Slowdown Of Atlantic Meridional Overturning Circulation Research Articles

Modulation of regional carbon uptake by AMOC and alkalinity changes in the subpolar North Atlantic under a warming climate

IntroductionThe slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and associated consequences on ocean carbon uptake could have large implications for the Earth’s climate system and its global carbon cycle.MethodsThis study analyzes ten Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and reveals that the regional carbon uptake in the subpolar North Atlantic under a high CO2 emission scenario moderately correlates with the decline in AMOC at 40°N. AMOC transports warm and salty subtropical waters to the subpolar regions.ResultsModels with stronger AMOC slowdown generally exhibit weaker surface warming and larger decline of surface salinity and alkalinity. We consider two plausible mechanisms linking the AMOC slowdown to the decline of regional CO2 uptake: the reduction in surface alkalinity and diminished subduction. The decline of surface salinity and alkalinity reduces the ocean’s capacity to buffer acids leading to a reduced CO2 uptake. This important contribution is unique to the North Atlantic. Diminished convective mixing and subduction of surface water can further decrease the downward transport of anthropogenic carbon, as also shown in previous research. The centennial trends of pCO2 are decomposed into four components driven by temperature, salinity, alkalinity and dissolved inorganic carbon, revealing that alkalinity and dissolved inorganic carbon are both significant contributors. The alkalinity-driven pCO2 essentially follows surface salinity, establishing the linkage between AMOC slowdown and alkalinity decline.DiscussionOur results indicate that alkalinity changes are important for the interplay between AMOC and the regional carbon sequestration ability across the late 20th and the entirety of the 21st century in the subpolar North Atlantic.

Diagnosing the causes of AMOC slowdown in a coupled model: a cautionary tale

Abstract. It is now established that the increase in atmospheric CO2 is likely to cause a weakening, or perhaps a collapse, of the Atlantic Meridional Overturning Circulation (AMOC). To investigate the mechanisms of this response in CMIP5 models, Levang and Schmitt (2020) have estimated the geostrophic streamfunction in these models offline and have decomposed the simulated changes into a contribution caused by the variations in temperature and salinity. They concluded that under a warming scenario, and for most models, the weakening of the AMOC is fundamentally driven by temperature anomalies, while freshwater forcing actually acts to stabilise it. However, given that both 3-D fields of ocean temperature and salinity are expected to respond to a forcing at the ocean surface, it is unclear to what extent the diagnostic is informative about the nature of the forcing. To clarify this question, we used the Earth system Model of Intermediate Complexity (EMIC), cGENIE, which is equipped with the C-GOLDSTEIN friction-geostrophic model. First, we reproduced the experiments simulating the Representative Concentration Pathway 8.5 (RCP8.5) warming scenario and observed that cGENIE behaves similarly to the majority of the CMIP5 models considered by Levang and Schmitt (2020), with the response dominated by the changes in the thermal structure of the ocean. Next, we considered hysteresis experiments associated with (1) water hosing and (2) CO2 increase and decrease. In all experiments, initial changes in the ocean streamfunction appear to be primarily caused by the changes in the temperature distribution, with variations in the 3-D distribution of salinity only partly compensating for the temperature contribution. These experiments also reveal limited sensitivity to changes in the ocean's salinity inventory. That the diagnostics behave similarly in CO2 and freshwater forcing scenarios suggests that the output of the diagnostic proposed in Levang and Schmitt (2020) is mainly determined by the internal structure of the ocean circulation rather than by the forcing applied to it. Our results illustrate the difficulty of inferring any information about the applied forcing from the thermal wind diagnostic and raise questions about the feasibility of designing a diagnostic or experiment that could identify which aspect of the forcing (thermal or haline) is driving the weakening of the AMOC.

Simulating AMOC tipping driven by internal climate variability with a rare event algorithm

This study investigates the possibility of Atlantic Meridional Overturning Circulation (AMOC) noise-induced tipping solely driven by internal climate variability without applying external forcing that alter the radiative forcing or the North Atlantic freshwater budget. We address this hypothesis by applying a rare event algorithm to ensemble simulations of present-day climate with an intermediate complexity climate model. The algorithm successfully identifies trajectories leading to abrupt AMOC slowdowns, which are unprecedented in a 2000-year control run. Part of these AMOC weakened states lead to collapsed state without evidence of AMOC recovery on multi-centennial time scales. The temperature and Northern Hemisphere jet stream responses to these internally-induced AMOC slowdowns show strong similarities with those found in externally forced AMOC slowdowns in state-of-the-art climate models. The AMOC slowdown seems to be initially driven by Ekman transport due to westerly wind stress anomalies in the North Atlantic and subsequently sustained by a complete collapse of the oceanic convection in the Labrador Sea. These results demonstrate that transitions to a collapsed AMOC state purely due to internal variability in a model simulation of present-day climate are rare but theoretically possible. Additionally, these results show that rare event algorithms are a tool of valuable and general interest to study tipping points since they introduce the possibility of collecting a large number of tipping events that cannot be sampled using traditional approaches. This opens the possibility of identifying the mechanisms driving tipping events in complex systems in which little a-priori knowledge is available.

Increased Asian aerosols drive a slowdown of Atlantic Meridional Overturning Circulation

Observational evidence and climate model experiments suggest a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) since the mid-1990s. Increased greenhouse gases and the declined anthropogenic aerosols (AAs) over North America and Europe are believed to contribute to the AMOC slowdown. Asian AAs continue to increase but the associated impact has been unclear. Using ensembles of climate simulations, here we show that the radiative cooling resulting from increased Asian AAs drives an AMOC reduction. The increased AAs over Asia generate circumglobal stationary Rossby waves in the northern midlatitudes, which shift the westerly jet stream southward and weaken the subpolar North Atlantic westerlies. Consequently, reduced transport of cold air from North America hinders water mass transformation in the Labrador Sea and thus contributes to the AMOC slowdown. The link between increased Asian AAs and an AMOC slowdown is supported by different models with different configurations. Thus, reducing emissions of Asian AAs will not only lower local air pollution, but also help stabilize the AMOC.

Analysis on the relationship between short-term variability and the weakening AMOC

The Atlantic Meridional Overturning Circulation (AMOC) is a vital component of the global climate system, playing a crucial role in transporting heat and salinity from the equator to the North Atlantic, which in turn keeps Europe significantly warmer than other regions at the same latitude. Despite the fact that long-term observations and models have demonstrated an AMOC slowdown in response to global warming since the industrial revolution, detecting this slowdown in the brief observational record has proven to be challenging. In this study, the author investigates the relationship between short-term variability and the weakening AMOC by analyzing satellite remote sensing data of sea surface salinity collected over a decade. As a consequence of global warming, the weakened AMOC transports less heat and salt from the equator to sub-polar regions, causing heat and salt anomalies. This evidence further corroborates the slowing of the AMOC in response to human-caused warming and emphasizes the variability on a scale of half a decade caused by the deceleration.

Weakened AMOC related to cooling and atmospheric circulation shifts in the last interglacial Eastern Mediterranean

There is limited understanding of temperature and atmospheric circulation changes that accompany an Atlantic Meridional Overturning Circulation (AMOC) slowdown beyond the North Atlantic realm. A Peqi’in Cave (Israel) speleothem dated to the last interglacial period (LIG), 129–116 thousand years ago (ka), together with a large modern rainfall monitoring dataset, serve as the base for investigating past AMOC slowdown effects on the Eastern Mediterranean. Here, we reconstruct LIG temperatures and rainfall source using organic proxies (TEX86) and fluid inclusion water d-excess. The TEX86 data show a stepwise cooling from 19.8 ± 0.2° (ca. 128–126 ka) to 16.5 ± 0.6 °C (ca. 124–123 ka), while d-excess values decrease abruptly (ca. 126 ka). The d-excess shift suggests that rainfall was derived from more zonal Mediterranean air flow during the weakened AMOC interval. Decreasing rainfall d-excess trends over the last 25 years raise the question whether similar atmospheric circulation changes are also occurring today.

Atlantic meridional overturning circulation increases flood risk along the United States southeast coast

The system of oceanic flows constituting the Atlantic Meridional Overturning Circulation (AMOC) moves heat and other properties to the subpolar North Atlantic, controlling regional climate, weather, sea levels, and ecosystems. Climate models suggest a potential AMOC slowdown towards the end of this century due to anthropogenic forcing, accelerating coastal sea level rise along the western boundary and dramatically increasing flood risk. While direct observations of the AMOC are still too short to infer long-term trends, we show here that the AMOC-induced changes in gyre-scale heat content, superimposed on the global mean sea level rise, are already influencing the frequency of floods along the United States southeastern seaboard. We find that ocean heat convergence, being the primary driver for interannual sea level changes in the subtropical North Atlantic, has led to an exceptional gyre-scale warming and associated dynamic sea level rise since 2010, accounting for 30-50% of flood days in 2015-2020.

Orbital Effect on North Atlantic Freshwater Forced Millennial‐Scale East Asian Summer Monsoon Variability During the Holocene

AbstractUnderstanding millennial‐scale East Asian summer monsoon (EASM) variability is critical for climate projection in East Asia in the context of slowing Atlantic Meridional Overturning Circulation (AMOC) caused by global warming. This study performs two groups of sensitivity experiments at 8.2 and 4.2 ka BP, both forced by the same North Atlantic freshwater hosing and corresponding orbital parameters using the Community Earth System Model, to explore the orbital effect on millennial‐scale EASM variability. Our simulations reveal that EASM variability differs significantly between these two cold events. During the 8.2 ka BP (4.2 ka BP) (0 BP = 1950 AD) cold event, North Atlantic freshwater forcing weakens the EASM, generating a meridional tripole (dipole) pattern of EASM precipitation anomalies. Furthermore, the same freshwater discharge causes a greater decline in the EASM, contrasting to a slight weaker AMOC slowdown, at 4.2 ka BP than at 8.2 ka BP. This suggests orbital parameters significantly influence the EASM response to North Atlantic cold events. The East Asian subtropical westerly jet, whose meridional position and intensity are controlled by summer insolation on an orbital timescale, plays a key role in this orbital effect. The westerly jet's meridional position determines the spatial pattern of EASM precipitation anomalies, while its intensity affects the amplitude of the EASM's response to freshwater forcing. Because of a larger EASM decline due to freshwater hosing with lower summer insolation, we anticipate a profound declining EASM precipitation in the future with low summer insolation if the AMOC continues to weaken.

Middle-Late Pleistocene Eastern Mediterranean nutricline depth and coccolith preservation linked to Monsoon activity and Atlantic Meridional Overturning Circulation

The eastern Mediterranean Sea lies under the influence of high- and low-latitude climatic systems. The northern part of the basin is affected by Atlantic depressions and continental and polar air masses that promote intermediate and deep-water formation. The southern part is influenced by subtropical conditions and monsoon activity. Monsoon intensification results in enhanced freshwater discharge from the Nile River and other (now dry) systems along the North African margin. This freshwater influx into the Mediterranean Sea reduces surface water buoyancy loss. Disentangling the influences of these diverse climatic forcings is hindered by inherent proxy data limitations and by interactions between the climatic forcings. Here we use a wealth of published and new paleoclimate records across Termination II to understand the impacts of the higher latitude and subtropical/monsoon climate influences on coccolithophore ecology and holococcolith preservation in Aegean Sea sediment core LC21. We then use these findings to interpret coccolith assemblage variations at Ocean Drilling Program Site 967 (located nearby LC21, at the Eratosthenes Seamount) during multiple glacial-interglacial cycles across the Middle Pleistocene (marine isotopic stages 14–9). The LC21 analysis suggests that holococcolith preservation was enhanced during Heinrich Stadial 11 (∼133 ka) and cold spell C26 (∼119 ka). These two events have been previously linked to cold conditions in the North Atlantic and Atlantic Meridional Overturning Circulation weakening. We propose that associated atmospheric perturbations over the Mediterranean Sea promoted deep-water formation, and thus holococcolith preservation. Similarly, in the Middle Pleistocene (MIS 14-9) of Site 967, we observe temporal coincidence between ten episodes of enhanced holococcolith preservation and episodes of Atlantic Meridional Overturning Circulation slowdown. In Site 967, we also identified repeated fluctuations in placoliths and in Florisphaera profunda, which indicate nutricline depth variations. The development of a deep chlorophyll maximum is associated with the North Africa and wet phases, as recently observed using elemental proxy records at Site 967, during the deposition of sapropel layers. A further deep chlorophyll maximum development is identified during MISs 12 and 10, as a result of pycnocline and nutricline shoaling within the lower part of the photic zone due to glacial sea-level lowering and water mass transport reduction at both the Gibraltar and Sicily Straits. Finally, enhanced holococcolith preservation during cold/dry events is clearly correlated to weakened monsoon activity in both Africa and Asia.

A data-model perspective on the Brazilian margin surface warming from the Last Glacial Maximum to the Holocene

The western South Atlantic along the Brazilian margin is an important region for the Atlantic Meridional Overturning Circulation (AMOC) because surface currents in this area transfer warm and salty waters from the Southern Hemisphere to the North Atlantic. Although the number of sea surface temperature (SST) reconstructions has grown in this region, it has been challenging to explain changes in different and sometimes proximal cores. To understand the SST evolution of the Brazilian margin from the Last Glacial Maximum (LGM) to the late Holocene, we present the first SST stack (BRSST stack) for this region. We compare the BRSST stack with the outputs of the transient climate model simulation TraCE-21ka. The BRSST stack shows an LGM cooling of 1.43 °C (from −1.31 to −1.55 °C, 2σ) relative to the late Holocene, followed by deglacial warming starting at ∼ 18.8 ka. TraCE-21ka simulates this early onset of the last deglaciation. Sensitivity experiments suggest that the input of meltwater from retreating ice sheets in the Northern Hemisphere triggered the post-LGM warming, which was subsequently sustained by increasing atmospheric CO2. Deglacial millennial-scale events of AMOC slowdown produced large-scale warming of the Brazilian margin not clearly distinguished by some previous studies. Analyzing our stack in its segregated components, i.e., the North Brazil Current (NBCSST stack) and Brazil Current (BCSST stack) – we noted in-phase warming at the onset of the last deglaciation, which is not found in TraCE-21ka simulations. We attributed this to underestimating the meltwater influence over the tropical Brazilian margin by the model. BRSST stack also presents episodes of abrupt cooling near periods of fast sea-level rise during the last deglaciation, which may be due to rapid AMOC reinvigoration or freshening of the Southern Ocean. Holocene climate resembles that recorded by other compilations, with no clear Holocene thermal maximum but presenting a cooling trend during the late Holocene possibly related to intensified volcanic activity. Our study indicates that the future human-induced SST change expected for the end of the 21st-century will overcome the background of natural climate variability of the last 22,000 years for the Brazilian margin.

The Forgotten Azores Current: A Long-Term Perspective

The Atlantic Meridional Overturning Circulation (AMOC) and its surface limb, the Gulf Stream, are in their weakest state since the last millennium. The consequences of this weakening in the Northeast Atlantic are not yet known. We show that the slowdown of the Gulf Stream in the 1960s, 1970s, and after 2000 may have caused a delayed weakening of the Azores Current. Concurrently, the Azores Front associated with the Azores Current migrated northward since the 1970s due to gradual changes in the Atlantic Multidecadal Oscillation and ocean heat content. We argue that the AMOC slowdown is also detectable in the low-energy region of the Northeast Atlantic and that the dynamics of Azores Current tightly connects to that of the dynamics of the Gulf Stream and AMOC on decadal and longer time scales.

The Role of a Weakened Atlantic Meridional Overturning Circulation in Modulating Marine Heatwaves in a Warming Climate

AbstractWe explore the effect of Atlantic meridional overturning circulation (AMOC) slowdown on global marine heatwaves (MHWs) under anthropogenic warming by maintaining AMOC strength in climate model simulations throughout the 21st century. The AMOC slowdown has an insignificant effect on global MHWs during the past four decades, except those over the North Atlantic warming hole (NAWH) where the weakened AMOC reduces the occurrence and duration of MHWs by about half by creating a cooler mean‐state sea surface temperature. As the AMOC continues weakening in current century, its effect becomes significant on MHWs in the North Atlantic and Pacific Oceans. The weakened AMOC induces a bipolar‐seesaw‐like change in MHW frequencies, with more frequent MHWs in the Southern Hemisphere but less frequent MHWs in the North Hemisphere except over the NAWH. The reason for the exception is that the NAWH region would enter a near‐permanent MHW state without an AMOC slowdown.

120 Years of AMOC Variability Reconstructed From Observations Using the Bernoulli Inverse

AbstractDetermining the long‐term nature of the Atlantic Meridional Overturning Circulation (AMOC) presents a major step in understanding ocean temperature forcing, and is crucial both for placing the recently observed AMOC slowdown in the context of climate change and for predicting future climate. We present a time series for AMOC strength over the last 120 years which is derived solely from observations. Application of the Bernoulli inverse to hydrography yields the general geostrophic circulation in the North Atlantic without requiring additional sea surface height information, allowing AMOC evaluation in the pre‐satellite era. AMOC varies on a multidecadal timescale, leading the Atlantic Multidecadal Variability (AMV) in sea surface temperature by 2.5 years. We consider the dynamics of the implied AMOC/AMV coupling, and hypothesize that the low‐frequency variability in both parameters is driven by large‐scale density anomalies circulating in the AMOC.

Mechanism of the Centennial Subpolar North Atlantic Cooling Trend in the FGOALS‐g2 Historical Simulation

AbstractA cold blob, manifested as a centennial cooling trend in sea surface temperature (SST), is observed in the mid‐latitude North Atlantic. The presence of the cold blob is hypothesized as an evidence of a slowdown of Atlantic Meridional Overturning Circulation (AMOC), based on paleoclimate proxies and global climate models (GCMs). However, the performance of GCMs in simulating the cold blob remains unsatisfactory in terms of the SST cooling rate and the spatial extent. This study investigates the forcing mechanism of the cold blob using the Flexible Global Ocean‐Atmosphere‐Land System Model Grid‐point version 2 (FGOALS‐g2), a GCM that reasonably simulates the observed cooling trend in the subpolar North Atlantic and its spatial pattern. Surface heat budget analysis suggests that the cold blob is largely a result of the imbalance between changes in the heat storage and surface turbulent heat fluxes, exhibiting a cooling and warming effect, respectively. Investigation of ocean heat content indicates that heat advection into the cold blob region has decreased, mainly due to ocean circulation changes. However, in the FGOALS‐g2, both the AMOC slowdown and the reduced meridional heat transport explain a limited portion (less than 50%) of the cold blob sea surface temperature anomaly trend. Overall, complementing existing studies that attribute the cold blob to an AMOC slowdown, our results suggest that additional processes, including subpolar gyre circulation and a synergy between the atmosphere and the ocean, are at work in the formation of the cold blob.

Impact of an AMOC weakening on the stability of the southern Amazon rainforest

The Atlantic Meridional Overturning Circulation (AMOC) and the Amazon rainforest are potential tipping elements of the Earth system, i.e., they may respond with abrupt and potentially irreversible state transitions to a gradual change in forcing once a critical forcing threshold is crossed. With progressing global warming, it becomes more likely that the Amazon will reach such a critical threshold, due to projected reductions of precipitation in tropical South America, which would in turn trigger vegetation transitions from tropical forest to savanna. At the same time, global warming has likely already contributed to a weakening of the AMOC, which induces changes in tropical Atlantic sea-surface temperature (SST) patterns that in turn affect rainfall patterns in the Amazon. A large-scale decline or even dieback of the Amazon rainforest would imply the loss of the largest terrestrial carbon sink, and thereby have drastic consequences for the global climate. Here, we assess the direct impact of greenhouse gas-driven warming of the tropical Atlantic ocean on Amazon rainfall. In addition, we estimate the effect of an AMOC slowdown or collapse, e. g. induced by freshwater flux into the North Atlantic due to melting of the Greenland Ice Sheet, on Amazon rainfall. In order to provide a clear explanation of the underlying dynamics, we use a simple, but robust mathematical approach (based on the classical Stommel two-box model), ensuring consistency with a comprehensive general circulation model (HadGEM3). We find that these two processes, both caused by global warming, are likely to have competing impacts on the rainfall sum in the Amazon, and hence on the stability of the Amazon rainforest. A future AMOC decline may thus counteract direct global-warming-induced rainfall reductions. Tipping of the AMOC from the strong to the weak mode may therefore have a stabilizing effect on the Amazon rainforest.

Centennial Changes in the Indonesian Throughflow Connected to the Atlantic Meridional Overturning Circulation: The Ocean's Transient Conveyor Belt

AbstractClimate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate.

Weakening Atlantic overturning circulation causes South Atlantic salinity pile-up

The Atlantic Meridional Overturning Circulation (AMOC) is an active component of the Earth’s climate system1 and its response to global warming is of critical importance to society. Climate models have shown an AMOC slowdown under anthropogenic warming since the industrial revolution2–4, but this slowdown has been difficult to detect in the short observational record5–10 because of substantial interdecadal climate variability. This has led to the indirect detection of the slowdown from longer-term fingerprints11–14 such as the subpolar North Atlantic ‘warming hole’11. However, these fingerprints, which exhibit some uncertainties15, are all local indicators of AMOC slowdown around the subpolar North Atlantic. Here we show observational and modelling evidence of a remote indicator of AMOC slowdown outside the North Atlantic. Under global warming, the weakening AMOC reduces the salinity divergence and then leads to a ‘salinity pile-up’ remotely in the South Atlantic. This evidence is consistent with the AMOC slowdown under anthropogenic warming and, furthermore, suggests that this weakening has likely occurred all the way into the South Atlantic. The slowdown in the Atlantic Meridional Overturning Circulation (AMOC) is remotely detected in an increasing South Atlantic salinity trend. This salinity pile-up is caused by reduced divergence of surface salinity transport under a weakened AMOC.

Global Pattern Formation of Net Ocean Surface Heat Flux Response to Greenhouse Warming

AbstractThis study examines global patterns of net ocean surface heat flux changes (ΔQnet) under greenhouse warming in an ocean–atmosphere coupled model based on a heat budget decomposition. The regional structure of ΔQnetis primarily shaped by ocean heat divergence changes (ΔOHD): excessive heat is absorbed by higher-latitude oceans (mainly over the North Atlantic and the Southern Ocean), transported equatorward, and stored in lower-latitude oceans with the rest being released to the tropical atmosphere. The overall global pattern of ΔOHD is primarily due to the circulation change and partially compensated by the passive advection effect, except for the Southern Ocean, which requires further investigations for a more definitive attribution. The mechanisms of North Atlantic surface heat uptake are further explored. In another set of global warming simulations, a perturbation of freshwater removal is imposed over the subpolar North Atlantic to largely offset the CO2-induced changes in the local ocean vertical stratification, barotropic gyre, and the Atlantic meridional overturning circulation (AMOC). Results from the freshwater perturbation experiments suggest that a significant portion of the positive ΔQnetover the North Atlantic under greenhouse warming is caused by the Atlantic circulation changes, perhaps mainly by the slowdown of AMOC, while the passive advection effect can contribute to the regional variations of ΔQnet. Our results imply that ocean circulation changes are critical for shaping global warming pattern and thus hydrological cycle changes.

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.

On the relationship between Atlantic meridional overturning circulation slowdown and global surface warming

According to established understanding, deep-water formation in the North Atlantic and Southern Ocean keeps the deep ocean cold, counter-acting the downward mixing of heat from the warmer surface waters in the bulk of the world ocean. Therefore, periods of strong Atlantic meridional overturning circulation (AMOC) are expected to coincide with cooling of the deep ocean and warming of the surface waters. It has recently been proposed that this relation may have reversed due to global warming, and that during the past decades a strong AMOC coincides with warming of the deep ocean and relative cooling of the surface, by transporting increasingly warmer waters downward. Here we present multiple lines of evidence, including a statistical evaluation of the observed global mean temperature, ocean heat content, and different AMOC proxies, that lead to the opposite conclusion: even during the current ongoing global temperature rise a strong AMOC warms the surface. The observed weakening of the AMOC has therefore delayed global surface warming rather than enhancing it.Social Media : The overturning circulation in the Atlantic Ocean has weakened in response to global warming, as predicted by climate models. Since it plays an important role in transporting heat, nutrients and carbon, a slowdown will affect global climate processes and the global mean temperature. Scientists have questioned whether this slowdown has worked to cool or warm global surface temperatures. This study analyses the overturning strength and global mean temperature evolution of the past decades and shows that a slowdown acts to reduce the global mean temperature. This is because a slower overturning means less water sinks into the deep ocean in the subpolar North Atlantic. As the surface waters are cold there, the sinking normally cools the deep ocean and thereby indirectly warms the surface, thus less sinking implies less surface warming and has a cooling effect. For the foreseeable future, this means that the slowing of the overturning will likely continue to slightly reduce the effect of the general warming due to increasing greenhouse gas concentrations.