model performed similarly. The spatial patterns of SLP found as the best predictors for all time-scales, which resulted from the wavelet decomposition, revealed different structures according to time-scale, showing possible different determinisms. More particularly, some low-frequency components ( 3.2-yr and 19.3-yr) showed a much wide-spread spatial extentsion across the Atlantic. Moreover, in accordance with other previous studies, the wavelet components detected in SLP and precipitation on interannual to interdecadal time-scales could be interpreted in terms of influence of the Gulf-Stream oceanic front on atmospheric circulation. Current works are now conducted including SST over the Atlantic in order to get further insights into this mechanism.

Fidelity of the Sr/Ca proxy in recording ocean temperature in the western Atlantic coral Siderastrea siderea

NASA Astrophysics Data System (ADS)

Kuffner, Ilsa B.; Roberts, Kelsey E.; Flannery, Jennifer A.; Morrison, Jennifer M.; Richey, Julie N.

2017-01-01

Massive corals provide a useful archive of environmental variability, but careful testing of geochemical proxies in corals is necessary to validate the relationship between each proxy and environmental parameter throughout the full range of conditions experienced by the recording organisms. Here we use samples from a coral-growth study to test the hypothesis that Sr/Ca in the coral Siderastrea siderea accurately records sea-surface temperature (SST) in the subtropics (Florida, USA) along 350 km of reef tract. We test calcification rate, measured via buoyant weight, and linear extension (LE) rate, estimated with Alizarin Red-S staining, as predictors of variance in the Sr/Ca records of 39 individual S. siderea corals grown at four outer-reef locations next to in-situ temperature loggers during two, year-long periods. We found that corals with calcification rates < 1.7 mg cm-2 d-1 or < 1.7 mm yr-1 LE returned spuriously high Sr/Ca values, leading to a cold-bias in Sr/Ca-based SST estimates. The threshold-type response curves suggest that extension rate can be used as a quality-control indicator during sample and drill-path selection when using long cores for SST paleoreconstruction. For our corals that passed this quality control step, the Sr/Ca-SST proxy performed well in estimating mean annual temperature across three sites spanning 350 km of the Florida reef tract. However, there was some evidence that extreme temperature stress in 2010 (cold snap) and 2011 (SST above coral-bleaching threshold) may have caused the corals not to record the temperature extremes. Known stress events could be avoided during modern calibrations of paleoproxies.