Atlantic Sea Surface Temperature Variability Research Articles

The Impact of Interactive Ocean Dynamics on Atlantic Sea Surface Temperature Variability

Abstract The impact of interactive ocean dynamics on internal variations of Atlantic sea surface temperature (SST) is investigated by comparing preindustrial control simulations of a fully coupled atmosphere–ocean–ice model to the same atmosphere–ice model with the ocean replaced by a motionless slab layer (henceforth slab ocean model). Differences in SST variability between the two models are diagnosed by an optimization technique that finds components whose variance differs as much as possible. This technique reveals that Atlantic SST variability differs significantly between the two models. The two components with the most extreme enhancement of SST variance in the slab ocean model resemble the tripole SST pattern associated with the North Atlantic Oscillation (NAO) and the Atlantic multidecadal variability (AMV) pattern. This result supports previous claims that ocean dynamics are not necessary for the AMV, although ocean dynamics lead to slight increases in the memory of both the AMV and the NAO tripole. The component with the most extreme enhancement of SST variance in the fully coupled model resembles the Atlantic Niño pattern, confirming the ability of our technique to isolate physical modes known to require ocean dynamics. The second component with more variance in the fully coupled model is a mode of subpolar SST variability. Both the reemergence of SST anomalies and changes in ocean heat transport lead to increased SST variance and memory in the subpolar Atlantic. Despite large differences in the mean and variability of SST, atmospheric variability is quite similar between the two models, confirming that most atmospheric variability is generated by internal atmospheric dynamics.

Investigating Atmospheric Responses to and Mechanisms Governing North Atlantic Sea Surface Temperatures over 10-Year Periods

Abstract North Atlantic sea surface temperature (SST) variability plays a critical role in modulating the climate system. However, characterizing patterns of North Atlantic SST variability and diagnosing the associated mechanisms is challenging because they involve coupled atmosphere–ocean interactions with complex spatiotemporal relationships. Here we address these challenges by applying a time-evolving self-organizing map approach to a long preindustrial coupled control simulation and identify a variety of 10-yr spatiotemporal evolutions of winter SST anomalies, including but not limited to those associated with the North Atlantic Oscillation–Atlantic multidecadal variability (NAO–AMV)-like interactions. To assess mechanisms and atmospheric responses associated with various SST spatiotemporal evolutions, composites of atmospheric and oceanic variables associated with these evolutions are investigated. Results show that transient-eddy activities and atmospheric circulation responses exist in almost all the evolutions that are closely correlated to the details of the SST pattern. In terms of the mechanisms responsible for generating various SST evolutions, composites of ocean heat budget terms demonstrate that contributions to upper-ocean temperature tendency from resolved ocean advection and surface heat fluxes rarely oppose each other over 10-yr periods in the subpolar North Atlantic. We further explore the potential for predictability for some of these 10-yr SST evolutions that start with similar states but end with different states. However, we find that these are associated with abrupt changes in atmospheric variability and are unlikely to be predictable. In summary, this study broadly investigates the atmospheric responses to and the mechanisms governing the North Atlantic SST evolutions over 10-yr periods. Significance Statement Climate variability in the North Atlantic Ocean has wide-ranging impacts on global and regional climate. However, the processes involved include interactions between the ocean and atmosphere that vary across both space and time, making it challenging to characterize and predict. Using a novel machine learning approach, this study identifies various time evolutions of North Atlantic sea surface temperature patterns over 10-yr periods. This includes evolutions with similar start states but different trajectories, which have important implications for predictability. Furthermore, we investigate the mechanisms responsible for these evolutions and how different sea surface temperature patterns affect atmospheric circulation through small-scale atmospheric disturbances. These new insights into the complex ocean–atmosphere interactions over time are critical for improving decadal prediction skill.

Connecting North Atlantic SST Variability to European Heat Events over the Past Decades

The occurrence of European heat events has increased during the last two decades. European heat events are responsible for social, economic and environmental damage and are projected to increase in magnitude, frequency and duration under global warming, strengthening the interest about the contribution of different mechanisms. Using the ERA5 reanalysis product, we go beyond case studies relating European heat events with cold North Atlantic sea surface temperatures (SSTs), and perform a systematic approach with a composite analysis to investigate the link between North Atlantic SSTs in a domain south of Greenland and the 2m air temperature (T2m) over central Europe. Composites of different North Atlantic SST states show that events with a negative tendency of North Atlantic SSTs are often followed by positive European T2m anomalies during summers when the North Atlantic SSTs are persistently low for several months. Enhanced lower–tropospheric baroclinicity in the North Atlantic is followed during these events by a slantwise ascent and an enhanced upper–tropospheric waveguide, promoting a downstream development of an European ridge. A combination of a wave number 3 pattern and regionally confined Rossby wave activity contribute to a trough–ridge pattern in the North Atlantic–European sector. A composite of European heat events further confirms the lagged statistical relationship between cold North Atlantic SSTs with a negative tendency and positive European T2m anomalies. A negative tendency of North Atlantic SSTs precedes 15 of 18 European heat events, and cold North Atlantic SST conditions are present during 14 of 18 European heat events.

Assessing the Spatiotemporal Patterns and Impacts of Droughts in the Orinoco River Basin Using Earth Observations Data and Surface Observations

Droughts impact the water cycle, ecological balance, and socio-economic development in various regions around the world. The Orinoco River Basin is a region highly susceptible to droughts. The basin supports diverse ecosystems and supplies valuable resources to local communities. We assess the spatiotemporal patterns and impacts of droughts in the basin using remote sensing data and surface observations. We use monthly precipitation (P), air temperature near the surface (T2M), enhanced vegetation index (EVI) derived from Earth observations, and average daily flow (Q) data to quantify drought characteristics and impacts. We also investigated the association between drought and global warming by correlating the drought intensity and the percentage of dry area with sea surface temperature (SST) anomalies in the Pacific (Niño 3.4 index), Atlantic (North Atlantic Index [NATL]), and South Atlantic Index [SATL]) oceans. We evaluate the modulating effect of droughts on the hydrological regime of the most relevant tributaries by calculating the trend and significance of the regional standardized precipitation index (SPI) and percentage area affected by dry conditions. El Niño events worsen the region’s drought conditions (SPI vs. Niño 3.4 index, r = −0.221), while Atlantic SST variability has less influence on the basin’s precipitation regime (SPI vs. NATL and SATL, r = 0.117 and −0.045, respectively). We also found that long-term surface warming trends aggravate drought conditions (SPI vs. T2M anomalies, r = −0.473), but vegetation greenness increases despite high surface temperatures (SPI vs. EVI anomalies, r = 0.284). We emphasize the irregular spatial-temporal patterns of droughts in the region and their profound effects on the ecological flow of rivers during prolonged hydrological droughts. This approach provides crucial insights into potential implications for water availability, agricultural productivity, and overall ecosystem health. Our study underlines the urgent need for adaptive management strategies to mitigate the adverse effects of droughts on ecosystems and human populations. The insights derived from our study have practical implications for developing strategies to address the impacts of droughts and ensure the protection of this ecologically significant region.

Buoyancy forcing: a key driver of northern North Atlantic sea surface temperature variability across multiple timescales

Abstract. Analyses of observational data (from year 1870 AD) show that sea surface temperature (SST) anomalies along the pathway of Atlantic Water transport in the North Atlantic, the Norwegian Sea and the Iceland Sea are spatially coherent at multidecadal timescales. Spatially coherent SST anomalies are also observed over hundreds of thousands of years during parts of the Pliocene (5.23–5.03, 4.63–4.43, and 4.33–4.03 Ma). However, when investigating CMIP6 (Coupled Model Intercomparison Project 6) SSP126 (Shared Socioeconomic Pathway) future scenario runs (next century) and other Pliocene time intervals, the following three additional SST relations emerge: (1) the Norwegian Sea SST anomaly is dissimilar to the North Atlantic and the Iceland Sea SST anomalies (Pliocene; 4.93–4.73 and 3.93–3.63 Ma), (2) the Iceland Sea SST anomaly is dissimilar to the North Atlantic and the Norwegian Sea SST anomalies (Pliocene; 3.43–3.23 Ma), and (3) the North Atlantic SST anomaly is dissimilar to the SST anomalies of the Norwegian and Iceland seas (future trend). Hence, spatially non-coherent SST anomalies may occur in equilibrium climates (Pliocene), as well as in response to transient forcing (CMIP6 SSP126 low-emission future scenario). Since buoyancy is a key forcing for the inflow of Atlantic Water to the Norwegian Sea, we investigate the impacts of buoyancy forcing on spatial relations between SST anomalies seen in the North Atlantic and the Norwegian and Iceland seas. This is done by performing a range of idealized experiments using the Massachusetts Institute of Technology general circulation model (MITgcm). Through these idealized experiments we can reproduce three out of four of the documented SST anomaly relations: being spatially coherent under weak to intermediate freshwater forcing over the Nordic Seas, the Iceland Sea being dissimilar to the North Atlantic and the Norwegian Sea under weak atmospheric warming over the Nordic Seas, and the North Atlantic being dissimilar to the Norwegian and Iceland seas under strong atmospheric warming over the Nordic Seas. We suggest that the unexplained SST anomaly relation, when the Norwegian Sea is dissimilar to the North Atlantic and the Iceland Sea, may reflect a response to a weakened Norwegian Atlantic Current compensated for by a strong Irminger Current or an expanded East Greenland Current.

Manifestation of the Early 20th Century Warming in the East-European Plain: Atmospheric Circulation Anomalies and Its Connection to the North Atlantic SST and Sea Ice Variability

A study of the climatic characteristics and annual runoff of the Volga and Severnaya Dvina rivers demonstrates that, on the East European Plain (EEP), Early Twentieth Century Warming (ETCW) manifested in a multiyear drought between 1934 and 1940; this drought has no analogues in this region in terms of intensity and duration according to Palmer’s classification, and caused extreme hydrological events. The circulation conditions during this event were characterized by an extensive anticyclone over Eastern Europe, combined with a cyclonic anomaly in the circumpolar region. An analysis of the spatial features of sea surface temperature (SST) anomalies indicate that the surface air temperature (SAT) anomalies in July on the EEP during ETCW were related not only to the North Atlantic (NA) warming and positive AMO phase, but also to a certain spatial pattern of SST anomalies characteristic of the 1920–1950 period. The difference between the SST anomalies of the opposite sign in the different NA zones, used as the indicator of the obtained spatial pattern, shows the quite close relations between the July SAT anomalies on the EEP and the atmospheric circulation patterns responsible for them. The positive phase of the Atlantic Multidecadal Oscillation (AMO) and the expansion of the subtropical high-pressure belt to the north and to the east can be considered as global-scale drivers of this phenomenon. The AMO also impacts the sea ice cover in the Barents–Kara Sea region, which, in turn, could have led to specific atmospheric circulation patterns and contributed to droughts on the EEP in the 1930s.

Carbon monoxide profile variability over the Manaus Metropolitan Region and its relations with biomass burning

The present study analyzes the temporal variability of carbon monoxide (CO) over the Manaus Metropolitan Region (MMR) and its relations with nearby fires based on data obtained by the environmental satellite AQUA, for the 2003–2020 period. For this purpose, wavelet transform analyses and wavelet coherence analyses were used. The results show a well-defined seasonal behavior, with an increase and decrease in mean CO concentrations during dry and wet seasons, respectively. Semiannual and annual scales represent around 95 % of CO temporal variability in lower troposphere (500 to 1,000 hPa) and are associated with rains and fires dynamics in the region. In terms of interannual variability, multiple variability scales (1.2–2, 2.5–3 and 4.5–6 years) were observed, which explain around 10–15 % of concentration variability near surface. The results suggest that climatic variations, associated with the tropical Pacific and Atlantic sea surface temperature variations, on these different time scales, affect rain dynamics and, consequently, fires and CO concentration. Specifically, in 2015/16, the combined effect from different variability scales acted to prolong the dry period over the region, which contributed to increase fires and the CO to reach higher values compared to previous years. These results show a new aspect of the importance of evaluating the combined effect of different climate variability scales on CO concentrations in the atmosphere.

An Integrated View of Organic Biomarker‐Based Sea Surface Temperature Changes in the Subarctic Pacific Since the Last Ice Age

AbstractThe phase relationships of sea surface temperature (SST) changes between the North Pacific and North Atlantic during deglacial millennial‐scale climate events have been of great interest. However, uncertainties remain partly due to the sparsity of deglacial SST records in the North Pacific. This study presents a new high‐resolution ‐SST record spanning the entire last deglaciation from core LV63‐41‐2 retrieved from the Northwestern Pacific off the Kamchatka Peninsula, which allows us to explore regional SST change patterns and associated driving mechanisms by compiling previously published SST data in the subarctic Pacific. The subarctic Pacific SST changes during the Bølling‐Allerød and Younger Dryas show in‐phase relationships in response to the North Atlantic SST variations, suggesting a dominant control of atmospheric teleconnections between both oceans. During Heinrich Stadial 1 (HS1) when the North Atlantic exhibited significant cooling, the subarctic Pacific SST developments are complex, showing gradual warming from the Last Glacial Maximum to HS1 in the Northwestern Pacific and cooling at the onset of HS1 in the Northeastern Pacific. We suggest that the inconsistent phase responses resulted from the combined effects of multiple processes, which involve an enhanced poleward advection of warm subtropical waters, cold meltwater inputs from the retreating Cordilleran Ice Sheet into the Northeastern Pacific, and a persistent La Niña‐like state in the tropical Pacific.

Modulating and Resetting Impacts of Different Volcanic Eruptions on North Atlantic SST Variations

AbstractThis study investigates the impact of volcanic eruption on the North Atlantic (NA) sea surface temperature (SST) during the last millennium based on both model simulations and reconstructions. Our focus is on the two types of relations between the states of SST and associated climate variables before and after the eruption: modulating and resetting. Here, modulating means that the posterior conditions depend on the initial SST and the volcano‐induced cooling is linearly superimposed on the initial SST phases, while resetting means that the posterior conditions are independent from the initial condition. Our analysis demonstrates that equatorial and northern volcanic eruptions larger than 30 Tg could reset NA SST, and there is a limitation of SST decrease after eruption, indicating the largest SST decrease does not exceed a certain value. While equatorial volcanic eruptions larger than 100 Tg and northern volcanic eruptions larger than 30 Tg can reset subsurface temperature changes. In contrast, even the largest volcanic eruption during the last millennium cannot reset the Atlantic Meridional Overturning Circulation.

Impact of Sea Surface Temperature Variability at Different Ocean Basins on Dust Activities in the Gobi Desert and North China

AbstractDust activities are closely related to local meteorological conditions partly driven by remote ocean variability. Here, we investigate the impact of tropical Pacific, North Atlantic, and Arctic ocean variability on dust activities in the Gobi Desert and North China and attempt to quantify their contributions. Three individual Sea Surface Temperature (SST) forcing experiments were performed to simulate the teleconnection from the three ocean basins. Two random forest regression models were established to link the meteorological variables to boreal Spring Dust Activity Frequency (SDAF). Results show that with SST (sea ice concentration for the Arctic) anomaly, tropical Pacific SST variability results in the most change in SDAF in the Gobi Desert, whereas the North Atlantic SST variability plays a more critical role in North China. Considering the observed SST variability, the tropical Pacific becomes more critical in affecting dust activities in both regions.

Precipitation in Northeast Mexico Primarily Controlled by the Relative Warming of Atlantic SSTs

AbstractReconstructing hydroclimate over the Common Era is essential for understanding the dominant mechanisms of precipitation change and improving climate model projections, which currently suggest Northeast Mexico will become drier in the future. Tree‐ring reconstructions have suggested regional rainfall is primarily controlled by Pacific sea‐surface temperatures (SSTs). However, tree ring records tend to reflect winter‐spring rainfall, and thus may not accurately record total annual precipitation. Using the first multiproxy speleothem record spanning the last millennium, combined with results from an atmospheric general circulation model, we demonstrate mean annual rainfall in Northeast Mexico is highly sensitive to Atlantic SST variability. Our findings suggest future precipitation in Northeast Mexico is more dependent upon the warming of Tropical Atlantic SSTs relative to the Tropical Pacific.

The Evolving Role of External Forcing in North Atlantic SST Variability over the Last Millennium

AbstractAtlantic multidecadal variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic Ocean basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea surface temperature (SST) anomalies that have similar characteristics to the observed AMV. However, there is an active debate as to whether these internal fluctuations driven by coupled atmosphere–ocean variability remain influential to the AMV on multidecadal time scales in our modern, anthropogenically forced climate. Here we provide evidence from multiple large ensembles of climate models, paleoreconstructions, and instrumental observations of a growing role for external forcing in the AMV. Prior to 1850, external forcing, primarily from volcanoes, explains about one-third of AMV variance. Between 1850 and 1950, there is a transitional period, where external forcing explains one-half of AMV variance, but volcanic forcing only accounts for about 10% of that. After 1950, external forcing explains three-quarters of AMV variance. That is, the role for external forcing in the AMV grows as the variations in external forcing grow, even if the forcing is from different sources. When forcing is relatively stable, as in earlier modeling studies, a higher percentage of AMV variations are internally generated.

Regional Signatures of Forced North Atlantic SST Variability: A Limited Role for Aerosols and Greenhouse Gases

AbstractPrior studies that argue external forcings drive Atlantic Multidecadal Variability (AMV) use linear detrending to remove the global warming trend from North Atlantic sea surface temperatures (SSTs). The linear detrending method, however, aliases residual nonlinear, global‐scale warming, affecting the interpretation of results. Here we revisit the role of external forcing in AMV using large climate model ensembles, examining the influences of greenhouse gases (GHGs) and industrial aerosols—the two dominant 20th‐century external forcing agents—on North Atlantic SSTs separately from their respective global response. Our approach shows that GHGs have little to no influence unique to the North Atlantic, while industrial aerosols exert only modest influences as part of a broader loading over the Northern Hemisphere. We demonstrate that a prominent role for external forcings on AMV is an artifact of the linear detrending method that disappears when their global responses common to the World Ocean are correctly accounted for.

Diagnosing Two‐Way Coupling in Decadal North Atlantic SST Variability Using Time‐Evolving Self‐Organizing Maps

AbstractDecadal variability in the North Atlantic plays a critical role in modulating regional and global climate. To identify the complex spatiotemporal patterns associated with decadal variability and diagnose mechanisms responsible spatially and over time simultaneously, we debut a novel application of a machine learning method—evolution self‐organizing maps. This time‐evolving framework is applied to a Community Earth System Model pre‐industrial simulation to identify 10‐year consecutive spatiotemporal evolutions of winter sea surface temperature (SST). Here we focus on a single evolution that transitions from SST patterns typically associated with a positive North Atlantic Oscillation (NAO) to a positive Atlantic Multidecadal Variability to a weak negative NAO and find that it can occur over just a 10‐year period. This method facilitates a new examination of buoyancy‐driven and wind‐driven ocean circulations as well as ocean‐atmosphere transient‐eddy feedbacks that confirms the importance of coupled atmosphere‐ocean dynamics in producing this decadal variability.

Understanding the Complicated Relationship Between ENSO and Wintertime North Tropical Atlantic SST Variability

AbstractThe El Niño–Southern Oscillation (ENSO) is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST) during its decaying spring, providing an essential source of NTA predictability. However, the positive ENSO–NTA SST relationship is absent during boreal winter, impeding accurate anticipation of wintertime NTA SST fluctuations. We find that this absence is mainly due to different NTA preconditions. Taking into account the previous summer NTA SST state as an initial condition, a robust relationship is established between ENSO and wintertime NTA SST variability. Furthermore, we show that the NTA initial conditions are in part traced to the ENSO preconditions. Besides, the simultaneous local atmospheric conditions play a role in shaping the initial NTA SST variabilities, especially in the developing summer of El Niño events. The understanding of the complicated ENSO–wintertime NTA SST relationship has implications for exploring trans‐basin climate interactions and associated climate predictability.

Seasonality in the relationship between equatorial-mean heat content and interannual eastern equatorial Atlantic sea surface temperature variability

Interannual sea surface temperature (SST) variations in the tropical Atlantic Ocean lead to anomalous atmospheric circulation and precipitation patterns with important ecological and socioeconomic consequences for the semiarid regions of sub-Saharan Africa and northeast Brazil. This interannual SST variability is characterized by three modes: an Atlantic meridional mode featuring an anomalous cross-equatorial SST gradient that peaks in boreal spring; an Atlantic zonal mode (Atlantic Niño mode) with SST anomalies in the eastern equatorial Atlantic cold tongue region that peaks in boreal summer; and a second zonal mode of variability with eastern equatorial SST anomalies peaking in boreal winter. Here we investigate the extent to which there is any seasonality in the relationship between equatorial warm water recharge and the development of eastern equatorial Atlantic SST anomalies. Seasonally stratified cross-correlation analysis between eastern equatorial Atlantic SST anomalies and equatorial heat content anomalies (evaluated using warm water volume and sea surface height) indicate that while equatorial heat content changes do occasionally play a role in the development of boreal summer Atlantic zonal mode events, they contribute more consistently to Atlantic Niño II, boreal winter events. Event and composite analysis of ocean adjustment with a shallow water model suggest that the warm water volume anomalies originate mainly from the off-equatorial northwestern Atlantic, in agreement with previous studies linking them to anomalous wind stress curl associated with the Atlantic meridional mode.

Mechanisms of Decadal North Atlantic Climate Variability and Implications for the Recent Cold Anomaly

AbstractDecadal sea surface temperature (SST) fluctuations in the North Atlantic Ocean influence climate over adjacent land areas and are a major source of skill in climate predictions. However, the mechanisms underlying decadal SST variability remain to be fully understood. This study isolates the mechanisms driving North Atlantic SST variability on decadal time scales using low-frequency component analysis, which identifies the spatial and temporal structure of low-frequency variability. Based on observations, large ensemble historical simulations, and preindustrial control simulations, we identify a decadal mode of atmosphere–ocean variability in the North Atlantic with a dominant time scale of 13–18 years. Large-scale atmospheric circulation anomalies drive SST anomalies both through contemporaneous air–sea heat fluxes and through delayed ocean circulation changes, the latter involving both the meridional overturning circulation and the horizontal gyre circulation. The decadal SST anomalies alter the atmospheric meridional temperature gradient, leading to a reversal of the initial atmospheric circulation anomaly. The time scale of variability is consistent with westward propagation of baroclinic Rossby waves across the subtropical North Atlantic. The temporal development and spatial pattern of observed decadal SST variability are consistent with the recent observed cooling in the subpolar North Atlantic. This suggests that the recent cold anomaly in the subpolar North Atlantic is, in part, a result of decadal SST variability.

The importance of inter‐basin atmospheric teleconnection in the SST footprint of Atlantic multidecadal oscillation over western Pacific

Western Pacific sea surface temperature (SST) multidecadal fluctuations are synchronized to the Atlantic multidecadal oscillation (AMO) phenomenon during the instrumental period. The possible mechanism of the inter-basin synchronization of multidecadal SST variability still remains a matter of discussion regarding the roles of external radiative forcing and internal inter-basin interaction. Here we address this issue using simulations of CMIP5 coupled models and a partially coupled model experiment with prescribed SSTs over the North Atlantic. Observational analysis suggests that in association with the warm AMO phase, prominent SST warming occurs over the western Pacific, accompanied by anomalous low pressures and ascending motion that maintain the warm SST anomalies through positive feedback of local air–sea interaction. The upward motion in the western Pacific corresponds to a significant intensification of Pacific zonal Walker circulation, which is coupled with an increase in the zonal gradient of atmospheric temperature aloft. The CMIP5 model simulated externally forced component of AMO-related changes in western Pacific SST is weak, associated with little change in Pacific zonal circulation showing an absence of anomalous upward motion over the western Pacific, and this is primarily resultant from the negligible changes in the zonal gradient of atmospheric temperature as a direct thermal response to the external radiative forcing. By contrast, the partially coupled model simulation forced by the Atlantic SST variations reasonably reproduces the observed AMO-related changes in western Pacific SST and pan-tropical atmospheric circulation. A surface radiation budget analysis for the western Pacific shows contrasting roles of AMO-related surface incoming solar radiation between the reanalysis/partially coupled model simulation and the forced signal in CMIP5, further confirming the key role of dynamically induced inter-basin atmospheric teleconnection in the multidecadal SST footprint of AMO over the western Pacific.

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.

Annually resolved Atlantic sea surface temperature variability over the past 2,900 y

Global warming due to anthropogenic factors can be amplified or dampened by natural climate oscillations, especially those involving sea surface temperatures (SSTs) in the North Atlantic which vary on a multidecadal scale (Atlantic multidecadal variability, AMV). Because the instrumental record of AMV is short, long-term behavior of AMV is unknown, but climatic teleconnections to regions beyond the North Atlantic offer the prospect of reconstructing AMV from high-resolution records elsewhere. Annually resolved titanium from an annually laminated sedimentary record from Ellesmere Island, Canada, shows that the record is strongly influenced by AMV via atmospheric circulation anomalies. Significant correlations between this High-Arctic proxy and other highly resolved Atlantic SST proxies demonstrate that it shares the multidecadal variability seen in the Atlantic. Our record provides a reconstruction of AMV for the past ∼3 millennia at an unprecedented time resolution, indicating North Atlantic SSTs were coldest from ∼1400-1800 CE, while current SSTs are the warmest in the past ∼2,900 y.