Strengthening ITF and weakening AMOC: time series evidence of trends and causal pathways to Agulhas variability

Abstract. Agulhas Leakage transports relatively warm and salty Indian Ocean waters into the Atlantic Ocean and as such is an important component of the global ocean circulation. These waters are part of the upper limb of the Atlantic meridional overturning circulation (AMOC), and Agulhas Leakage variability has been linked to AMOC variability. Agulhas Leakage is expected to increase under a warming climate due to a southward shift in the Southern Hemisphere westerlies, which could further influence the AMOC dynamics. This study uses a set of high-resolution preindustrial control, historical and transient simulations with the Community Earth System Model (CESM) with a nominal horizontal resolution of 0.1° for the ocean and sea ice and 0.25° for the atmosphere and land. At these resolutions, the model represents the necessary scales to investigate Agulhas Leakage transport variability and its relation to the AMOC. The simulated Agulhas Leakage transport of 19.7 ± 3 Sv lies well within the observed range of 21.3 ± 4.7 Sv. A positive correlation between the Agulhas Current and the Agulhas Leakage is shown, meaning that an increase of the Agulhas Current transport leads to an increase in Agulhas Leakage. The Agulhas Leakage impacts the strength of the AMOC through Rossby wave dynamics that alter the cross-basin geostrophic balance with a time lag of 2–3 years. Furthermore, the salt transport associated with the Agulhas Leakage influences AMOC dynamics through the salt–advection feedback by reducing the AMOC's freshwater transport at 34° S. The Agulhas Leakage transport indeed increases under a warming climate due to strengthened and southward-shifting winds. In contrast, the Agulhas Current transport decreases due to a decrease in the Indonesian Throughflow and the strength of the wind-driven subtropical gyre. The increase in the Agulhas Leakage is accompanied by a higher salt transport into the Atlantic Ocean, which could play a role in the stability of the AMOC via the salt–advection feedback.

The interocean exchange of water from the South Atlantic with the Pacific and Indian Oceans is examined using the output from the ocean general circulation model for the Earth Simulator (OFES) during the period 1980–2006. The main objective of this paper is to investigate the role of the interocean exchanges in the variability of the Atlantic meridional overturning circulation (AMOC) and its associated meridional heat transport (MHT) in the South Atlantic. The meridional heat transport from OFES shows a similar response to AMOC variations to that derived from observations: a 1 Sv (1 Sv ≡ 106 m3 s−1) increase in the AMOC strength would cause a 0.054 ± 0.003 PW increase in MHT at approximately 34°S. The main feature in the AMOC and MHT across 34°S is their increasing trends during the period 1980–93. Separating the transports into boundary currents and ocean interior regions indicates that the increase in transport comes from the ocean interior region, suggesting that it is important to monitor the ocean interior region to capture changes in the AMOC and MHT on decadal to longer time scales. The linear increase in the MHT from 1980 to 1993 is due to the increase in advective heat converged into the South Atlantic from the Pacific and Indian Oceans. Of the total increase in the heat convergence, about two-thirds is contributed by the Indian Ocean through the Agulhas Current system, suggesting that the warm-water route from the Indian Ocean plays a more important role in the northward-flowing water in the upper branch of the AMOC at 34°S during the study period.

The Indonesian Throughflow (ITF) is projected to slow down under anthropogenic warming. Several mechanisms—some mutually conflicting—have been proposed but the detailed processes causing this slowdown remain unclear. By turning on/off buoyancy and wind forcings globally and in key regions, this study investigates the dynamical adjustments underlying the centennial ITF slowdown in the global oceans and climate models. Our results show that the projected weakened ITF transport in the top 1500 m is dominated by remote anomalous buoyancy forcing in the North Atlantic Ocean. Specifically, surface freshening and warming over the North Atlantic Ocean slow the Atlantic meridional overturning circulation (AMOC), and the resultant dynamic signals propagate through the coastal-equatorial waveguide into the southeastern Indian Ocean and western Pacific Ocean, causing the reduction of ITF transport over a deep layer. In contrast, the anomalous surface buoyancy flux in the Indo-Pacific affects the ocean temperature and salinity in a shallow upper layer, resulting in ITF changes in forms of high baroclinic mode structure with negligible impacts on the net ITF transport. A vertical partitioning index is proposed to distinguish the remote forcing via the AMOC and regional forcing in the Indo-Pacific Ocean, which could be useful for monitoring, attributing, and predicting the changing ITF transport under global warming.

<p>A distinct multi-centennial climate variability signal is apparent in the EC-Earth3 model 2000-year pre-industrial control simulation. This variability arises primarily in the North Atlantic basin and appears to be closely associated with Atlantic Meridional Overturning Circulation (AMOC). It is mainly modulated by the ocean heat transport and freshwater exchange between the Arctic Ocean and the North Atlantic. When a stronger AMOC occurs, it is coherent with anomalous anticyclonic surface currents in the Arctic and cyclonic surface currents in Greenland, Iceland, Norwegian Seas and Labrador Seas. The increased heat in the subpolar gyre region strengthens the oceanic surface evaporation, resulting in a saltier deep convection region and hence strengthens the deep-water formation. Meanwhile, stronger AMOC transports more ocean heat into the Arctic and melts the sea-ice, causing more freshwater to enter the Arctic. The AMOC strength and freshwater accumulation in the Arctic both reach their peaks in about 50 years. Then, in the following 50 years, the freshwater in Arctic slowly pours into the Greenland-Iceland-Labrador Seas, weakens the subpolar gyre, inhibits deep-water formation and eventually weakens the AMOC. Finally, the oscillation shifts to the opposite phase. These physical processes sustain a 160-200 year variability of AMOC, which is considered as the main driver of the multi-centennial climate variability signal in our simulation.</p> <p> </p> <p>In high CO2 forced climates, here simulated with climate sensitivity experiments with alternate CO2 levels of 400 and 560 ppm respectively, the multi-centennial variability of AMOC is present but has a suppressed amplitude. AMOC variability under 400 ppm CO2 forcing shows a similar frequency band as that in the pre-industrial simulation, with enhanced Arctic-Atlantic salinity anomaly exchange. Under 560 ppm CO2 forcing, the AMOC variability shows a lower frequency band. Here, alongside the Arctic-Atlantic salinity exchange, there are also salinity anomalies propagating from the south Atlantic to the north Atlantic. This leads to a longer maintaining of meridional inter-basin exchanges in the entire Atlantic and Arctic. The decrease of Arctic sea-ice under stronger radiative forcing will cause more freshwater to enter the North Atlantic, slow down the deep-water flow, and thus suppress the AMOC strength. Meanwhile the mechanism that sustains AMOC variability, which was inferred from the pre-industrial simulation, will also change as less sea ice in the North Atlantic and Arctic lead to a more well mixed Arctic-Atlantic salinity anomaly exchange.</p> <p>These experiments indicate that the dynamics of the meridional inter-basin exchange in the north Atlantic and its influence on the salinity are essential components to the centennial climate variability and should be considered when assessing future North Atlantic climate.</p>

The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle. The AMOC has been shown to be weakening in recent years1; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD 1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA—sourced from melting glaciers and thickened sea ice that developed earlier in the LIA—weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet. Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here. The data presented here is the supporting data for Thornalley et al. 2018 (see details below) and is derived from cores KNR-178-56JPC and KNR-178-48JPC. It includes the mean sortable silt size, details of radiocarbon dating, the % nps and binned sub-surface temperature reconstructions.

Coupled modeling studies have recently shown that the existence of the glacial ice sheets intensifies the Atlantic meridional overturning circulation (AMOC). However, most models show a strong AMOC in their simulations of the Last Glacial Maximum (LGM), which is biased compared to reconstructions that indicate both a weaker and stronger AMOC during the LGM. Therefore, a detailed investigation of the mechanism behind this intensification of the AMOC is important for a better understanding of the glacial climate and the LGM AMOC. Here, various numerical simulations are conducted to focus on the effect of wind changes due to glacial ice sheets on the AMOC and the crucial region where the wind modifies the AMOC. First, from atmospheric general circulation model experiments, the effect of glacial ice sheets on the surface wind is evaluated. Second, from ocean general circulation model experiments, the influence of the wind stress change on the AMOC is evaluated by applying wind stress anomalies regionally or at different magnitudes as a boundary condition. These experiments demonstrate that glacial ice sheets intensify the AMOC through an increase in the wind stress at the North Atlantic mid-latitudes, which is induced by the North American ice sheet. This intensification of the AMOC is caused by the increased oceanic horizontal and vertical transport of salt, while the change in sea ice transport has an opposite, though minor, effect. Experiments further show that the Eurasian ice sheet intensifies the AMOC by directly affecting the deep-water formation in the Norwegian Sea.

This study describes the intra- to interannual variability of the Atlantic meridional overturning circulation (AMOC) and the relative dynamical contributions to the total variability in an eddy-resolving 1/12° resolution ocean model. Based on a 53-yr-long hindcast and two 4-yr-long ensembles, we assess the total AMOC variability as well as the variability arising from small differences in the ocean initial state that rapidly imprints on the mesoscale eddy fields and subsequently on large-scale features. This initial-condition-dependent variability will henceforth be referred to as “chaotic” variability. We find that intra-annual AMOC fluctuations are mainly driven by the atmospheric forcing, with the chaotic variability fraction never exceeding 26% of the total variance in the whole meridional Atlantic domain. To understand the nature of the chaotic variability we decompose the AMOC (into its Ekman, geostrophic, barotropic, and residual components). The barotropic and geostrophic AMOC contributions exhibit strong, partly compensating fluctuations, which are linked to chaotic spatial variations of currents over topography. In the North Atlantic, the largest chaotic divergence of ensemble members is found around 24°, 38°, and 64°N. At 26.5°N, where the AMOC is monitored by the RAPID–MOCHA array, the chaotic fraction of the AMOC variability is 10%. This fraction is slightly overestimated with the reconstruction methodology as used in the observations (∼15%). This higher fraction of chaotic variability is due to the barotropic contribution not being completely captured by the monitoring system. We look at the strong AMOC decline observed in 2009/10 and find that the ensemble spread (our measure for chaotic variability) was not particularly large during this event. Significance Statement The ocean is characterized by ubiquitous swirls (eddies) with diameters ranging from more than 100 km (low latitudes) to a few tens of kilometers (high latitudes). There is limited predictability of the timing and location of such eddies. They introduce unpredictable (“chaotic”) variability, which affects the ocean circulation on a wide range of spatial and temporal scales. Any observations of ocean currents contain a fraction of chaotic variability. However, it is, in general, not possible to quantify this chaotic variability from observations. Here we use a set of simulations performed with a state-of-the-art ocean computer model to estimate the fraction of chaotic variability in the amount of warm northward flowing near-surface seawater that delivers large amounts of heat to the North Atlantic, known to scientists as the Atlantic meridional overturning circulation (AMOC). We find that about 10%–25% of the AMOC variance is likely to be chaotic.

The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging.

Climate models consistently project a weakening Indonesian Throughflow (ITF) under future warming, but its driving mechanisms remain incompletely explained. Prior studies primarily attribute the ITF decline to changes in the Atlantic Meridional Overturning Circulation (AMOC) and Indo‐Pacific winds, but these two processes account for only a fraction of the simulated ITF weakening (50%–100% across models). Here we show that Southern Ocean surface warming and freshening explain this missing fraction. Enhanced dense‐to‐light water mass transformation in the Southern Ocean produces upper‐ocean volume convergence that propagates globally via wave dynamics, slowing the ITF in a manner analogous to the AMOC‐driven impacts. We further quantified the combined AMOC and Southern Ocean contributions and find that these bipolar processes explain, on average, 89% of the projected ITF decline in climate models. Our results reveal critical linkages between high‐latitude climate change and low‐latitude circulation, positioning the ITF as an indicator of global overturning adjustments.

Climate 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.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken by the end of this century across all future scenarios considered by the IPCC Sixth Assessment Report. Consequently, the climate system will likely be influenced not only by continued global warming but also by the effects of a reduced AMOC. In this study, we assess the impact of AMOC weakening on extreme cold events in winter and extreme warm events in summer over Europe, using targeted sensitivity experiments with the EC-Earth3 climate model. Starting from a fully coupled ocean-atmosphere simulation with an artificially weakened AMOC, we conducted a series of atmosphere-only integrations with prescribed sea surface temperatures and sea-ice cover to isolate the atmospheric response to both moderate and strong reductions in AMOC strength.Our results show that during boreal winter, a weakened AMOC induces average cooling over Europe and intensified cold extremes. However, the cooling at the Northern Hemisphere’s high latitudes intensifies the near-surface temperature meridional gradient at high northern latitudes. The enhanced meridional temperature gradient strengthens the jet stream, which in turn reduces the frequency of atmospheric blocking over the North Atlantic and northwestern Europe. Since wintertime blocking is typically associated with prolonged cold spells, this mechanism leads to a paradoxical reduction in such events despite the overall cooling.In boreal summer, the weakening AMOC causes widespread cooling across the Northern Hemisphere, especially over the North Atlantic. While most of Europe experiences a decrease in extreme warm events, Eastern Europe and western Russia emerge as exceptions, with an increased frequency of heatwaves. As the AMOC weakens, maximum cooling during boreal summer occurs over the North Atlantic, reducing the temperature gradient at higher latitudes. Consequently, the jet stream weakens which facilitates the development of atmospheric blocking patterns. These blocking, through mechanisms such as subsidence warming and increased shortwave radiation under clear skies, contribute to more frequent heatwaves in the region.Our findings underscore the pivotal role of large-scale ocean circulation in shaping regional climate extremes. As the AMOC is expected to weaken in the coming decades, understanding its interaction with atmospheric dynamics is essential for improving projections of future climate risks, particularly the compound effects of global warming and ocean circulation changes on European weather extremes.

Detecting changes in the transport of the Gulf Stream and the Atlantic overturning circulation from coastal sea level data: The extreme decline in 2009–2010 and estimated variations for 1935–2012

The Tibetan Plateau (TP) over the Eurasian continent has significant effects on both regional and global climate. It can even affect the remote Atlantic meridional overturning circulation (AMOC), as shown in this study. Through coupled modeling experiments, we demonstrate that removing the TP immediately weakens the meridional wind over East Asia, resulting in stronger westerlies in the midlatitudes. The stronger westerlies enhance the southward Ekman flow and surface latent and sensible heat losses in the subpolar North Atlantic, cooling the surface ocean and leading to stronger North Atlantic deep-water formation and stronger AMOC during the first few decades after the TP removal. At the same time, accompanying the weakened trade winds in the tropical Pacific, more moisture is transported from the tropical Pacific to the North Atlantic, freshening the surface ocean and triggering a weakening of the AMOC. The AMOC weakening in turn results in southward expansion and melting of sea ice, providing more freshwater to the North Atlantic, which furthers the weakening of the AMOC. The positive feedback between the AMOC and sea ice eventually leads to AMOC shutdown. We illustrate that there would be no AMOC without the TP. These results call for a revisiting of how ocean circulation and global climate may have responded to the TP uplift and other tectonic changes on the geological time scale.

Due to large northward heat transport, the Atlantic meridional overturning circulation (AMOC) strongly affects the climate of various regions. Its internal variability has been shown to be predictable decades ahead within climate models, providing the hope that synchronizing ocean circulation with observations can improve decadal predictions, notably of the North Atlantic subpolar gyre (SPG). Climate predictions require a starting point which is a reconstruction of the past climate. This is usually performed with data assimilation methods that blend available observations and climate model states together. There is no unique method to derive the initial conditions. Moreover, this can be performed using full-field observations or their anomalies superimposed on the model's climatology to avoid strong drifts in predictions. How critical ocean circulation drifts are for prediction skill has not been assessed yet. We analyze this possible connection using the dataset of 12 decadal prediction systems from the World Meteorological Organization Lead Centre for Annual-to-Decadal Climate Prediction. We find a variety of initial AMOC errors within the predictions related to a dynamically imbalanced ocean states leading to strongly displaced or multiple maxima in the overturning structures. This likely results in a blend of what is known as model drift and initial shock. We identify that the AMOC initialization influences the quality of the SPG predictions. When predictions show a large initial error in their AMOC, they usually have low skill for predicting internal variability of the SPG for a time horizon of 6-10 years. Full-field initialized predictions with low AMOC drift show better SPG skill than those with a large AMOC drift. Nevertheless, while the anomaly-initialized predictions do not experience large drifts, they show low SPG skill when skill also present in historical runs is removed using a residual correlation metric. Thus, reducing initial shock and model biases for the ocean circulation in prediction systems might help to improve their prediction for the SPG beyond 5 years. Climate predictions could also benefit from quality-check procedure for assimilation/initialization because currently the research groups only reveal the problems in initialization once the set of predictions has been completed, which is an expensive effort.

Paleo proxy data suggest that the Atlantic meridional overturning circulation (AMOC) was shallower and weaker at the Last Glacial Maximum (LGM) than at present. In this study, we have identified the existence of a thermal threshold of the AMOC which may explain why many coupled climate models fail to simulate the weaker AMOC during the LGM. By using results obtained from a coupled climate model and conducting sensitivity simulations with an ocean general circulation model, we found that the sudden transition from the present‐day AMOC to the weaker glacial AMOC occurs when we gradually change the degree of surface cooling from present‐day to glacial conditions. This result is related to response of deep convection in the northern North Atlantic Ocean; moderate cooling enhances deep convection whereas sufficient cooling results in total covering of sea ice there and suppression of deep convection. The findings indicate the existence of a thermal threshold controlling the AMOC, where the present‐day‐type AMOC suddenly shifts to the weaker glacial AMOC once the surface cooling exceeds this threshold. We also demonstrate that wind stress forcing plays a critical role in controlling the value of the thermal threshold. Our study suggests that slight differences in the degree of surface cooling or wind stress forcing for LGM simulations could lead to the very different response of the AMOC during the LGM as reported in previous LGM simulations.