Tropical Atlantic Sea Surface Temperature Research Articles

A Synoptic Scale Perspective on Greenland Ice Core δ18O Variability and Related Teleconnection Patterns

The variability of stable oxygen isotope ratios (δ18O) from Greenland ice cores is commonly linked to changes in local climate and associated teleconnection patterns. In this respect, in this study we investigate ice core δ18O variability from a synoptic scale perspective to assess the potential of such records as proxies for extreme climate variability and associated weather patterns. We show that positive (negative) δ18O anomalies in three southern and central Greenland ice cores are associated with relatively high (low) Rossby Wave Breaking (RWB) activity in the North Atlantic region. Both cyclonic and anticyclonic RWB patterns associated with high δ18O show filaments of strong moisture transport from the Atlantic Ocean towards Greenland. During such events, warm and wet conditions are recorded over southern, western and central part of Greenland. In the same time the cyclonic and anticyclonic RWB patterns show enhanced southward advection of cold polar air masses on their eastern side, leading to extreme cold conditions over Europe. The association between high δ18O winters in Greenland ice cores and extremely cold winters over Europe is partly explained by the modulation of the RWB frequency by the tropical Atlantic sea surface temperature forcing, as shown in recent modeling studies. We argue that δ18O from Greenland ice cores can be used as a proxy for RWB activity in the Atlantic European region and associated extreme weather and climate anomalies.

Understanding the variability of the rainfall dipole in West Africa using the EC-Earth last millennium simulation

There is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

Impact of the Benguela coastal low-level jet on the southeast tropical Atlantic SST bias in a regional ocean model

The current generation of coupled ocean–atmosphere climate models, which are widely used for studying the dynamics of the climate system and to make projections of future climate, suffers from erroneously warm (exceeding 5 °C) sea surface temperature (SST) simulation in the eastern boundary upwelling systems (EBUS) in the Atlantic and Pacific oceans. While recent improvements in the horizontal resolution of coupled model components helped to alleviate this issue in the North and South Pacific and North Atlantic oceans, the warm bias in the Southeast Tropical Atlantic (SETA) region still remains above 5 °C. Recent studies have highlighted the importance of inaccuracies in the near-coastal winds, especially in the Benguela low-level coastal jet (BLLCJ), in the formation of this warm SST bias. In addition, unique ocean circulation features like the poleward flowing coastal current and the presence of a strong coastal SST front were argued to contribute to the complexity in simulating realistic SST in the SETA region. In this study, we investigate how ocean model resolution and the spatial structure and strength of the BLLCJ affect the SST warm bias in the SETA. We conducted a suite of ocean model experiments with varying horizontal resolutions (81–3 km) and different atmospheric forcing products. We found that the accuracy in the magnitude of BLLCJ’s alongshore component and in the spatial structure of BLLCJ are the primary factors in reducing the warm SST bias in SETA and the ocean model horizontal resolution is of only secondary importance. A weaker alongshore component of the BLLCJ leads to a weaker upwelling through Ekman transport and fails to generate a downwind equatorward surface jet in the ocean. When the core of the BLLCJ is too far offshore, the broad wind stress curl zone gives rise to anomalous poleward depth integrated flow through Sverdrup balance and weakens the wind stress curl induced Ekman pumping near the coast. Both the presence of a poleward current, which can transport warm equatorial waters farther south, and the absence of the equatorward surface jet contributes to the warm SST bias. Heat budget analysis shows that the contribution from the anomalous poleward flow is a bigger contributor to the warm bias than the weak upwelling. With more realistic, high-resolution winds, an ocean model resolution increase from 27 to 9 km slightly reduces the extent and magnitude of SST bias. However, no such improvement occurs with coarse resolution less realistic winds. Sensitivity experiments using atmospheric forcing fields from different sources confirm that the major contribution to the warm SST bias is in fact from the erroneous winds rather than other atmospheric forcing fields like net shortwave and downward longwave radiation. Further studies are needed to understand whether eddy heat advection or mean flow heat advection primarily cause the warm SST bias.

Different impacts of spring tropical Atlantic SST anomalies on Eurasia spring climate during the periods of 1970–1995 and 1996–2018

We investigate impacts of spring tropical Atlantic sea surface temperature anomalies (SSTAs) on Eurasian spring climate during the period of 1961–2018, using both observations and Community Atmosphere Model (CAM) simulations. Observations during the study period show that the tropical Atlantic sea surface temperature (SST) experienced an abrupt warming around 1995. Distinctive impacts of spring tropical Atlantic SSTAs on Eurasian spring surface air temperature (SAT) and precipitation during the period 1970–1995 (P1) and 1996–2018 (P2) are revealed. During P1, spring tropical Atlantic SSTAs were highly correlated with spring SAT anomalies over Northern Europe, Southeast China, and Indochina Peninsula. These high correlations disappeared during P2; instead, high correlations dominated over North-Northeastern China, and Central Siberia in spring. The high correlation patterns between spring tropical Atlantic SSTAs and spring Eurasian precipitation of these two periods also differed. The interannual relationship between spring tropical Atlantic SSTAs and Eurasian SAT/precipitation during P2 was mainly due to the existence of an anomalous atmospheric wave train across the tropical Atlantic and Eurasia, which was absent during P1. These observational results are confirmed by a set of CAM simulations.

Atlantic and Pacific tropics connected by mutually interactive decadal-timescale processes

Decadal climate prediction presumes there are decadal-timescale processes and mechanisms that, if initialized properly in models, potentially provide predictive skill more than one or two years into the future. Candidate mechanisms involve Pacific decadal variability and Atlantic multidecadal variability, elements of which involve slow fluctuations of tropical Pacific and Atlantic sea surface temperatures (SSTs) from positive anomalies (positive phase) to negative anomalies (negative phase). Here we use model experiments to show that there tends to be a weak opposite-sign SST response in the tropical Pacific when observed SSTs are specified in the Atlantic, while there is a weak same-sign SST response in the tropical Atlantic when observed SSTs are specified in the tropical Pacific. Net surface heat flux in the Atlantic and ocean dynamics in the Pacific play contrasting roles in the ocean response to specified SSTs in the respective basins. We propose that processes in the Pacific and Atlantic are sequentially interactive through the atmospheric Walker circulation along with contributions from midlatitude teleconnections for the Atlantic response to the Pacific. Atmospheric Walker circulation results in a two-way interaction between decadal-scale sea surface temperature variability in the Atlantic and Pacific, according to pacemaker climate modelling experiments.

Performance of the IPCC AR6 models in simulating the relation of the western North Pacific subtropical high to the spring northern tropical Atlantic SST

AbstractThis study examines the relationship between the spring northern tropical Atlantic (NTA) sea surface temperature (SST) and its following summer western North Pacific subtropical high (WNPSH) in historical simulations of 20 coupled models of the IPCC AR6. These models well simulate the spring NTA SST, as well as the WNPSH‐related atmospheric and precipitation anomalies over the Asian monsoon region. Ensemble mean of these models reproduces the observed relation and processes linking the spring NTA SST to the summer WNPSH. In the ensemble mean, spring NTA SST warming persists to the following summer, and induces an anomalous Walker circulation with an ascent over the tropical Atlantic and a descent over the tropical central Pacific. The associated precipitation decrease over the tropical central Pacific induces an anomalous low‐level anticyclone over the WNP via a Rossby wave atmospheric response. Meanwhile, spring NTA SST warming leads to easterly wind anomalies over the Indian Ocean (IO) via a Kelvin wave atmospheric response and causes SST warming there via reducing wind speeds. Then, the precipitation anomalies induced by the IO SST warming result in an enhancement of the WNPSH. There exists a large diversity of the NTA SST‐WNPSH relationship among the models. This diversity is related to the difference in the climatological mean IO precipitation. The models with large mean IO precipitation provide a favourable condition for the IO SST warming to bring large local precipitation anomalies and trigger strong easterly wind anomalies over the tropical WNP, and thus exert a strong impact on the WNPSH.

Increase in Lower Stratospheric Water Vapor in the Past 100 Years Related to Tropical Atlantic Warming

AbstractLower Stratospheric water vapor (SWV) is one of important drivers of global climate change. Increases and decreases in lower SWV have been found to strengthen and offset global warming effects, respectively. Using several data sets, we find that sea surface temperature (SST) warming in the past 100 years has caused an increase in SWV. SST warming over the tropical Indian Ocean and the western Pacific has resulted in a drier stratosphere. However, tropical Atlantic Ocean warming has resulted in a significantly wetter stratosphere and is the main contributor to the increasing trend of SWV in the past 100 years. The responses of Rossby and Kelvin waves over the Indian Ocean and western Pacific to Atlantic warming have led to a warmer tropopause temperature, resulting in more water vapor entering the stratosphere. This study suggests that SWV trend may simply be the result of a game between warm pool SST and tropical Atlantic SST changes.

Origin of Weakened Interannual Sea Surface Temperature Variability in the Southeastern Tropical Atlantic Ocean

AbstractObservations and reanalysis products are used to investigate the substantial weakening in the southeastern tropical Atlantic sea surface temperature (SST) variability since 2000. Relative to 1982–1999, the March‐April‐May SST variability in the Angola‐Benguela area (ABA) has decreased by more than 30%. Both equatorial remote forcing and local forcing are known to play an important role in driving SST variability in the ABA. Compared to 1982–1999, since 2000, equatorial remote forcing had less influence on ABA SSTs, whereas local forcing has become more important. In particular, the robust correlation that existed between the equatorial zonal wind stress and the ABA SSTs has substantially weakened, suggesting less influence of Kelvin waves on ABA SSTs. Moreover, the strong correlation linking the South Atlantic Anticyclone and the ABA SSTs has reduced. Finally, multidecadal surface warming of the ABA could also have played a role in the weakening of the interannual SST variability.

The Record-Breaking 1933 Atlantic Hurricane Season

AbstractThe 1933 Atlantic hurricane season was extremely active, with 20 named storms and 11 hurricanes including 6 major (category 3+; 1-min maximum sustained winds ≥96 kt) hurricanes occurring. The 1933 hurricane season also generated the most accumulated cyclone energy (an integrated metric that accounts for frequency, intensity, and duration) of any Atlantic hurricane season on record. A total of 8 hurricanes tracked through the Caribbean in 1933—the most on record. In addition, two category 3 hurricanes made landfall in the United States just 23 h apart: the Treasure Coast hurricane in southeast Florida followed by the Cuba–Brownsville hurricane in south Texas. This manuscript examines large-scale atmospheric and oceanic conditions that likely led to such an active hurricane season. Extremely weak vertical wind shear was prevalent over both the Caribbean and the tropical Atlantic throughout the peak months of the hurricane season, likely in part due to a weak-to-moderate La Niña event. These favorable dynamic conditions, combined with above-normal tropical Atlantic sea surface temperatures, created a very conducive environment for hurricane formation and intensification. The Madden–Julian oscillation was relatively active during the summer and fall of 1933, providing subseasonal conditions that were quite favorable for tropical cyclogenesis during mid- to late August and late September to early October. The current early June and August statistical models used by Colorado State University would have predicted a very active 1933 hurricane season. A better understanding of these extremely active historical Atlantic hurricane seasons may aid in anticipation of future hyperactive seasons.

Resolution dependence of CO2-induced Tropical Atlantic sector climate changes

A long-standing problem in state-of-the-art climate models is the Tropical Atlantic (TA) warm sea surface temperature (SST) bias, which goes along with major biases in large-scale atmospheric circulation. Here we show that TA-sector climate changes forced by increasing atmospheric carbon dioxide (CO2) levels are sensitive to model resolution. Two versions of a climate model employing greatly varying atmospheric resolution and exhibiting very different warm bias strength are compared. The version with high atmospheric resolution features a small SST bias and simulates an eastward amplified SST warming over the equatorial Atlantic, in line with the observed SST trends since the mid-20th century. On the contrary, the version with coarse atmospheric resolution exhibits a large SST bias and projects relatively uniform SST changes across the equatorial Atlantic. In both model versions, the warming pattern resembles the pattern of interannual SST variability simulated under present-day conditions. Atmospheric changes also vastly differ among the two climate model versions. In the version with small SST bias, a deep atmospheric response is simulated with a major change in the Walker circulation and strongly enhanced rainfall over the equatorial region, whereas the atmospheric response is much weaker and of rather different character in the model with large SST bias. This study suggests that higher atmospheric resolution in climate models may enhance global warming projections over the TA sector.

Cooperative effects of tropical Pacific and Atlantic SST forcing in southern China winter precipitation variability

This study investigates the variability of winter precipitation over southern China (SC) and the impacts of sea surface temperature (SST) forcing in the tropical Pacific and Atlantic Oceans. The SC winter precipitation displays two major spatial modes: a same-sign mode with the maximum center located in southeastern China, and a north–south dipole mode with opposite variations between the lower reach of the Yangtze River and the southeastern coast of China. Both modes are associated with the variation of the western North Pacific (WNP) anticyclone. The same-sign mode is associated with canonical El Nino through the Pacific-East Asian teleconnection in the lower troposphere. The north–south dipole mode is subject to the impact of the western Pacific warm pool associated with the central Pacific El Nino and equatorial Atlantic forcing. The influence of equatorial Atlantic warming is through an upper-tropospheric zonal wave pattern from the tropical Atlantic to the mid-latitudes of East Asia. Both the tropical Pacific and Atlantic oceanic forcing induce a north–south dipole pattern of winter precipitation anomalies over SC by affecting the northward extension of the WNP anticyclone. Moreover, the observed statistical relationship between the dipole mode and the equatorial Atlantic SST anomalies is present in hindcasts/forecasts of the NCEP Climate Forecast System version 2 (CFSv2), confirming a potential source of the predictability of the north–south dipole-like variation of the winter precipitation in SC.

An early-warning indicator for Amazon droughts exclusively based on tropical Atlantic sea surface temperatures

Droughts in tropical South America have an imminent and severe impact on the Amazon rainforest and affect the livelihoods of millions of people. Extremely dry conditions in Amazonia have been previously linked to sea surface temperature (SST) anomalies in the adjacent tropical oceans. Although the sources and impacts of such droughts have been widely studied, establishing reliable multi-year lead statistical forecasts of their occurrence is still an ongoing challenge. Here, we further investigate the relationship between SST and rainfall anomalies using a complex network approach. We identify four ocean regions which exhibit the strongest overall SST correlations with central Amazon rainfall, including two particularly prominent regions in the northern and southern tropical Atlantic. Based on the time-dependent correlation between SST anomalies in these two regions alone, we establish a new early-warning method for droughts in the central Amazon basin and demonstrate its robustness in hindcasting past major drought events with lead-times up to 18 months.

The Southeastern Tropical Atlantic SST Bias Investigated with a Coupled Atmosphere–Ocean Single-Column Model at a PIRATA Mooring Site

AbstractWarm sea surface temperature (SST) biases in the tropical Atlantic Ocean form a longstanding problem in coupled general circulation models (CGCMs). Considerable efforts to understand the origins of these biases and alleviate them have been undertaken, but state-of-the-art CGCMs still suffer from biases that are very similar to those of the generation of models before. In this study, we use a powerful combination of in situ moored buoy observations and a new coupled ocean–atmosphere single-column model (SCM) with parameterization that is identical to that of a three-dimensional CGCM to investigate the SST bias. We place the SCM at the location of a Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) mooring in the southeastern tropical Atlantic, where large SST biases occur in CGCMs. The SCM version of the EC-Earth state-of-the-art coupled GCM performs well for the first five days of the simulation. Then, it develops an SST bias that is very similar to that of its three-dimensional counterpart. Through a series of sensitivity experiments we demonstrate that the SST bias can be reduced by 70%. We achieve this result by enhancing the turbulent vertical ocean mixing efficiency in the ocean parameterization scheme. The underrepresentation of vertical mixing in three-dimensional CGCMs is a candidate for causing the warm SST bias. We further show that surface shortwave radiation does not cause the SST bias at the location of the PIRATA mooring. Rather, a warm atmospheric near-surface temperature bias and a wet moisture bias contribute to it. Strongly nudging the atmosphere to profiles from reanalysis data reduces the SST bias by 40%.

Relationship between Rainfall Pattern in North Central and Southern Nigeria and Some Tropical Climate Systems

Monthly rainfall data from meteorological stations in Nigeria are analyzed from 1951 to 1992, to establish their relationship with some Tropical climate systems. The climate systems include Tropical South Atlantic (TSA) sea surface temperature index, North Atlantic Ocean (NAO) atmospheric index, Tropical North Atlantic (TNA) sea surface temperature index, Central India Precipitation (CIP) and and Outgoing Longwave Radiation Anomaly (OLRA)). The rainfall pattern was given in terms of the August break (quantified in terms of the monsoon break intensity (MBI) and annual rainfall anomaly index (RAI). Partial correlation analysis was carried out to determine the effect of the tropical climate systems on the linear association between them and the stations in Nigeria. Our results show that the rainfall anomalies although sometimes intense do not have predictable patterns. The tele connection between CIP and total rainfall in Nigeria suggests that the rainfall patterns in Nigeria is likely to be modulated by the Tropical Easterly Jet (TEJ) connecting rainfall pattern in Central India to that in Nigeria. The August break is observed to be highly variable and does not show a clear discernable pattern of variability. Its variability may be connected with multiple forcings from ocean and mesoscale circulations.

Pacific decadal oscillation remotely forced by the equatorial Pacific and the Atlantic Oceans

The Pacific Decadal Oscillation (PDO), the leading mode of Pacific decadal sea surface temperature variability, arises mainly from combinations of regional air-sea interaction within the North Pacific Ocean and remote forcing, such as from the tropical Pacific and the Atlantic. Because of such a combination of mechanisms, a question remains as to how much PDO variability originates from these regions. To better understand PDO variability, the equatorial Pacific and the Atlantic impacts on the PDO are examined using several 3-dimensional partial ocean data assimilation experiments conducted with two global climate models: the CESM1.0 and MIROC3.2m. In these partial assimilation experiments, the climate models are constrained by observed temperature and salinity anomalies, one solely in the Atlantic basin and the other solely in the equatorial Pacific basin, but are allowed to evolve freely in other regions. These experiments demonstrate that, in addition to the tropical Pacific’s role in driving PDO variability, the Atlantic can affect PDO variability by modulating the tropical Pacific climate through two proposed processes. One is the equatorial pathway, in which tropical Atlantic sea surface temperature (SST) variability causes an El Niño-like SST response in the equatorial Pacific through the reorganization of the global Walker circulation. The other is the north tropical pathway, where low-frequency SST variability associated with the Atlantic Multidecadal Oscillation induces a Matsuno-Gill type atmospheric response in the tropical Atlantic-Pacific sectors north of the equator. These results provide a quantitative assessment suggesting that 12–29% of PDO variance originates from the Atlantic Ocean and 40–44% from the tropical Pacific. The remaining 27–48% of the variance is inferred to arise from other processes such as regional ocean-atmosphere interactions in the North Pacific and possibly teleconnections from the Indian Ocean.

Contributions of tropical-extratropical oceans to the prediction skill of ENSO after 2000

The skill of most ENSO prediction models has declined significantly since 2000. This decline may be due to a weakening of the correlation between tropical predictors and ENSO. Moreover, the effects of extratropical ocean variability on ENSO have increased during this period. To improve ENSO predictability, the authors investigate the influence of the extratropical Atlantic and Pacific oceans on ENSO during the pre-2000 and post-2000 periods, and find that the influence of the northern tropical Atlantic sea surface temperature (NTA SST) on ENSO has significantly increased since 2000. Furthermore, there is a much earlier and stronger correlation between NTA SST and ENSO over the central-eastern Pacific during June-July-August in the post-2000 period compared with the pre-2000 period. The extratropical Pacific SST predictors for ENSO retain an approximate 10-month lead time after 2000. The authors use SST signals in the extratropical Atlantic and Pacific to predict ENSO using a statistical prediction model. This results in a significant improvement in ENSO prediction skill and an obvious decrease in the spring predictability barrier phenomenon of ENSO. These results indicate that extratropical Atlantic and Pacific SSTs can make substantial contributions to ENSO prediction, and can be used to enhance ENSO predictability after 2000.

The Impact of Changes in Tropical Sea Surface Temperatures over 1979–2012 on Northern Hemisphere High-Latitude Climate

AbstractWhile rapid changes in Arctic climate over recent decades are widely documented, the importance of different driving mechanisms is still debated. A previous study proposed a causal connection between recent tropical Pacific sea surface temperature (SST) trends and circulation changes over northern Canada and Greenland (NCG). Here, using the HadGEM3-A model, we perform a suite of sensitivity experiments to investigate the influence of tropical SSTs on winter atmospheric circulation over NCG. The experiments are forced with observed SST changes between an “early” (1979–88) and “late” period (2003–12) and applied across the entire tropics (TropSST), the tropical Pacific (PacSST), and the tropical Atlantic (AtlSST). In contrast to the previous study, all three experiments show a negative 200-hPa eddy geopotential height (Z200) anomaly over NCG in winter, which is similar to the response in AMIP experiments from four other climate models. The positive Z200 NCG anomaly in ERA-Interim between the two periods is inside the bounds of internal variability estimated from bootstrap sampling. The NCG circulation anomaly in the TropSST experiment is associated with a Rossby wave train originating from the tropical Pacific, with an important contribution coming from the tropical Atlantic SSTs connected via an atmospheric bridge through the tropical Pacific. This generates anomalous upper-level convergence and a positive Rossby wave source anomaly near the North Pacific jet exit region. Hence, while a tropics–Arctic teleconnection is evident, its influence on recent Arctic regional climate differs from observed changes and warrants further research.

Impacts of different types of El Niño and La Niña on northern tropical Atlantic sea surface temperature

The present study investigates the dependence of the northern tropical Atlantic (NTA) sea surface temperature (SST) response to El Nino and La Nina events on the decay time and amplitude of tropical Pacific SST anomalies. It is found that the NTA SST response to La Nina events displays a notable difference between late and early decaying events, similar to that in response to El Nino events, but with a weaker signal. Latent heat flux is a dominant term in the NTA SST change in preceding winter-early spring in both El Nino and La Nina events and in the difference of the NTA SST anomaly between late and early decaying El Nino and La Nina events. The zonal and meridional advections have an opposite effect on the NTA SST warming in late decaying El Nino events. Although the warming in the NTA region is similar in late decaying moderate and strong El Nino events, the distribution of the SST anomalies in the mid-latitude North Atlantic Ocean shows a notable difference between the two types of late decaying El Nino events. The SST anomalies also display difference in the early decaying weak and moderate El Nino events. Surface heat flux differences are largely attributed to wind differences.

Reintensification of the Anomalous Western North Pacific Anticyclone during the El Niño Modoki Decaying Summer: Relative Importance of Tropical Atlantic and Pacific SST Anomalies

AbstractThe El Niño Modoki–induced anomalous western North Pacific anticyclone (WNPAC) undergoes an interesting reintensification process in the El Niño Modoki decaying summer, the period when El Niño Modoki decays but warm sea surface temperature (SST) anomalies over the tropical North Atlantic (TNA) and cold SST anomalies over the central-eastern Pacific (CEP) dominate. In this study, the region (TNA or CEP) in which the SST anomalies exert a relatively important influence on reintensification of the WNPAC is investigated. Observational analysis demonstrates that when only anomalous CEP SST cooling occurs, the WNPAC experiences a weak reintensification. In contrast, when only anomalous TNA SST warming emerges, the WNPAC experiences a remarkable reintensification. Numerical simulation analysis demonstrates that even though the same magnitude of CEP SST cooling and TNA warming is respectively set to force the atmospheric general circulation model, the response of the WNPAC is still much stronger in the TNA warming experiment than in the CEP cooling experiment. Further analysis demonstrates that this difference is caused by the distinct location of the effective tropical forcing between the CEP SST cooling and TNA SST warming for producing a WNPAC. The CEP cooling-induced effective anomalous diabatic cooling is located in the central Pacific, by which the forced anticyclone becomes gradually weak from the central Pacific to the western North Pacific. Thus, a weak WNPAC is produced. In contrast, as the TNA SST warming–induced effective anomalous diabatic cooling is just located in the western North Pacific via a Kelvin wave–induced Ekman divergence process, the forced anticyclone is significant and powerful in the western North Pacific.

Forcing of western tropical South Atlantic sea surface temperature across three glacial-interglacial cycles

Abstract The western tropical Atlantic (WTA) supplies warm and saline waters to the upper-limb of the Atlantic Meridional Overturning Circulation (AMOC) and may store excess heat and salinity during periods of AMOC slowdown. Since previous sea surface temperature (SST) reconstructions from the WTA typically focus on the Last Glacial Maximum and the last deglaciation, additional long-term records spanning multiple glacial-interglacial transitions are needed in order to elucidate the drivers of long-term WTA SST variability. We performed Mg/Ca analyses on the surface-dwelling planktic foraminifera Globigerinoides ruber (pink) from a sediment core raised from the southern WTA to reconstruct SST changes over the past 322 kyr. We evaluate the relative importance of atmospheric pCO2, AMOC strength and trade-wind intensity in driving the thermal evolution of the WTA across three glacial-interglacial cycles. Our results indicate a lack of pronounced glacial-interglacial variability in the SST record, prompting us to exclude atmospheric pCO2 as a direct driver of SST variations in the southern WTA. Similarly, we conclude that variations in AMOC strength also likely did not have a strong influence on long-term WTA SST, based on the low and relatively stable interhemispheric SST gradient over the past 322 kyr. Our results reveal high-amplitude variability in zonal SST gradients within the (sub)tropical South Atlantic and similarities between the long-term patterns of the intrahemispheric meridional SST gradient and our SST record. Based on these findings, we propose that changes in the intrahemispheric meridional SST gradient modulate southeast trade wind intensity, which in turn drives variations in zonal SST gradients and southern WTA SSTs.