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The Rockall Trough (RT) accommodates the warmest and saltiest branch of the North Atlantic Current, which delivers water from the Gulf Stream into the marine environment around western Europe. In addition, the European Slope Current (ESC) carries warm water northward along the eastern boundary of the RT, and exchange between the ESC and the continental shelf is a dominant factor in determining the oceanographic conditions around the British Isles. However, the picture of the mean circulation and variability in the RT is still emerging, with a continuous observational campaign still in its relative infancy. The ESC, in particular, is poorly constrained by ship‐based, mooring, and satellite observations. In this paper, we examine the RT circulation and volume transport using a temporally extended and spatially expanded observing network. Six years of continuous mooring occupation reveal that a large‐amplitude, basin‐scale freshening event, previously detected south of Iceland around 2015, impacted the RT around 2017. Geostrophic transport was greatly reduced during this period, driven by a concurrent subsurface temperature increase at the western boundary. The circulation regained strength during the latter part of the record. We gathered 110 glider transects over 22 months which capture the ESC velocity field in unprecedented detail. The data are sufficient to characterize both the mean state and the emergent seasonal variability of the ESC, and reveal the year‐round presence of a southward countercurrent at depth. Variability in the strength and structure of this previously unstudied feature modulates net northward transport in the eastern boundary current system. Plain Language Summary: The Rockall Trough (RT) is a key pathway for warm and salty water flowing northward, a process which plays a key role in dictating the western European climate. However, the amount of water flowing northward, and how it changes over time, is still rather uncertain. In particular, the narrow boundary current in the eastern RT has previously proven difficult to measure. In this paper, we use 6 years of continuous oceanographic observations in the RT to monitor the flow. We also used underwater robots, called gliders, to focus on the narrow current in the eastern boundary. The data gathered give the best description of the ocean currents in the region to date. The study period coincided with a period of profound change across the subpolar North Atlantic, with a strong freshwater signal detected in the RT around 2017. The glider observations give a detailed view of the narrow eastern boundary current and how it varies seasonally, and also revealed the presence of a southward‐flowing current at depth. Key Points: Eastern boundary current system captured in unprecedented detail using 22 months of near‐continuous glider observationsLarge‐scale freshening event in subpolar North Atlantic impacted Rockall Trough (RT) around 2017Interannual variability in poleward transport driven by subsurface temperature anomalies at the western flank of the RT [ABSTRACT FROM AUTHOR]

Using GFDL's new coupled model SPEAR, we have developed a decadal coupled reanalysis/initialization system (DCIS) that does not use subsurface ocean observations. In DCIS, the winds and temperature in the atmosphere, along with sea surface temperature (SST), are restored to observations. Under this approach the ocean component of the coupled model experiences a sequence of surface heat and momentum fluxes that are similar to observations. DCIS offers two initialization approaches, called A1 and A2, which differ only in the atmospheric forcing from observations. In A1, the atmospheric winds/temperature are restored toward the JRA reanalysis; in A2, surface pressure observations are assimilated in the model. Two sets of coupled reanalyses have been completed during 1961–2019 using A1 and A2, and they show very similar multi‐decadal variations of the Atlantic Meridional Overturning Circulation (AMOC). Two sets of retrospective decadal forecasts were then conducted using initial conditions from the A1 and A2 reanalyses. In comparison with previous prediction system CM2.1, SPEAR‐A1/A2 shows comparable skill of predicting the North Atlantic subpolar gyre SST, which is highly correlated with initial values of AMOC at all lead years. SPEAR‐A1 significantly outperforms CM2.1 in predicting multi‐decadal SST trends in the Southern Ocean (SO). Both A1 and A2 have skillful prediction of Sahel precipitation and the associated ITCZ shift. The prediction skill of SST is generally lower in A2 than A1 especially over SO presumably due to the sparse surface pressure observations. Plain Language Summary: In this paper we document the development and skill assessment of a new coupled decadal prediction system. We have developed a decadal coupled reanalysis/initialization system that does not use subsurface ocean observations. Under this approach the ocean component of the coupled model experiences a sequence of surface heat and momentum fluxes that are similar to observations. The new system offers two initialization approaches which differ only in the atmospheric forcing from observations. One approach utilizes the three‐dimensional reanalysis data, while the another only uses the surface pressure observations. We document the process‐based diagnosis of the improved representation of multi‐decadal variations of the Atlantic Meridional Overturning Circulation associated with the observed North Atlantic Oscillation forcing. The skill assessment of the new decadal retrospective forecasts versus observations supports that the proposed approach is feasible for the decadal climate prediction, increasing diversity of modeling tools for the decadal climate initialization and predictability research. The experimental decadal prediction using the new system will be used for the real‐time decadal climate outlooks. Key Points: Development of GFDL's new decadal coupled reanalysis/initialization system is documentedThe system captures the multi‐decadal variations of Atlantic Meridional Overturning Circulation associated with North Atlantic OscillationThe new system significantly outperforms its predecessor in predicting multi‐decadal SST trends in the Southern Ocean [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) observations in the North Atlantic since 1870 reveal a region of enhanced warming off the northeastern coast of North America, and a region of cooling to the south of Greenland. It has been hypothesized that these adjacent SST trends are a result of long‐term changes in the buoyancy‐driven ocean circulation—a slowdown of the Atlantic Meridional Overturning Circulation. The impacts of these historical SST trends on the atmosphere are estimated using idealized atmospheric general circulation model experiments in which the global atmosphere is exposed to modern climatological forcing minus the aforementioned regional SST trends. The local response includes a negative North Atlantic Oscillation tendency and southward shift of the wind forcing for the subtropical gyre. Due to planetary wave propagation, the regional SST trends also induce a northward shift of the intertropical convergence zone over the Indian Ocean. Implications for climate feedbacks and projections are discussed. Plain Language Summary: The ocean surface is warming over most of the planet, with a couple of notable exceptions. One is over the North Atlantic Ocean, just south of Greenland, where the temperature of the ocean surface has actually been cooling by about 1°C since 1870. Nearby, extending off the northeastern coastline of North America is a region where the ocean surface has been warming much faster than the global ocean on average—by about 1.5°C since 1870. This side‐by‐side pair of accelerated warming and cooling trends is likely due to a slowdown of the ocean's overturning circulation, which carries large amounts of heat energy from the tropics toward the poles. This paper reveals the effects of those unusual ocean temperature trends in the North Atlantic on the atmospheric circulation using a computer model of the global atmosphere. The model simulations indicate that these ocean temperature trends are causing shifts in the jet stream, changing the way the winds propel the upper ocean currents, and even have impacts quite far away by moving the tropical rain belt northward in the Indian Ocean. Key Points: The impact of historical sea surface temperature trends in the North Atlantic since 1870 is simulated with a global atmospheric modelAdjacent warming and cooling trends in the North Atlantic Ocean induce a negative NAO‐like response and southward‐shifted gyre forcingPropagation of planetary waves away from the North Atlantic induces a northward‐shifted intertropical convergence zone in the Indian Ocean [ABSTRACT FROM AUTHOR]

Scientific and societal interest in the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and U.S. East Coast sea level has intensified over the past decade, largely due to (1) projected, and potentially ongoing, enhancement of sea level rise associated with AMOC weakening and (2) the potential for observations of U.S. East Coast sea level to inform reconstructions of North Atlantic circulation and climate. These implications have inspired a wealth of model‐ and observation‐based analyses. Here, we review this research, finding consistent support in numerical models for an antiphase relationship between AMOC strength and dynamic sea level. However, simulations exhibit substantial along‐coast and intermodel differences in the amplitude of AMOC‐associated dynamic sea level variability. Observational analyses focusing on shorter (generally less than decadal) timescales show robust relationships between some components of the North Atlantic large‐scale circulation and coastal sea level variability, but the causal relationships between different observational metrics, AMOC, and sea level are often unclear. We highlight the importance of existing and future research seeking to understand relationships between AMOC and its component currents, the role of ageostrophic processes near the coast, and the interplay of local and remote forcing. Such research will help reconcile the results of different numerical simulations with each other and with observations, inform the physical origins of covariability, and reveal the sensitivity of scaling relationships to forcing, timescale, and model representation. This information will, in turn, provide a more complete characterization of uncertainty in relevant relationships, leading to more robust reconstructions and projections. Plain Language Summary: Sea level along the U.S. East Coast is influenced by changes in the density and currents of the North Atlantic Ocean. Indeed, there are simple theoretical considerations that relate indices of basin‐scale flow to coastal sea level. Such a relationship could be leveraged to predict future sea level changes and coastal flooding given an expected change in climate and ocean circulation. Alternatively, it could be used to reconstruct ocean circulation from sea level measurements. This paper reviews the nature of this relationship and whether, and when, it is evident in climate models and observations. Although the current generation of large‐scale climate and ocean models generally show an antiphase relationship between basin‐scale ocean current strength and coastal sea level, the spatial pattern of sea level change differs from theory and between models. Supported by existing and emerging research, the authors hypothesize that these deviations result from important physical processes occurring on the continental shelf and slope, and the complexities of the 3‐dimensional ocean circulation. A quantitative assessment of the importance of these processes is critical for understanding past and future climate and sea level changes in this heavily populated and vulnerable region. Key Points: The relationship between the AMOC and coastal sea level is important to flood risk projections and ocean circulation reconstructionsThe amplitude and pattern of sea level variability associated with AMOC variations is location, forcing, timescale, and model dependentFuture research should address the complex spatiotemporal structure of AMOC and the role of near‐coast ageostrophic processes [ABSTRACT FROM AUTHOR]

The Irminger Current (IC), flowing northeastward along the western flank of the Reykjanes Ridge, is an important component in the overturning of the North Atlantic subpolar gyre. A 2‐year (2014–2016) time series from moored observations shows that the IC consists of two highly variable current cores. A subsampling experiment, using an ocean reanalysis, showed that this current's variability is adequately captured by the array. The two current cores contribute nearly equally to the mean volume transport. The total 2‐year mean transport was found to be 10.6 Sv with a standard deviation of daily (monthly) values of 9.2 Sv (4.4 Sv) and a standard error of 1.4 Sv. Mean heat and freshwater transport were 0.21 PW and −22.5 mSv, respectively (Sref = 34.92). The volume transport is strongest in spring, and the average over the first year (8.6 Sv) was lower than that of the second year (12.4 Sv), coinciding with an increase in the density gradient over the array in the second year. The variability of the total transport is dominated by variations in the western core, while the warmer, saltier eastern core contributes more to the heat and (negative) freshwater transport. During the two observed winters, which were marked by exceptional deep convection in the central Irminger Sea, mixed layer deepening down to 400 m depth and outcropping of the 27.7 kg m3 isopycnal were observed within the IC.Plain Language Summary: The Irminger Current flows northeastward along the Reykjanes Ridge in the Irminger Sea. It transports warm saline water originating from the North Atlantic Current in the upper layers and deep waters coming from the overflows on the Iceland‐Scotland Ridge. This paper describes the first year‐round moored observations of velocity, temperature, and salinity of this current system. These new observations show that the Irminger Current consists of two cores that contribute equally to the mean transport. The core closer to the top of the ridge is more stable and transports warmer, more saline water, while the core closer to the center of the Irminger Basin is more variable in location and transport. During the observed winters, strong atmospheric cooling created deep mixed layers down to 400 m at the mooring locations. This observed cooling and densification of the upper ocean waters indicate that this current system is an important contributor to the overturning in the Northeastern Atlantic.Key Points: First year‐round observations of the two‐core Irminger Current over the Reykjanes Ridge show a mean transport of 10.6 ± 1.4 Sv (st error)IC transports are highly variable with a std of 9.2 Sv (daily values) and 4.4 Sv (monthly values); the western core is the most variableMixing to densities near 27.7 kg m−3 occurred over the array in both winters (deepest in the west), contributing to the overturning [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is predicted to weaken under global warming. Whether it is caused by heat flux or freshwater flux is under debate. Here we separate these two processes in changing the AMOC under global warming. The simulated AMOC is weakened during the first 600 years and then gradually recovered to its initial state, with heat flux and freshwater feedback dominating at different timescales. Global warming immediately puts freshwater into the Southern Ocean, which triggers the initial AMOC weakening via altering surface temperature. Concurrently, the extensive heat into the ocean surface increases the temperature over the subpolar North Atlantic, reducing the deep convection and thus the AMOC in the subsequent 50–150 years. Meanwhile, the Arctic sea ice melt leads to the AMOC shutdown. Subsequently, the salinity accumulation in the subtropical North Atlantic propagating northward to restart the North Atlantic deep convection is responsible for the AMOC recovery. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is projected to slow down under anthropogenic warming. However, whether this weakening is caused by heat or freshwater flux is of great debate. Here we use a fully coupled Earth System model to separate the role of direct heat flux forcing and freshwater feedback on AMOC behavior under idealized anthropogenic forcing. The simulated AMOC slows down during the first 600 years and then gradually recovers to its initial strength, with heat flux and freshwater feedback dominating at different timescales: The initial AMOC weakening during the first 50 years is caused by the freshwater feedback, its subsequently weakening during 50–150 years is dominated by direct heat flux forcing, while its continuous weakening and recovery is again attributed by freshwater feedback. This work quantifies the individual role of heat and freshwater flux in AMOC change, highlighting their different contributions at different timescales. Key Points: The first study to separate the direct heat effect from freshwater feedback on the thermohaline circulation under global warmingThe initial Atlantic meridional overturning circulation (AMOC) weakening is driven by freshwater feedback, while its continuous weakening is attributed to heat flux forcingThe AMOC collapse and recovery after 200 years is driven by the freshwater feedback [ABSTRACT FROM AUTHOR]

Neodymium (Nd) isotope ratios are potentially valuable for paleo‐ocean circulation reconstructions because they trace water‐mass transport and mixing in the present‐day oceans. Moreover, the Atlantic and Pacific global end‐member Nd‐isotope values can be constrained through time, and at in‐between sites their proportions vary with past climate changes. However, an important source of uncertainty about Nd‐isotopes' applicability for paleo‐circulation studies arises from the inability to constrain past Nd‐concentrations of the end‐members. Here we address this "paleo‐[Nd] problem" through a Bayesian analysis. Results show that even large variability in end‐member Nd‐concentrations over the Pleistocene is unlikely to significantly impact their concentration ratio, therefore end‐member concentration changes likely have only small impacts on Nd‐isotope ratios at in‐between locations. The results support their applicability to reconstruct past Atlantic paleo‐circulation. In addition, a Nd‐isotope mass balance for Antarctic Bottom Water shows that its present‐day Nd‐isotope values are consistent with the input from its sources. Plain Language Summary: Ocean circulation transfers heat around the Earth and characterizing its past variability is critical for understanding climate changes. Neodymium (Nd) isotopes are often used to trace past ocean circulation because they mimic deep ocean water mass mixing, and the North Atlantic and North Pacific global ocean mixing end‐member water mass compositions can be constrained through time. Therefore, temporal changes in Nd‐isotope ratios at intermediate locations like the Equatorial and South Atlantic are assumed to approximate changes in the proportions contributed by the global end‐members and thus reflect ocean circulation changes. However, Nd‐isotope ratios at intermediate locations are also sensitive to changes in Nd‐concentrations, which cannot be constrained in the global ocean end‐member water masses. This has caused the applicability of Nd‐isotope ratios for tracing paleo‐ocean circulation to be questioned. Our study examines the sensitivity of Nd‐isotope ratios at locations in‐between the global end‐members to changes in their Nd‐concentrations over recent interglacial‐glacial cycles. Results show that even substantial changes in their Nd‐concentrations likely have little impact on the Nd‐isotope ratios at intermediate locations, indicating that given the right intermediate location choices, Nd‐isotope ratios indeed reflect past deep ocean mixing, thus supporting their use to reconstruct past ocean circulation. Key Points: Neodymium (Nd) isotopes trace modern water mass mixing, but unconstrained end‐member Nd concentrations limit their paleocirculation useA Bayesian approach tests effects of Nd‐concentration changes in global water mass end‐members on Nd‐isotopes at intermediate locationsResults predict small impacts on Nd‐isotopes and constrain projected uncertainties in past ocean end‐member mixing reconstructions [ABSTRACT FROM AUTHOR]

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Since climate model studies project a decline of the Atlantic Meridional Overturning Circulation (AMOC) in the 21st century, monitoring AMOC changes remains essential. While AMOC variability is expected to be coherent across latitudes on longer than decadal timescales, connectivity on inter‐annual and seasonal timescales is less clear. Model studies and observational estimates disagree on the regions and timescales of meridional connectivity and AMOC observations at multiple latitudes are needed to study its connectivity. We calculate basin‐wide AMOC volume transports (1993–2018) from measurements of the North Atlantic Changes (NOAC) array at 47°N, combining data from moored instruments with hydrography and satellite altimetry. The mean NOAC AMOC is 17.2 Sv exhibiting no long‐term trend. Both the unfiltered and low‐pass filtered NOAC AMOC show a significant correlation with the RAPID‐MOCHA‐WBTS AMOC at 26°N when the NOAC AMOC leads by about one year. Plain Language Summary: In the North Atlantic Ocean, currents transport heat and salt from the warm subtropical regions to the colder and less saline subpolar regions. This current system is part of the Atlantic Meridional Overturning Circulation (AMOC). This enormous northward heat transport associated with the AMOC impacts regional and global climate. In a warming world, climate models project a reduction of the AMOC, affecting, for example, air temperature and rain patterns over the continents. Continuous observations are required to investigate whether climate models realistically simulate the AMOC and assess the validity of these projections. We calculate the AMOC from moored observations, hydrography, and satellite data at 47°N from 1993 to 2018. In this period, the AMOC at 47°N does not show a trend. The 26°N AMOC lags the 47°N AMOC by about one year, indicating that the AMOC evolution at these two latitudes is connected. Key Points: We present a 25‐year observational Atlantic Meridional Overturning Circulation (AMOC) volume transport record at 47°N with a mean basin‐wide AMOC transport of 17.2 SvThe inter‐annual variability at 47°N is similar to the RAPID AMOC at 26°N, while the monthly variability is much stronger at 47°NThe AMOC time series at 47°N and 26°N show a significant lag correlation when the AMOC at 47°N leads by about one year [ABSTRACT FROM AUTHOR]

Future projections indicate the Atlantic Meridional Overturning Circulation (AMOC) will weaken and shoal in response to global warming, but models disagree widely over the amount of weakening. We analyze projected AMOC weakening in 27 CMIP6 climate models, in terms of changes in three return pathways of the AMOC. The branch of the AMOC that returns through diffusive upwelling in the Indo‐Pacific, but does not later upwell in the Southern Ocean (SO), is particularly sensitive to warming, in part, because shallowing of the deep flow prevents it from entering the Indo‐Pacific via the SO. The present‐day strength of this Indo‐Pacific pathway provides a strong constraint on the projected AMOC weakening. However, estimates of this pathway using four observationally based methods imply a wide range of AMOC weakening under the SSP5‐8.5 scenario of 29%–61% by 2100. Our results suggest that improved observational constraints on this pathway would substantially reduce uncertainty in 21st century AMOC decline. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents that move warm surface waters from the south to the north of the Atlantic Ocean where they cool, sink, and return southward at depth. Changes in the AMOC would have wide‐ranging impacts on our climate. It is predicted to weaken as the climate warms during the 21st century, but the extent of weakening varies among different climate models. We show that AMOC weakening is greatest in models that have a large exchange of water between the AMOC and the Indo‐Pacific Ocean along a specific pathway. The magnitude of this ocean pathway, inferred from four observation‐based estimates of the global overturning circulation, is uncertain. By using these estimates and analyzing the relationship between the aforementioned ocean pathway and AMOC weakening across many climate models, we can predict how the real‐world AMOC will change. Our findings indicate that by 2100, under a high greenhouse gas emission scenario, the AMOC will weaken by 29%–61%. This highlights the importance of reducing differences between observational estimates of the ocean's overturning pathways to reduce uncertainty in future AMOC weakening and to improve the representation of these pathways in climate models. Key Points: The magnitude of 21st century Atlantic Meridional Overturning Circulation (AMOC) weakening in CMIP6 models is highly correlated with an AMOC pathway into the Indo‐Pacific OceanThe real‐world "Indo‐Pacific diffusive" AMOC pathway inferred from observation‐based estimates is used to constrain future AMOC weakeningUnder high‐end greenhouse gas forcing, AMOC weakening based on this emergent constraint relationship ranges from 29% to 61% by 2100 [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is frequently used to diagnose the state of the North Atlantic circulation, but as an integrated quantity the AMOC strength does not necessarily mirror changes in the individual circulation components. Here, we investigate future circulation changes in the subtropical North Atlantic (26.5°N) in CMIP6 models, diagnosing the relationship between the Gulf Stream, Deep Western Boundary Current (DWBC), gyre recirculation, and the integrated AMOC response. Under continued high emissions, we find a multi‐model mean Gulf Stream weakening of 29% (11.2 Sv) and a DWBC weakening of 47% (8.5 Sv) by the end of the century. However, 33% (3.7 Sv) of the Gulf Stream weakening is due to changes in wind stress and therefore not simply a compensating effect for reduced high‐latitude water mass transformation and a weaker DWBC. Our findings have implications for how we understand the dynamics of future North Atlantic circulation changes. Plain Language Summary: Climate models show circulation in the North Atlantic Ocean to weaken under continued high greenhouse gas emissions. How this weakening is expressed in the Gulf Stream, Deep Western Boundary Current (DWBC), and subtropical gyre circulation remains little explored. Here, we use models that contributed to the Coupled Model Intercomparison Project Phase 6 to investigate potential future changes in the subtropical North Atlantic Ocean. Averaging over 14 models, we find the Gulf Stream to weaken by 29% and the DWBC to weaken by 47% by the end of the century. As much as 33% of the Gulf Stream weakening is due to changes in the large‐scale winds, while the remaining fraction is compensating for the weakening DWBC. Our findings have implications for understanding the interconnectedness of the different circulation branches and their relationship to the large‐scale Atlantic Meridional Overturning Circulation. Key Points: Future Atlantic Meridional Overturning Circulation decline is deconstructed by quantifying changes in the Gulf Stream, Deep Western Boundary Current (DWBC), and gyre recirculationIn a high emission scenario, Coupled Model Intercomparison Project Phase 6 models show a Gulf Stream weakening of 29% and a DWBC weakening of 47% at 26.5°N by the end of the century33% of the Gulf Stream weakening is due to changes in winds [ABSTRACT FROM AUTHOR]

Understanding how the transport of gases and aerosols responds to climate change is necessary for policy making and emission controls. There is considerable spread in model projections of tracer transport in climate change simulations, largely because of the substantial uncertainty in projected changes in the large‐scale atmospheric circulation. In particular, a relationship between the response of tropospheric transport into the high latitudes and a shift of the midlatitude jet has been previously established in an idealized modeling study. To test the robustness of this relationship, we analyze the response of a passive tracer of northern midlatitude surface origin to abrupt 2xCO2 and 4xCO2 in a comprehensive climate model (Goddard Institute for Space Studies E2.2‐G). We show that a poleward shift of the northern midlatitude jet and enhanced eddy mixing along isentropes on the poleward flank of the jet result in decreased tracer concentrations over the midlatitudes and increased concentrations over the Arctic. This mechanism is robust in abrupt 2xCO2 and 4xCO2 simulations, the nonlinearity to CO2 forcing, and two versions of the model with different atmospheric chemistry. Preliminary analysis of realistic chemical tracers suggests that the same mechanism can be used to provide insights into the climate change response of anthropogenic pollutants. Plain Language Summary: Pollutants such sulfate aerosols, soot, and carbon monoxide are transported by atmospheric flows from the northern midlatitude surface to higher altitudes and the Arctic. Here we study how this transport responds to climate change by using a passive tracer without chemistry. During northern winter, the westerly jet accelerates and shifts poleward under increased CO2 concentration. This leads to more mixing that brings cleaner air from the subtropical surface to the midlatitude troposphere but also polluted air from the midlatitude surface to the Arctic troposphere. This pattern is robust in tracers with and without chemistry, suggesting that transport changes play an important role in shaping the response of pollutant distributions to climate change. It also suggests that reducing the uncertainty of the midlatitude jet response will facilitate more accurate projection of pollutant transport in a warming climate. Key Points: Response of tropospheric tracer transport from the northern hemisphere (NH) midlatitude surface to increased CO2 depends on the midlatitude jet responseChanges in isentropic eddy mixing associated with the midlatitude jet dominate the response of the transport to NH high latitudesA poleward shift of the NH midlatitude jet associated with Atlantic Meridional Overturning Circulation weakening leads to less tracer in the midlatitudes and more in the Arctic [ABSTRACT FROM AUTHOR]

Caribbean through‐flow accounts for two‐thirds of the Florida Current and consequently is an important conduit of heat and salt fluxes in the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). Considering there is evidence that up to one‐half of the Florida Current originates as South Atlantic Water (SAW), determining the distribution of SAW throughout the Caribbean Island passages is important as this constitutes the major pathway for cross‐equatorial AMOC return flow. The Anegada Passage (AP) is a major pathway for subtropical gyre inflow and suggested to be a potential SAW inflow pathway worth revisiting. Here, we present glider‐based observations of temperature, salinity and subsurface velocity that represent the first observations of any type in the AP in nearly 20 years. An isopycnal water mass analysis is conducted to quantify the transport of water masses with South Atlantic or North Atlantic origin. Two potentially new aspects of AP transport are revealed. The total AP transport (−4.8 Sv) is shown to be larger than previously estimated, potentially by up to a factor of two. The transport of SAW through the AP (−1.66 Sv) is also shown to be larger than previously estimated, which represents 35% of the total transport reported here and 28% of the SAW entering the Caribbean north of the Windward Island Passages. These results indicate the AP may be an important pathway for cross‐equatorial AMOC return flow. These results also provide evidence that gliders with acoustic doppler profilers are a viable method for measuring island passage transport. Plain Language Summary: The Caribbean Sea through‐flow is a major transport pathway for heat and salt in the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). The presence of water masses (bodies of water with distinct temperature and salinity properties) that originated in the South Atlantic (SAW) in the Northern Hemisphere is indicative of cross‐equatorial AMOC return flow. Ship‐based observations in the 1990's identified major pathways for this AMOC return flow but there is still a substantial amount of SAW that is taking an unknown, alternate route northward. This study presents the first observations in nearly 20 years of temperature, salinity, and subsurface velocity in the Anegada Passage (AP). Here, we perform a water mass analysis that suggests the total transport and SAW transport through the AP is larger than previously estimated. This result is significant as the AP may be an important pathway for AMOC return flow. This study also shows that autonomous underwater gliders are a viable method for measuring island passage transport. Key Points: Total transport and transport of South Atlantic Water through the Anegada Passage (AP) may be larger than previously estimatedThe AP is a pathway for both Atlantic Meridional Overturning Circulation return flow and subtropical gyre recirculationGliders are an effective component of the global ocean observing system, needed for measuring subsurface water mass structure and transport [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is central to the climate of the Atlantic by redistributing mass, heat, and freshwater. Hydrographic sections help monitor its strength at different latitudes, and inverse box models provide estimates of AMOC, heat, and freshwater transports. We have used all available hydrographic zonal sections at 24.5°N and 30°S over the last 30 years to conclude that single section inverse models agree with monthly outputs from an ocean general circulation model at the time of the cruise. In contrast, inverse models using multiple sections at different latitudes and times of the year for each of the last three decades are more consistent with decadal averages from the same model. Therefore, solutions of inverse models with single sections are affected by aliasing and represent the state of the ocean at the time of cruise. However, aliasing is greatly reduced when using multiple sections to assess low‐frequency variability. Plain Language Summary: Heat and nutrients in the Atlantic Ocean are redistributed through a process called Atlantic meridional overturning circulation, which is being monitored to detect changes in its strength. Hydrographic data offer the possibility to assess this variability, although it can be influenced by the ocean dynamics happening at the time of the cruise. When comparing the results from inverse box models applied to single sections with numerical model output, the best fit appears for the time of the cruise. This reflects that the monthly variation affects the solutions of the inverse model, which can be understood as representative of the duration of the cruise. Inverse models using several sections from cruises carried out in different times of the year and different years average out the local temporal phenomena that can affect the results and better represent the decadal average of the numerical models. Key Points: The Atlantic meridional overturning circulation has been monitored with hydrographic data and now basin‐wide arrays enable the detection of high‐frequency variabilityInverse solutions from single sections are affected by aliasing, as they capture the circulation structure of the time of the cruiseInverse models with multiple sections at different latitudes and times agree with decadal averages from an ocean general circulation model [ABSTRACT FROM AUTHOR]

The Faroe‐Bank Channel (FBC) is a key gateway through which dense overflow water of the Nordic Seas supplies the lower limb of the Atlantic Meridional Overturning Circulation. Most recently, it was discovered that a deep jet through the Faroe‐Shetland Channel carries the bulk of this overflow water, but numerous questions regarding its structure, seasonality, and interannual variability as well as its linkage to atmospheric forcing remain poorly understood. A realistic high‐resolution ocean reanalysis (GLORYS12; 1993–2018) is, therefore, employed to address these questions. We first confirm that the Faroe‐Shetland Channel Jet is a permanent feature in GLORYS12 as well as in an ensemble of low‐resolution reanalyses. On seasonal time scales, we find a strong transport covariability between this deep jet and the observed FBC overflow. On interannual time scales, the strength of this deep jet is governed by the wind‐forced circulation in the Nordic Seas. Due to the largely barotropic structure of these flows, they have a signature detectable in satellite sea‐surface heights. Further, we suggest that the structure of the deep jet is qualitatively consistent with a geostrophic dynamical model that accounts for along‐isobath density variations. This study indicates that GLORYS12 is a promising product to study the dense water pathways and dynamics in the Nordic Seas. Plain Language Summary: Our knowledge about ocean circulation at the sea surface has increased over the past couple of decades thanks to satellite observations. The deep ocean circulation, however, remains poorly understood and requires multiple observational platforms to piece it together. Recently, a rapid current was discovered in a very dynamic region, i.e., the Faroe‐Shetland Channel. This deep flow is found to connect to one of the main gateways that feed the deep North Atlantic Ocean with the dense waters of the Nordic Seas, and is, therefore, important for the strength of the Atlantic Meridional Overturning Circulation. We seek here to learn more about its characteristics and drivers using a high‐resolution ocean reanalysis, i.e., a model that assimilates ocean observations. When compared to multiple observational platforms, the ocean reanalysis is found to perform reasonably well. The location of this deep jet is confirmed to be robust both in this high‐resolution ocean reanalysis as well as in low‐resolution ocean reanalyses. We also show that variations in transport of this deep jet are largely governed by the wind field in the Nordic Seas. Because of this, the variations associated with the transport of this deep jet can be detected from satellite‐derived sea‐surface heights. Key Points: The Faroe‐Shetland Channel Jet is a robust feature across ocean reanalyses, both high‐resolution and low‐resolutionThe deep flows in the Faroe‐Shetland Channel are regulated by the wind‐forced circulation in the Nordic SeasThe high‐resolution ocean reanalysis, GLORYS12, is a promising product to study the overflow pathways/dynamics in the Nordic Seas [ABSTRACT FROM AUTHOR]

Limited constraints on the variability of the deep‐water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe‐Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (−12.6 and −14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ −18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ −31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene. Plain Language Summary: The Labrador Sea is an important region for deep water formation and for the ocean circulation in the Atlantic region. Over the last 32 thousand years, numerous discharges from melting glaciers added freshwater to the Labrador Sea which could help understand the future effects of current melting glaciers. This information is necessary to better constrain climate predictions in order to gauge the effects on the Global Ocean Water Circulation. However, past deep water production in the Labrador is still poorly constrained, complicating reconstruction of the Atlantic Meridional Overturning Circulation on different timescales. In this study we investigated changes in deep and intermediate water mass circulation patterns over the last 32 kyr based on the radiogenic Nd isotope compositions that serve as a water mass circulation proxy. Analysis of four marine sediment cores show that the deposition of large volumes of detrital carbonates during studied period had a large effect on the recorded in the sediment column signals. New data suggest active water mass circulation during the maximum extent of glacial ice sheets. The modern day ocean circulation patterns have emerged during the Late Holocene (6 ka). Key Points: Estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador SeaDissolution of detrital dolostones in the water column during Heinrich stadials at least partially controlled ɛNd signaturesDuring the LGM generally more radiogenic signatures possibly indicate active water mass advection and mixing in the Labrador Sea [ABSTRACT FROM AUTHOR]

Several large‐scale components of the climate system may undergo a rapid transition as critical conditions are exceeded. These tipping elements are also dynamically coupled, allowing for a global domino effect under global warming. Here we focus on such cascading events involving the Greenland Ice Sheet (GIS), the West Antarctica Ice Sheet (WAIS) and the Atlantic Meridional Overturning Circulation (AMOC). Using a conceptual model, we study the combined tipping behavior due to three dominant feedbacks: the marine ice sheet instability for the WAIS, the height‐surface mass balance feedback for the GIS and the salt‐advection feedback for the AMOC. We show that, in a realistic parameter range of the model, a tipping of the WAIS can inhibit cascading events by preserving the AMOC stability. Plain Language Summary: In the climate system, the interaction of specific components known as tipping elements are thought to be able to induce a global domino effect, or cascading tipping. In this study, we present a conceptual model containing the most strongly interacting components, namely the Atlantic Meridional Overturning Circulation (AMOC), the Greenland Ice Sheet and the West Antarctica Ice Sheet. We find that the stability of this system as a whole is strongly modified when interactions are included. Especially, while a Greenland Ice Sheet collapse destabilizes the AMOC, the model shows that a collapse of the West Antarctica Ice Sheet might prevent a global cascading event by stabilizing the AMOC. Key Points: A conceptual model of interacting Atlantic Meridional Overturning Circulation (AMOC), Greenland Ice Sheet and West Antarctica Ice Sheet (WAIS) is presentedInteractions between these tipping elements strongly modifies the stability of the whole systemA collapse of the WAIS can prevent tipping of the AMOC [ABSTRACT FROM AUTHOR]

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020. Plain Language Summary: The Gulf Stream is the only pathway by which the ocean transports warm water to high latitudes in the North Atlantic and Nordic Seas where it cools, sinks and flows back south at intermediate depths from the subpolar North Atlantic and at greater depths from the Nordic Seas. This circulation pattern, the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate. But the Gulf Stream is also the wester boundary current to the horizontal circulation driven by the trade winds and westerlies at low and mid‐latitudes. The Gulf Stream also drives large recirculations to its south and north. In this study we use hydrographic data taken since the 1930s to calculate the poleward flow in the Gulf Stream, while canceling out the recirculation and wind‐driven transports to estimate the strength of the north‐flowing portion of the AMOC. There is evidence for a 2.0 Sv Gulf Stream slow‐down between 1930 and 2020. Whether and to what extent this reflects a slowdown of the AMOC or wind‐driven circulation cannot be established with certainty. Our estimate of a 0.4 Sv AMOC decrease is reported with low confidence. Key Points: Direct measurements along the Oleander line (∼37°N) show that the depth of the Atlantic Meridional Overturning Circulation (AMOC) maximum is close to 1,000 mNet northward transport between North America and Bermuda equals 41.1 ± 0.4 Sv of which the AMOC accounts for about 18.4 ± 0.6 SvNet northward transport NW of Bermuda slowed by 2 ± 0.8 Sv between 1930 and 2020 with 0.4 ± 0.6 Sv attributed to an AMOC decline [ABSTRACT FROM AUTHOR]

The Irminger Sea is one of the few regions in the ocean where deep (>1,000 m) convection occurs. Convection is followed by restratification during summer, when the stratification of the water column is reestablished and the convectively formed water is exported at depth. There are currently no descriptions of interannual variability and physical drivers of restratification in the Irminger Sea. We investigate restratification in the upper 600 m of the central Irminger Sea using reanalysis data for the years 1993–2019. We find distinctly different restratification processes in the upper 100 m (the upper layer) and the water below it (the lower layer). In the upper layer, the stratification is dominated by a seasonal cycle that matches the cycle of the surface heat flux. In 2010 and 2019, there were peaks in upper layer restratification, which could partly be related to strong atmospheric heat and freshwater fluxes. Greenland runoff likely also contributed to the high restratification, although this contribution could not be quantified in the present study. In the lower layer there is strong interannual variability in stratification, caused by variability both in the convection and the restratification strength. The restratification strength is strongly correlated with the eddy kinetic energy in the eastern Irminger Sea, suggesting that lower layer restratification is driven by lateral advection of warm, saline waters through Irminger Current eddies. In the future, surface warming and freshening of the Irminger Sea due to anthropogenic climate change are expected to increase upper layer stratification, potentially inhibiting convection. Plain Language Summary: In the Irminger Sea, cold winters can cause the ocean's surface waters to cool enough that they start mixing downward. The mixing can continue for months and eventually form a dense water mass of more than 1 km deep. This process is called deep convection. During summer, the dense water gets exported at depth, and the water column will go back to its original structure with lighter waters over denser waters. This is called restratification. The formation and export of dense waters in the Irminger Sea plays an important role in the global ocean circulation. Here we study what drives the restratification in the Irminger Sea, and how it varies in time. We find that in the upper 100 m the restratification is influenced by heating from the atmosphere, which warms the surface waters and makes them lighter. Additionally, rainfall or ice melt which make the surface waters fresher can contribute to restratification. Below the surface layer, restratification happens through transport of warm and saline waters from the eastern Irminger Sea. With rising ocean temperatures due to climate change, the Irminger Sea might see more restratification in the future, which might slow down global oceanic heat transport. Key Points: Restratification in the Irminger Sea is distinctly different in the upper 100 m of the water column than belowIn the upper layer, summers with exceptionally high stratification had strong atmospheric heat and freshwater gainIn the lower layer, restratification is driven by lateral advection of warm and saline waters through Irminger Current eddies [ABSTRACT FROM AUTHOR]

The Subpolar North Atlantic plays a critical role in the formation of the deep water masses which drive Atlantic Meridional Overturning Circulation (AMOC). Labrador Sea Water (LSW) is formed in the Labrador Sea and exported predominantly via the Deep Western Boundary Current (DWBC). The DWBC is an essential component of the AMOC advecting deep waters southward, flowing at depth along the continental slope of the western Atlantic. By combining sustained hydrographic observations from the Labrador Sea to 26.5°N, we investigate the signal propagation and advective timescales of LSW via the DWBC from its source region to the Tropical Atlantic through various approaches using robust neutral density classifications. Two individually defined LSW classes are observed to advect on timescales that support a new plausible hydrographically observed advective pathway. We find each LSW class to advect on independent timescales, and validate a hypothesized alternative‐interior advection pathway branching from the DWBC by observing the arrival of LSW outside of the DWBC in the Bermuda basin on timescales similar to arriving at 26.5°N, 10–15 yr after leaving the source region. Advective timescales estimated herein indicate that this interior pathway is likely the main advective pathway; it remains uncertain whether a direct pathway plays a significant advective role. Using LSW convective signals as advective tracers along the DWBC permits the estimation of advective timescales from the subpolar to tropical latitudes, illuminating deep water advection pathways across the North Atlantic and the lower‐limb of AMOC as a whole. Plain Language Summary: The Deep Western Boundary Current (DWBC) exports cold and dense deep waters formed in the Subpolar North Atlantic to the tropics, and therefore plays a primary role in global ocean circulation and heat balance. We focus here on Labrador Sea Water (LSW), a water mass formed through wintertime mixing events within the Subpolar North Atlantic characterized by distinctive low‐temperature and low‐salinity signatures. By following the passage of these signatures through several locations, we investigate the pathways and spreading timescales of LSW from its source region toward the subtropical North Atlantic by the DWBC. We find two distinct LSW masses to reach the same location on independent timescales, and observe LSW in the Central Atlantic just prior to or on the same timescale as being observed in the Tropical Atlantic. These findings indicate that an alternative‐interior export pathway branching from the DWBC is likely to exist, exporting LSW away from the continental slope and into the Atlantic interior rather than following a direct equatorward route. Estimating advective timescales and pathways of the DWBC using LSW aid in the present understanding and future prediction of overturning circulation in the Atlantic Ocean. Key Points: Advection timescales of the Deep Western Boundary Current (DWBC) are inferred from Labrador Sea Water (LSW) convective minima in the North AtlanticThe two LSW classes examined are observed to advect on different timescales, likely due to varying advective pathwaysAn alternative interior‐advective pathway observed as a bifurcation in the DWBC advects water to the interior [ABSTRACT FROM AUTHOR]

The change in anthropogenic CO2 (Canth) in the Atlantic Ocean is linked to the Atlantic Meridional Overturning Circulation (AMOC), that redistributes Canth meridionally and in depth. We have employed direct biogeochemical measurements and hydrographic data from the last 30 years, adjusted using inverse models for each decade with both physical and biogeochemical constraints. We then have computed the meridional transports and the vertical transports between two sections at the interphases by advection and diffusion. We have focused on the repeated sections at three latitudes—30°S, 24, and 55°N, dividing the Atlantic into two boxes. We have divided the net transport into upper, deep and abyssal layers, with an upper and abyssal northward transport of Canth and a southward component in deep layers. The change in time in the net transports of Canth appears to be mainly due to modifications in the transport of upper layers. The lower layer of the AMOC, a combination of deep and abyssal waters, maintain more consistent transports in time. Vertical advection plays an important role in the North Atlantic, exporting Canth from upper to deep layers. In the South Atlantic, the newly formed Antarctic Bottom Water exports Canth from abyssal to deep layers. The strong gradient in Canth concentration at the interphase of upper and deep layers results in a strong vertical diffusion. Plain Language Summary: Rising levels of CO2 in seawater may affect the calcifying organisms that depend on the balance of different carbon species. Therefore, the amount of anthropogenic carbon (Canth) that is captured by the ocean has been closely monitored. Using hydrographic and biogeochemical observations from a set of oceanographic cruises covering the Atlantic basin for the last three decades, we have built three models to investigate the state and evolution of Canth at different depths of the Atlantic Ocean. Those results helped us determine the strength of the meridional circulation of Canth in the Atlantic and the importance of vertical fluxes in the sequestering of Canth in the ocean. Key Points: Three decades of inverse modeling of meridional and vertical transports of Canth in the AtlanticThe change in the net transport of Canth appear from the variability in upper layersOverturning and diffusion contribute almost equally to the interior storage of Canth [ABSTRACT FROM AUTHOR]

The interaction between ocean circulation and ice‐sheet dynamics plays a key role in the Quaternary climate. Compared with the surface and deep regions of the Atlantic Meridional Overturning Circulation (AMOC), the study of intermediate depths during key time periods, such as Heinrich Stadials (HSs), remains poorly documented, especially for the northeast Atlantic. Here we use benthic foraminiferal assemblage data to trace palaeoenvironmental changes from ~32 to 14 ka cal bp at ~1000 m depth in the Bay of Biscay. Our results highlight the high sensitivity of foraminifera, with species‐specific responses, to continental (European Ice Sheet dynamics) and marine (AMOC) forcing factors during the last three HSs. In general, HSs were characterised by the concomitant presence of meso‐oligotrophic and anoxia indicator species and the low abundance of high‐energy indicator species. This confirms an overall sluggish intermediate circulation during the three HSs in the northeast Atlantic. HS1 is distinctive for its abundance of high‐organic flux indicator species during its early phase. This is consistent with the fact that HS1 was by far the most important period of ice‐sheet retreat and meltwater release into the ocean over the studied time interval. Finally, foraminifera depict the mid‐HS2 reventilation event due to regional glacier instabilities. [ABSTRACT FROM AUTHOR]

Under the ongoing and potential risks from anthropogenic warming, net negative carbon dioxide (CO2) emissions are inevitable to stabilize or recover the Earth's climate. It is important not only to understand climate irreversibility in response to CO2 removal but also to understand how fast each component of the climate system will recover to its original state. Based on idealized CO2 ramp‐up and ‐down ensemble simulations, here we show that the initial buoyancy states of the Arctic Ocean, such as upper ocean salinity and density, are vital to determining how fast Arctic and global mean temperatures will recover on a centennial time scale. The denser initial Arctic oceanic condition is linked to faster recovery of the Atlantic meridional overturning circulation (AMOC) in the ramp‐down period, which is further accelerated by strong positive AMOC‐salt‐advection feedback. Faster AMOC recovery can delay Arctic temperature recovery by transporting warmer water into the northern subpolar Atlantic during the ramp‐down period. In addition, denser Arctic water enhances vertical mixing, which also results in delayed Arctic cooling under a strong vertical temperature gradient in the subpolar‐to‐polar Atlantic. Our findings suggest that the Arctic's initial states have a centennial memory for the future Arctic and global climate changes. Plain Language Summary: Understanding whether anthropogenic global warming is irreversible, and how fast our climate system will recover to its original state is of great socio‐economic benefit as this is closely related to future climate adaptation and policy decisions. In this study, we investigate what determines the recovery speed of the Arctic temperature. We found that the most uncertain region in response to carbon dioxide (CO2) removal is the Arctic. The Arctic temperature shows a strong inter‐ensemble spread when the CO2 concentration returned to the present level. This different Arctic temperature in each ensemble member can be explained by the density and salinity of the Arctic Ocean about a century ago. For example, a denser Arctic Ocean condition in the early CO2 reduction pathway slows down the recovery of the Atlantic meridional overturning circulation (AMOC) in the presence of positive AMOC‐salt‐advection feedback. Because faster AMOC recovery can transport more warm water into the Atlantic sector of the Arctic, Arctic cooling as a result of CO2 reduction is delayed. Our findings suggest that the Arctic's initial states have a centennial memory for the future Arctic and global climate changes. Key Points: The carbon dioxide removal ensemble simulation shows that the Arctic climate changes exhibit the largest ensemble spread, suggesting the most uncertain regionThe initial buoyancy condition of the Arctic Ocean has a centennial memory for the future recovery of Arctic temperatureThe denser Arctic Ocean is linked to faster recovery of the Atlantic meridional overturning circulation (AMOC) and Arctic temperature, with strong AMOC‐salt‐advection feedback [ABSTRACT FROM AUTHOR]

Multidecadal variability of Atlantic Meridional Overturning Circulation (AMOC) has been reconstructed by various proxies, simulated in climate models, and linked to multidecadal Arctic salinity variability. Here we construct a simple conceptual model to understand the two‐way interactions of the Arctic with multidecadal AMOC variability through a delayed oscillator mechanism. We revise Stommel's Two‐Box Model by including an advective time delay for the Arctic density/salinity anomalies to reach the subpolar North Atlantic and a coupled negative feedback between the AMOC and the freshwater flux entering the Arctic through atmosphere and/or sea ice responses. Self‐sustained multidecadal AMOC oscillations exist in the revised Stommel's Two‐Box model if the oceanic advective time delay is longer than the oscillation threshold, and the periods of the AMOC delayed oscillator depend crucially on this advective time delay. The coupled freshwater feedback provides additional delayed negative feedback and reduces the advective time delay threshold required for the oscillations. Plain Language Summary: Stommel's Two‐Box Model provides a pioneering and powerful theoretical framework to study the mechanisms of steady Atlantic Meridional Overturning Circulation (AMOC) states and abrupt AMOC changes. However, Stommel's Two‐Box Model itself does not include a self‐sustained multidecadal AMOC oscillation solution. In our study, we revise Stommel's Two‐Box model by considering the delayed oceanic advective time lag for the Arctic density/salinity anomalies to reach the subpolar North Atlantic and the dependence of the freshwater flux entering the Arctic on the AMOC strength. The revised Stommel's Two‐Box Model is able to obtain the AMOC delayed oscillator at multidecadal timescales and suggests the important role of the Arctic salinity anomalies and associated delayed negative feedback in the multidecadal AMOC variability. The regimes and multidecadal periods of the AMOC delayed oscillator depend crucially on the advective time scale of the Arctic signal reaching the subpolar North Atlantic. The AMOC‐related coupled freshwater feedback provides additional delayed negative feedback and reduces the threshold of the advective time delay needed for the multidecadal AMOC oscillations. Key Points: Multidecadal Atlantic Meridional Overturning Circulation (AMOC) variability exists in revised Stommel's Two‐Box Model with advective time delay and coupled freshwater feedback includedRegimes/periods of the AMOC delayed oscillator depend on the advective time lag of Arctic salinity signal and freshwater feedback strengthThe model suggests an important role of the Arctic salinity and delayed negative freshwater feedback in the multidecadal AMOC variability [ABSTRACT FROM AUTHOR]

Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer‐term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient‐like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle. Key Points: We reconstructed seawater cadmium in the Florida Straits over the HoloceneCd gradually declined in the last 8,000 years, economically explained by weakening AMOC but inconsistent with geostrophic transport estimatesMore intense sampling over last 2,000 years suggests little Cd variability on centennial timescales, consistent with weak AMOC variability [ABSTRACT FROM AUTHOR]

A significant fraction of the Eulerian‐mean downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) occurs along the subpolar North Atlantic continental slopes and is maintained by along‐boundary densification and large‐scale geostrophic balance. We here use Argo and shipboard hydrography data to map the 2002–2015 long‐term mean density field along the boundary via a dedicated optimal interpolation tool. The overall downstream densification implies an Eulerian‐mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. A clear regional pattern emerges with downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross‐basin estimates confirm that vertical overturning transport across the marginal seas of the subpolar North Atlantic mainly occurs along the continental slopes, and suggest the usefulness of hydrographic data in providing quantitative information about the sinking branch of the AMOC. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC), a critical component of the Earth's climate system due to its role in redistributing heat and freshwater between low and high latitudes, is anticipated to decline over the next century. The downwelling of surface waters in the subpolar North Atlantic that feeds the lower limb of AMOC is a vital yet vulnerable process. As revealed by previous theoretical and modeling work, the overall downstream densification along the boundary results in a significant boundary downwelling. Here, the density along the western boundary between Denmark Strait and Flemish Cap is reconstructed to provide a first observation‐based description of the regional and seasonal distribution of this boundary‐focused downwelling in the subpolar North Atlantic. This study not only provides valuable insights into how to improve existing ocean circulation theories of overturning but also contributes to a solid benchmark for evaluating how climate models simulate the sinking branch of the AMOC. Key Points: The long‐term mean full‐depth density field of the subpolar North Atlantic's boundary is reconstructed from hydrography dataThe along‐boundary densification results in a 2.12 ± 0.43 Sv Eulerian‐mean downwelling between Denmark Strait and Flemish CapA first observation‐based regional and seasonal distribution of near‐boundary Eulerian‐mean downwelling is provided [ABSTRACT FROM AUTHOR]

Using the climate model MIROC4m, we simulate self‐sustained oscillations of millennial‐scale periodicity in the climate and Atlantic meridional overturning circulation under glacial conditions. We show two cases of extreme climatic precession and examine the mechanism of these oscillations. When the climatic precession corresponds to strong (weak) boreal seasonality, the period of the oscillation is about 1,500 (3,000) years. During the stadial, hot (cool) summer conditions in the Northern Hemisphere contribute to thin (thick) sea ice, which covers the deep convection sites, triggering early (late) abrupt climate change. During the interstadial, as sea ice is thin (thick), cold deep‐water forms and cools the subsurface quickly (slowly), which influences the stratification of the North Atlantic Ocean. We show that the oscillations are explained by the internal feedbacks of the atmosphere‐sea ice‐ocean system, especially subsurface ocean temperature change and salt advection feedback with a positive feedback between the subpolar gyre and deep convection. Plain Language Summary: Ice core data indicate the existence of millennial‐scale climate fluctuations, known as Dansgaard‐Oeschger (DO) events, with global‐scale impacts during the last glacial period. Evidence suggests that DO events are closely related to abrupt changes in the Atlantic meridional overturning circulation (AMOC). Here, using the climate model, MIROC4m, we simulate DO‐like oscillations of millennial‐scale periodicity with a mid‐glacial atmospheric CO2 level and suitable obliquity and glacial ice sheet topography but without external freshwater forcing. When the AMOC is in a weak (strong) state, subsurface temperatures in the North Atlantic increase (reduce) gradually, which weakens (strengthens) stratification of the North Atlantic Ocean and leads to AMOC recovery (collapse). At the same time, salt advection with the North Atlantic subpolar gyre changes greatly with the abrupt changes in the AMOC and works as a positive feedback. Furthermore, we examine the direct effects of the precessional component of orbital forcing on the AMOC by comparing the two cases of extreme climatic precession, which can change the seasonality of the atmosphere‐ocean system. Our simulations show that when the precession shifts to stronger boreal seasonality, the period of the oscillation shortens through changes in sea ice thickness and subsurface temperature in the North Atlantic. Key Points: Dansgaard‐Oeschger‐like oscillations of millennial‐scale are simulated with a coupled climate model, MIROC4m, under glacial conditionsThe period of the simulated oscillations shortens as the climatic precession shifts to stronger boreal seasonalityThe period of the oscillations can be influenced by the seasonal sea ice change and the subsurface temperature change in the North Atlantic [ABSTRACT FROM AUTHOR]

The Irminger Current (IC) on the western flank of the Reykjanes Ridge is an important contributor to the northward transport in the Atlantic Meridional Overturning Circulation. Here, we combine 28 years of Copernicus Marine Environment Monitoring Service (CMEMS) ocean reanalysis data with 6 years of mooring data to investigate variability in volume transport of the IC. We found a mean volume transport for the IC of 11.6 Sv between 1993 and 2020 revealing the dominance of the IC in the total Irminger Sea northward volume transport (20.3 Sv). We found a significant decrease (−3.7 Sv) in volume transport of the IC until 2011 followed by a steeper (but shorter, leading to +2.7 Sv) increase until 2020. These changes across the Irminger Sea section are dominated by the IC, which in turn are driven by changes in the density gradient across the Irminger Sea related to convection. On decadal and interannual time scales the IC volume transport is well correlated with the decrease in the sea surface height difference over the basin. In 2019, a temporary intensification of the western IC core led to an exceptionally strong volume transport of the IC with 20 Sv. This was caused by density changes within the IC boundaries due to the presence of mesoscale eddies. Thus, IC transport variability is a superposition of basin‐wide to local processes that influence the velocity field on different time scales. Plain Language Summary: The Irminger Current (IC) transports relatively warm and saline water northwards along the Mid Atlantic Ridge toward Iceland, where one part continues further north and the rest turns around to flow south along Greenland. We investigate changes in the IC's northward transport using a combination of direct observations from moorings and a longer reanalysis time series. We found that the IC is dominating the northward volume transport in the eastern Irminger Sea. The IC is also driving the long‐term decrease in the total northward volume transport. Its intensity depends on changes in the basin‐wide density gradient between sea surface height (SSH) minimum in the convection area of the central Irminger Sea and the SSH maximum on top of the Mid Atlantic Ridge. This study shows that the high variability in transport can be explained by a combination of processes acting on different time scales. Changes in the IC and the central Irminger Sea are crucial to help us monitor changes in the overturning of the eastern subpolar North Atlantic. Key Points: The Irminger Current (IC) is a major contributor to the northward volume transport in the Irminger SeaThe IC's volume transport decreased from 1993 to 2011 followed by an increase from 2012 to 2020Basin‐wide to local changes in the density field contribute to transport variability at different time scales [ABSTRACT FROM AUTHOR]

The Tibetan Plateau (TP) is important in shaping the global climate. To understand how its thermal condition may affect the Atlantic meridional overturning circulation (AMOC), we conduct a series of albedo forcing experiments to simulate the TP warming and cooling conditions. The results show that the TP warming can immediately generate anticyclone in the subtropical North Atlantic, leading to more water vapor transporting from the North Atlantic to eastern tropical Pacific. This results in less precipitation over the North Atlantic and increases the sea surface salinity. The latter enhances the deep‐water formation in the subpolar North Atlantic and thus the AMOC. In contrast, the TP cooling weakens the AMOC via opposite processes. These findings imply that the AMOC change can be affected not only by local factors but also remote forcing from Asia. Plain Language Summary: The Tibetan Plateau (TP) plays a vital role in regulating the regional and global climate. By far, most studies focus on the impacts of TP thermal effect on atmospheric change. Few efforts have been devoted to investigate the TP thermal effect on ocean circulation. In this study, we artificially reduce and increase the surface albedo over the TP region to simulate the TP warming and cooling conditions. The results show that the TP thermal conditions can affect the Atlantic meridional overturning circulation (AMOC) strength by altering the inter‐basin moisture transport between the North Atlantic and tropical Pacific. The TP heating immediately generates a westward Rossby wave train that can arrive at the North Atlantic, resulting in anomalous high‐pressure system over there. This helps to bring more moisture from the North Atlantic to eastern tropical Pacific. The moisture divergence over the North Atlantic increases the sea surface salinity, which is responsible for the enhanced deep‐water formation and thus the AMOC. In contrast, the TP cooling leads to AMOC weakening via opposite processes. Our study implies that the 7°C TP warming at the end of 21st century under the representative concentration pathway 8.5 (RCP8.5) scenario can enhance the AMOC by about 10%. Key Points: The Tibetan Plateau (TP) heating (cooling) can cause enhanced moisture transport from the North Atlantic (Pacific) to Pacific (North Atlantic)The TP heating (cooling) can strengthen (weaken) the Atlantic meridional overturning circulation (AMOC)The TP warming at the end of 21st century under the RCP8.5 scenario can lead to a 10% AMOC enhancement [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) plays a vital role in global climate, redistributing heat, and freshwater. It is predicted to decline due to anthropogenic climate change, with major implications for global climate. Accurately assessing AMOC strength with in situ observations has inspired a number of dedicated observing systems in the Atlantic since the 2000s. However, no consensus has been reached on whether the slowdown of the AMOC and its associated heat and freshwater transports is occurring. These dedicated systems are too recent to detect long‐term trends. We have analyzed hydrographic data from zonal sections across the Atlantic for 30 years that predate and overlap the era of AMOC observations. Our results show no changes in the AMOC for all sections analyzed over the whole Atlantic for the last 30 years. We also find an increased export of freshwater from the South Atlantic associated with an increase in upper salinity. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is the oceanic process by which upper warm waters flow northward and cold deep waters flow southward. The AMOC has a large effect on European and global climate. Models have predicted a decline of its strength due to anthropogenic climate change. Across‐ocean systems monitoring the currents on the water column have yet to find this slowdown. We have analyzed hydrographic data collected for the last 30 years and have built a model for each decade of the circulation of the Atlantic, and found no changes in time in the Atlantic Ocean for each hydrographic section. Also, our results present an increase in the amount of freshwater leaving the South Atlantic. Key Points: No changes at any latitude between hydrographic sections carried out in the last 30 yearsThe major contributor to the Atlantic Meridional Overturning Circulation at the subpolar North Atlantic is the Eastern subbasin for the three decadesThe increase of southward freshwater overturning transport at 30°S indicates a bistable state of the AMOC [ABSTRACT FROM AUTHOR]

The mooring observations of the Overturning in the Subpolar North Atlantic Program reveal a significant freshening of the Iceland Scotland overflow waters that did not involve the Nordic Seas, the source of the dense Deep North Atlantic Water (Devana et al., 2021, https://doi.org/10.1029/2021GL094396). Their study suggests that this freshening at depth in the Iceland Basin stems from the largest upper‐ocean freshening event in 120 years that rapidly communicated through entrainment with the Iceland Scotland Overflow Waters. This communication, which is very likely driven by strong wintertime heat losses, strongly adds to our thinking that the progression of this extreme freshening event is providing us with a natural tracer that is helping to identify and understand key processes that determine the strength and variability of the overturning circulation and its sensitivity to ongoing climate change. Continued monitoring of the overturning in the North Atlantic is therefore necessary. Plain Language Summary: The largest surface freshening event in 120 years observed in 2016 in the eastern Subpolar North Atlantic is now found to have rapidly communicated with the deep waters that sustain the Atlantic overturning circulation, a rich system of ocean currents that is key to the European and global climate. Key Points: Overturning in the Subpolar North Atlantic Program moorings reveal a rapid communication between the largest upper‐ocean freshening event and the Iceland Scotland Overflow WatersSalinity variations of the Nordic Seas bottom waters reveal no connection to this recent deep ocean freshening in the Iceland BasinThe extreme rate of freshening has provided us with a natural tracer that is helping us identify and understand key Atlantic Meridional Overturning Circulation processes [ABSTRACT FROM AUTHOR]

The multiple equilibrium behavior of the Atlantic meridional overturning circulation (AMOC) is related to the mode transition of the AMOC, which played an important role in abrupt glacial climate changes. However, the difference between modern and glacial climates is not well understood. Here, we examined the structure of the multiple equilibria of the AMOC under the modern and glacial climates through hysteresis experiments using an earth system model of intermediate complexity. The glacial AMOC exhibited a shallower AMOC mode but not the off mode, and a narrower range of multiple equilibria than the modern AMOC. This glacial hysteresis behavior occurred associated with a shift of deep water formation due to changes in surface air temperature and sea ice, whereas that of the modern AMOC is attributed to the basin‐scale salinity feedback. These features were commonly found in two glacial simulations with different strengths of the AMOC at the Last Glacial Maximum. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is the Atlantic part of the density‐driven deep ocean circulation and contributes to the warm climate in the North Atlantic through meridional heat transport. The AMOC is known to have multiple modes and transitions between them are deeply connected to abrupt climate changes during glacial periods. We compared the structures of the AMOC modes during present‐day and glacial climates by imposing a gradually changing freshwater flux on the North Atlantic using an earth system model of intermediate complexity. The glacial AMOC shows a shallow‐AMOC mode that is absent in the modern AMOC. In addition, the range of the bistable regime is narrower in the glacial climate than in the modern climate. These features are explained by different mechanisms that drive mode transitions between the modern and glacial climates. In the present‐day climate, salt transport into the Atlantic by the AMOC itself acts as dominant feedback for mode transitions. In the glacial climate, however, a stable shallow‐AMOC mode appears because of the changes in the deep water formation site caused by atmospheric cooling. Interactions among deep water formation, air temperature, and sea ice are important for the glacial mode shift. Key Points: The glacial Atlantic meridional overturning circulation (AMOC) shows different hysteresis behaviors from the modern AMOC in an earth system model of intermediate complexityThe differences in hysteresis are commonly found in two simulations showing different reproducibility of the glacial AMOCAn atmospheric thermal feedback becomes important rather than basin‐scale salinity feedback for the glacial AMOC mode transitions [ABSTRACT FROM AUTHOR]

We present a simple smoother designed for smooth data adjustments in sequentially generated reanalysis products by utilizing knowledge of future assimilation increments. A decay time parameter is applied to the smoother increments to account for memory decay timescales in the ocean. The result is different from simply time smoothing the reanalysis itself as only the increments are smoothed so the reanalysis product can retain high frequency variability that is being internally generated by the model and the atmospheric forcing. The smoother is applied first to the Lorenz 1963 model and then to the daily Met Office GloSea5 Global ¼° ocean reanalysis during 2016. Results show significant improvement over the original reanalysis in the 3D temperature and salinity state and variability, as well as in the sea surface height (SSH) and ocean currents. Comparisons are made directly against temperature, salinity and SSH observations, as well as independent 15 m drifter velocities. The impact on the time variability of conservative quantities, particularly ocean heat and salt content, as well as kinetic energy and the Atlantic Meridional Overturning Circulation (AMOC), is also demonstrated. Plain Language Summary: The ocean observing system is sparse and ocean circulation is slow relative to the atmosphere, giving the system a longer memory of state properties. Therefore an ocean reanalysis could seek to use more "future data" to help produce the best historical ocean state reconstruction for a given day. Conventional sequential assimilation approaches used operationally make no use of "future" data outside the window of current data analyzed for forecasting. Here we introduce a new approach for application to large global ocean reanalysis systems that have only been run in "forward mode" (using past data), using the history of stored data increments to produce a more physically plausible time‐evolving ocean state with smoother temporal adjustments toward the available observations. Key Points: A novel temporal smoother algorithm is designed for improving ocean reanalysis products by utilizing the stored history of assimilation incrementsHigh frequency variability internally generated by the model and the atmospheric forcing is retained in the outputThe smoother significantly improves the reanalysis temperature, salinity, velocity, and sea surface height fields [ABSTRACT FROM AUTHOR]

We explore the climate system response to abrupt CO2 forcing, spanning the range 1× to 8×CO2, with two state‐of‐the‐art coupled atmosphere‐ocean‐sea‐ice‐land models: the NASA Goddard Institute for Space Studies Model E2.1‐G (GISS‐E2.1‐G) and the Community Earth System Model (CESM‐LE). We find that the effective climate sensitivity is a non‐monotonic function of CO2 in both models, reaching a minimum at 3×CO2 for GISS‐E2.1‐G, and 4×CO2 for CESM‐LE. A similar non‐monotonic response is found in Northern Hemisphere surface temperature, sea‐ice, precipitation, the latitude of zero precipitation‐minus‐evaporation, and the strength of the Hadley cell. Interestingly, the Atlantic meridional overturning circulation collapses when non‐monotonicity appears and does not recover for larger CO2 forcings. Analyzing the climate response over the same CO2 range with slab‐ocean versions of the same models, we demonstrate that the climate system's non‐monotonic response is linked to ocean dynamics. Plain Language Summary: We perform runs with two different models using CO2 concentrations in the atmosphere higher (from 1× to 8×CO2) relative to pre‐industrial conditions, in order to explore how the effective climate sensitivity (ECSeff) and the entire climate system change with increasing CO2. We show that ECSeff is a non‐monotonic function of CO2, minimizing at 3×CO2 in one model and 4×CO2 in the other. A similar non‐monotonic response appears in precipitation, sea‐ice, the edge of the dry zone, and Hadley cell strength. Interestingly, the Atlantic Meridional Overturning Circulation, which brings warm water into the North Atlantic, also shuts down at the same forcings when ECSeff is minimum and does not recover for higher forcings. We further show that the non‐monotonic response of the climate system stems from changes in ocean dynamics. Key Points: We examine the response of the climate system to abrupt 1.5×, 2×, 3×, 4×, 5×, 6×, 7×, and 8×CO2 forcing with two different coupled modelsClimate sensitivity, sea‐ice extent, global precipitation and the atmospheric circulation respond non‐monotonically across this range of CO2The non‐monotonicity of the response is associated with changes in ocean dynamics, notably over the North Atlantic [ABSTRACT FROM AUTHOR]

The IPSL‐CM6‐LR atmosphere‐ocean coupled model exhibits a pronounced multicentennial variability of the Atlantic meridional overturning circulation (AMOC), driven by delayed freshwater accumulation and release in the Arctic. The AMOC fluctuations are preceded by salinity‐driven density anomalies in the main deep convection sites in the Labrador and Greenland seas. During a strong AMOC, a combination of reduced sea ice volume and anomalous currents reduces the freshwater export from the Arctic and leads instead to a slow accumulation of freshwater in the central Arctic. Simultaneously, the saltier Atlantic inflow through the Barents Sea results in a positive salinity anomaly in the Eastern Arctic subsurface. When the surface Central Arctic freshwater pool finally reaches the Lincoln Sea, the oceanic currents around Greenland reorganize, leading to the export of the anomalous Arctic freshwater to the North Atlantic, enhancing the stratification in deep convection sites. The AMOC then decreases, positive salinity anomalies appear in the Central Arctic, and the variability switches to the opposite phase. These AMOC and sea ice fluctuations have broader climate impacts, with a strong AMOC leading to a mean warming of about 0.4°C north of 20°N, reaching up to 1°C in the Arctic lower troposphere during winter. In all seasons, a northward displacement of the intertropical convergence zone is also simulated. Plain Language Summary: The North Atlantic Ocean is known to have large climate fluctuations emerging from the different components of the climate system and their interactions. These fluctuations play a crucial role in the North American and European climate or the Arctic sea ice. A proper understanding of such internal variations is key to attribute the observed climate changes to anthropogenic activities or to assess the skill of decadal forecast systems. However, investigations of the long‐term basin‐scale variations are restrained by the limited instrumental observations. Therefore, an atmosphere‐ocean general circulation model is used here to explore the low‐frequency variability. This model simulates a large variability in the North Atlantic with a period between 1 century and 1 millennium. We found that this variability owes its existence to the freshwater exchanges between the North Atlantic and Arctic. Such North Atlantic variability has important impacts, as typical positive anomalies of the Atlantic oceanic northward heat transport reduce the sea ice, warm up the whole Northern Hemisphere by 0.4°C, and shift northward the intertropical convergence zone. Key Points: A centennial to multicentennial variability emerges from the North Atlantic and Arctic Ocean in the IPSL‐CM6A‐LR modelAMOC oscillations are driven by freshwater build‐up and release in the ArcticThe interplay between sea ice and oceanic freshwater export modulates the salinity anomalies in the Arctic [ABSTRACT FROM AUTHOR]

The upper branch of the Atlantic meridional overturning circulation (AMOC) plays a critical role in ocean circulation and climate change, yet its variability during the last glacial period is poorly documented. Here, we investigate the northward‐flowing Glacial Eastern Boundary Current (GEBC) in the North Atlantic, known today as the European Slope Current, and representing the easternmost portion of the upper branch of the AMOC. Based on flow speed and isotopic records, we show that Dansgaard‐Oeschger (D‐O) interstadials (stadials) correspond to a faster (weaker) GEBC during the ∼50–15 ka period. This, by analogy to present‐day conditions, suggests enhanced (reduced) strength of the subpolar gyre and, by extension, of northern‐sourced water production and AMOC during D‐O interstadials (stadials). Concomitant fluctuations of both the GEBC and the European Ice Sheet between ∼30 and 17 ka suggest an active role of the upper branch of AMOC in the poleward transport of heat and freshwater to the northern North Atlantic, with direct impacts on deep water formation and AMOC strength. We explore these ice‐sheet—AMOC interactions and the associated abrupt climate changes over the last glacial period and the last deglaciation. Key Points: The strength of the upper branch of Atlantic meridional overturning circulation (AMOC) is reflected in variations of the Glacial Eastern Boundary Current along the European marginSedimentological data show a Dansgaard‐Oeschger interstadial/faster‐stadial/slower flow patternFaster upper AMOC in the east basin corresponds with increased deep return flow in the west [ABSTRACT FROM AUTHOR]

Changes in ocean ventilation, controlled by both overturning rates and air‐sea gas exchange, are thought to have played a central role in atmospheric CO2 rise across the last deglaciation. Here, we constrain the nature of Atlantic Ocean ventilation changes over the last deglaciation using radiocarbon and stable carbon isotopes from two depth transects in the Atlantic basin. Our findings broadly cohere with the established pattern of deglacial Atlantic overturning change, and underline the existence of active northern sourced deep‐water export at the Last Glacial Maximum (LGM). We find that the western Atlantic was less affected by incursions of southern‐sourced deep water, as compared to the eastern Atlantic, despite both sides of the basin being strongly influenced by the air‐sea equilibration of both northern and southern deep‐water end‐members. Ventilation at least as strong as modern is observed throughout the Atlantic during the Bølling‐Allerød (BA), implying a "flushing" of the entire Atlantic water column that we attribute to the combined effects of Atlantic meridional overturning circulation (AMOC) reinvigoration and increased air‐sea equilibration of southern sourced deep‐water. This ventilation "overshoot" may have counteracted a natural atmospheric CO2 decline during interstadial conditions, helping to make the BA a "point of no return" in the deglacial process. While the collected data emphasize a predominantly indirect AMOC contribution to deglacial atmospheric CO2 rise, via far field impacts on convection in the Southern Ocean and/or North Pacific during Heinrich Stadial 1 and the Younger Dryas, the potential role of the AMOC in centennial CO2 pulses emerges as an important target for future work. Key Points: Increased ventilation ages occurred at all sites during the northern hemisphere stadials Heinrich Stadial 1, and the Younger DryasThe entire Atlantic was flushed with well‐ventilated waters at the onset of the Bølling‐AllerødDeglacial CO2 rise linked to abrupt releases from the Atlantic and more sustained release from the Southern Ocean and/or Pacific [ABSTRACT FROM AUTHOR]

The difference between North Atlantic subpolar gyre sea surface temperatures (SPG SSTs) and hemispheric‐ or global‐scale surface temperatures has been utilized as an index of centennial‐timescale changes in Atlantic Meridional Overturning Circulation (AMOC) strength. Here, using Community Earth System Model ensembles, we show that surface temperature‐based indices (STIs) proposed to date largely reflect global‐scale temperature trends and thus do not reflect dynamical relationships with AMOC. More broadly, we find that relationships between STIs, SPG SSTs, and AMOC strength differ greatly in significance and magnitude over different time periods because they are dependent upon the nature of external forcing. In the twentieth century, characterized by offsetting greenhouse gas and aerosol forcing, the relationship between SSTs and AMOC strength varies widely and changes sign across a 20‐member ensemble. We conclude that STIs and SPG SSTs are poor predictors of centennial‐timescale AMOC strength variations. Plain Language Summary: The short observational record of the Atlantic Meridional Overturning Circulation (AMOC) limits our ability to assess changes in its strength over the instrumental and preinstrumental periods. Indirect proxies of ocean circulation are thus required to make inferences about past trends, for example, those over the past century. Several previous analyses have used surface temperature indices to interpret twentieth century AMOC trends. However, the robustness of this indirect AMOC proxy, including its sensitivity to time period, timescale, and/or climate state, has not been assessed. We use two state‐of‐the art climate model ensembles to assess AMOC/surface temperature relationships over century timescales, finding a strong dependence upon time period and climate forcing. Our results clarify the origins of discrepancies in AMOC/surface temperature relationships and suggest that interpretations of twentieth century climate and ocean circulation change based on surface temperature indices are limited. Key Points: Previously proposed surface temperature indices (STIs) of AMOC strength are dominated by global temperature trendsSTIs are poor predictors of AMOC strength outside their calibration period, calling into question previous interpretations of twentieth century trendsOver centennial timescales, AMOC/STI relationships are sensitive to the nature of external forcing and unforced variability [ABSTRACT FROM AUTHOR]

The North Atlantic warming hole (NAWH) is referred to as a reduced warming, or even cooling, of the North Atlantic during an anthropogenic‐driven global warming. A NAWH is predicted by climate models during the 21st century, and its pattern is already emerging in observations. Despite the known key role of the North Atlantic surface temperatures in setting the Northern Hemisphere climate, the mechanisms behind the NAWH are still not fully understood. Using state‐of‐the‐art climate models, we show that anthropogenic aerosol forcing opposes the formation of the NAWH (by leading to a local warming) and delays its emergence by about 30 years. In agreement with previous studies, we also demonstrate that the relative warming of the North Atlantic under aerosol forcing is due to changes in ocean heat fluxes, rather than air‐sea fluxes. These results suggest that the predicted reduction in aerosol forcing during the 21st century may accelerate the formation of the NAWH. Plain Language Summary: Anthropogenic aerosols are particles suspended in the atmosphere, which were released due to anthropogenic activity. These particles have a general cooling effect on the Earth due to their interactions with radiation and with clouds. Here we show that the surface temperature in the North Atlantic Ocean is predicted to increase due to aerosol forcing (despite the global cooling). This trend is the opposite of the surface temperature trend predicted due to increase in greenhouse gases (global warming with a warming "hole" in the North Atlantic, trend known as the North Atlantic warming hole—NAWH). Using state‐of‐the‐art climate models, we show that aerosol forcing delays the formation of the NAWH by about 30 years. This trend could have important climatic impacts due to the key role of the North Atlantic surface temperatures in setting the Northern Hemisphere's climate and due to the predicted reduction in aerosol forcing in the next few decades. Key Points: Using CMIP6 and CESM‐LE simulations, we show that aerosol forcing generates a global cooling trend with a warming in the North AtlanticThis trend opposes the North Atlantic warming hole trend due to greenhouse gasesAerosol forcing delays the formation of the North Atlantic warming hole by about 30 years [ABSTRACT FROM AUTHOR]

The East Asian summer monsoon and winter monsoon (EASM and EAWM) are two important components of the East Asian monsoon system. However, the relationship between the two components during the Holocene remains debated in the observation. Here, we use a set of simulations in CCSM3 to investigate the EASM‐EAWM relation across different timescales during the Holocene. It is shown that the intensities of EASM and EAWM are positively correlated at orbital timescale but negatively correlated from millennial down to multidecadal timescales. The positive correlation at orbital timescales is forced by the seasonal insolation forcing, while the negative correlation at millennial and shorter timescales is caused mainly by the internal variability of the Atlantic Meridional Overturning Circulation (AMOC) and the subsequent teleconnection to East Asia via land‐sea thermal contrast. Plain Language Summary: The East Asian summer monsoon (EASM) and East Asian winter monsoon (EAWM) are important factors that affect regional climate. However, it has remained controversial whether EASM and EAWM vary in‐phase or out of phase in the Holocene. Our simulations show that EASM correlates with EAWM positively at orbital timescale due to the seasonal precessional forcing but negatively correlated from multidecadal to millennial timescales because of the impact of the Atlantic Meridional Overturning Circulation (AMOC). Key Points: Anticorrelations are found in EASM and EAWM variations from multidecadal to millennial timescales during the HoloceneThe negative EASM‐EAWM relation is likely caused by the AMOC [ABSTRACT FROM AUTHOR]

To study the role of the Atlantic meridional overturning circulation (AMOC) in transient climate change, we perform an abrupt CO2‐doubling experiment using a coupled atmosphere‐ocean‐ice model with a simple geometry that separates the ocean into small and large basins. The small basin exhibits an overturning circulation akin to the AMOC. Over the simulated 200 years of change, it stores heat at a faster rate than the large basin by 0.6 ± 0.2 W m−2. We argue that this is due to the small basin MOC. However, we find that as the MOC weakens significantly, it has little impact on the small basin's heat storage rate. We suggest this is due to the effects of both compensating warming patterns and interbasin heat transports. Thus, although the presence of a MOC is important for enhanced heat storage, MOC weakening is surprisingly unimportant. Plain Language Summary: The oceans take up the vast majority of the excess heat energy due to global warming. An important large‐scale ocean circulation is the Atlantic meridional overturning circulation (AMOC). Under global warming, it has been suggested that this circulation is important for storing excess heat energy in the deep ocean. Some evidence suggests that the AMOC has weakened since the mid‐twentieth century, and surface warming may intensify in response to the Atlantic storing less excess heat energy in the deep ocean. In order to examine this process more closely, we use a simple computer model of a world with no land masses and only two ocean basins: a small basin with a circulation similar to the AMOC, and a large basin without, similar to the Pacific. We approximate global warming by increasing the CO2 in the model atmosphere and find that the small basin stores heat at a faster rate than the large basin. The small basin's overturning circulation does weaken, but this weakening does not affect the small basin's heat storage rate. This is a surprising result and casts doubt on the idea that AMOC changes will significantly enhance surface warming as the Earth warms. Key Points: A deep meridional overturning circulation (MOC) connects the ocean surface to its interior, enhancing heat storage rate under global warmingThis circulation in the Atlantic (AMOC) may enable its enhanced heat storage rate relative to the Pacific in recent decadesMOC weakening does not necessarily impact ocean heat storage rate due to compensating physical processes [ABSTRACT FROM AUTHOR]

Four hydrographic cruises carried out between ~26.5 and 31°N in the eastern North Atlantic Subtropical Gyre in fall (2016 and 2017) and spring (2017 and 2018) are used to identify water masses and infer oceanic circulation. Geostrophic velocities are initially adjusted by referencing them to data from a Lower Acoustic Doppler Current Profiler (LADCP) and later to velocities estimated with an inverse box model. The distribution of the intermediate water masses (700 to 1,400 m depth) varies seasonally. Antarctic Intermediate Water (AAIW) comprises the largest contributor to the seasonal cycle in the intermediate water masses. Circulation of the Canary Current (CC) differs in fall and spring. In fall, the CC flows southward through the western islands and recirculates south of the archipelago, subsequently flowing northward through the passage between the eastern islands and Africa. North of Lanzarote, the recirculated CC intensified as it is joined by a southeasterly branch of the CC north of Lanzarote. In spring, the net transport of the CC is southward. High interannual variability in mass transport is evident in both spring and fall as a result of the position of the current, with its easternmost (westernmost) position found in spring (fall) 2018 (2016). At intermediate levels, highly variable northward/southward transport is apparent in fall over the African slope, with the Intermediate Poleward Under Current (IPUC) only present in 2017. Plain Language Summary: The Canary Current is part of the eastern boundary of the North Atlantic Subtropical Gyre as well as the upwelling system off NorthWest Africa. Its location plays a major role in the Atlantic Meridional Overturning Circulation, which controls the global climate. Therefore, we have studied the Canary Current. Its circulation pattern and its seasonal and interannual variability have been inferred using data from four hydrographic cruises (two carried out in fall and two in spring) around the region of the Canary Islands. From these cruises we have computed the mass transport of the current and identified the water masses present on it. The results show a high seasonal variation in the intermediate water masses (those on the depth range from ~700 to 1,400 m depth). The mass transport of the current also shows a seasonal variability at surface levels (<700 m depth) and at intermediate levels. The current flows southward through the islands and the passage between the islands and Africa in spring, but in fall, the CC reverses south of Fuerteventura and flows northward through the passage between the archipelago and Africa. Interannually, the mass transport also presents differences, being higher in fall 2017 and spring 2018. Key Points: High seasonal and interannual variabilities of the Canary Current transport over the African slopeInterannual variability of the Intermediate Poleward UnderCurrentHigh interannual variability in the seasonal cycle of the Atlantic Meridional overturning Circulation in the eastern boundary [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) has been, and will continue to be, a key factor in the modulation of climate change both locally and globally. However, there remains considerable uncertainty in recent AMOC evolution. Here, we show that the multimodel mean AMOC strengthened by approximately 10% from 1850–1985 in new simulations from the 6th Coupled Model Intercomparison Project (CMIP6), a larger change than was seen in CMIP5. Across the models, the strength of the AMOC trend up to 1985 is related to a proxy for the strength of the aerosol forcing. Therefore, the multimodel difference is a result of stronger anthropogenic aerosol forcing on average in CMIP6 than CMIP5, which is primarily due to more models including aerosol‐cloud interactions. However, observational constraints—including a historical sea surface temperature fingerprint and shortwave radiative forcing in recent decades—suggest that anthropogenic forcing and/or the AMOC response may be overestimated. Key Points: The Atlantic Meridional Overturning Circulation (AMOC) is important in modulating climate change, but its past evolution is uncertainIn CMIP6 historical runs, the AMOC strengthens by ~10% as a response to increased forcing from aerosol indirect effects, unlike in CMIP5Observational constraints suggest that anthropogenic forcing and/or the AMOC response may be overestimated [ABSTRACT FROM AUTHOR]

We explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multimodel ensemble. Comparison with RAPID and SAMBA observations suggests that the ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850–2014) are generally neutral, but many models exhibit an AMOC peak around the 1980s. Ensemble mean AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34–45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a circulation pattern in the Atlantic Ocean that is an important component of the climate system, due to its ability to redistribute and sequester heat and carbon. An accurate representation of the AMOC is a critical test for climate models and essential for building confidence in their projections. Here we investigate the AMOC in 27 climate models that contributed simulations to the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that many models reproduce the observed AMOC quite well, but there are still several models in which the AMOC is too weak or too strong. Most models suggest a slight upward trend in the AMOC from 1850 to the 1980s. Simulations representing different scenarios for future socioeconomic development suggest a stronger AMOC decline compared to previous assessments. Using direct measurements of the AMOC since 2004 and an emerging across‐model relationship between AMOC decline in the 21st century and their present‐day mean state, we find that the majority of CMIP6 models point to an end of century AMOC weakening of 34–45% of its present‐day strength. A smaller group of models projects much less weakening of only up to 30% of its present state. Key Points: AMOC mean strength is well reproduced by the CMIP6 multimodel mean, but large model spread persistsProjected AMOC decline by the end of the 21st century shows weak dependence on the SSP scenariosAn emergent constraint between AMOC strength and projected decline suggests possible AMOC decline between 34% and 45% by 2100 [ABSTRACT FROM AUTHOR]

The isotopic composition of neodymium dissolved in seawater consists of a distal, advected component that reflects water mass mixing and circulation but near land can also contain a large local component originating from terrestrial sources such as aeolian or fluvial material. In order to use Nd isotopes to reconstruct paleocirculation, it is important to detect any local influences on the seawater signal recorded in deep sea sediments. Here we present rare earth element (REE) and Nd isotope (εNd) records from the deep Caribbean for two well‐studied time intervals in the Late Pliocene and Early Pleistocene. We measured trace element and REE compositions of weakly cleaned foraminifera to investigate if the Nd isotope signal from the same samples contained a local component. We find distinct changes in REE compositions across glaciations that are consistent with increases in the supply of local terrestrial material to the basin likely the results of glacially driven changes in sea level. Despite these larger terrestrial inputs, the Ce anomaly (Ce/Ce*) became more pronounced during glaciations indicating a better deep Caribbean ventilation. Short negative Nd isotope excursions occurred during three of the four studied glaciations, independently of any other proxy indicators for changes in ocean circulation suggesting that inputs from local terrigenous sources of Nd controlled the signal. We recommend that studies that aim to use εNd as a paleocirculation tracer routinely measure REE compositions of the authigenic phase to identify any possible terrestrial influence on the signal. Key Points: REEs can be used to identify local influences on the Nd isotope record in the deep CaribbeanWe find a distinct terrestrial REE signal in the deep Caribbean associated with Late Pliocene and Pleistocene closure events of the Central American SeawayCe/Ce* records a ventilation signal despite input of terrestrial material [ABSTRACT FROM AUTHOR]

The poleward flow of Atlantic Water in the Nordic Seas forms the upper limb of the meridional overturning circulation driving an important heat transport. The Norwegian Atlantic Front Current along the Mohn Ridge between the Greenland and Norwegian Seas is characterized for the first time, using repeated sections over 14 months from autonomous underwater gliders and two research cruises. The Norwegian Atlantic Front Current follows the 2,550‐m isobath with a width of about 60 km and absolute geostrophic velocities peaking at about 0.45 m s−1. The mean transport of Atlantic Water is 4.6 ± 0.2 Sv (equivalent to temperature transport of 100 ± 6 TW). Seasonal variability was observed with an amplitude of 0.9 Sv and maximum values in the fall. The deep currents at 1,000 m explained most of this seasonal variation and were anticorrelated with time‐integrated wind stress curl over the Lofoten Basin. Part of this flow might recirculate within the Lofoten Basin, while the rest continues toward the Arctic. Key Points: Poleward Atlantic Water transport is 4.6±0.2 Sv along the Mohn RidgeSeasonality in deep currents leads to a maximum transport in the fallVolume transport variations were anticorrelated with wind stress curl sign over the Lofoten Basin [ABSTRACT FROM AUTHOR]

The Lagrangian method—where current location and intensity are determined by tracking the movement of flow along its path—is the oldest technique for measuring the ocean circulation. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to continuously track freely drifting surface buoys throughout the ice‐free oceans and begin to construct basin‐scale, and eventually global‐scale, maps of the surface circulation. At about the same time, development of acoustic methods to track neutrally buoyant floats below the surface led to important new discoveries regarding the deep circulation. Since then, Lagrangian observing and modeling techniques have been used to explore the structure of the general circulation and its variability throughout the global ocean, but especially in the Atlantic Ocean. In this review, Lagrangian studies that focus on pathways of the upper and lower limbs of the Atlantic Meridional Overturning Circulation (AMOC), both observational and numerical, have been gathered together to illustrate aspects of the AMOC that are uniquely captured by this technique. These include the importance of horizontal recirculation gyres and interior (as opposed to boundary) pathways, the connectivity (or lack thereof) of the AMOC across latitudes, and the role of mesoscale eddies in some regions as the primary AMOC transport mechanism. There remain vast areas of the deep ocean where there are no direct observations of the pathways of the AMOC. Plain Language Summary: Measuring ocean currents by following their paths—rather than by observing them at a fixed location—is the oldest method for exploring ocean currents. For centuries, mariners used compilations of ship drift data to map out the location and intensity of surface currents along major shipping routes of the global ocean. In the mid‐20th century, technological advances in electronic navigation allowed oceanographers to track freely drifting surface buoys continuously throughout the ice‐free oceans and begin to construct global maps of the surface currents. At about the same time, the development of methods using sound to track floats below the surface led to important new discoveries regarding the currents bar below the surface. Since then, these techniques have been used to explore the location and strength of currents, and how they change in time, throughout the world's oceans, but especially in the Atlantic Ocean. Computer simulations of the oceans are also used to calculate the pathways of virtual surface drifters and subsurface floats. In this review, studies that use these flow‐following methods to measure the pathways of the north‐south shallow and deep currents that make up the Atlantic Meridional Overturning Circulation (also sometimes referred to as the Great Ocean Conveyor) have been gathered together to illustrate aspects of the conveyor system that are uniquely captured by this technique. These include the importance of large recirculations, or "waiting areas," where water circulates around and around before moving onto the next segment of the conveyor. Also, these techniques for observing ocean currents illustrate how disconnected some parts of the conveyor are, and how swirling, translating pools of water ~100 km in diameter, and not continuous currents, help to move water along the conveyor's path. Vast areas of the deep ocean remain where there are no direct observations of the pathways of the conveyor. Key Points: Lagrangian methods are critical to describing the three‐dimensional structure of the Atlantic Meridional Overturning CirculationFluid parcel trajectories reveal the lack of meridional connectivity in both the shallow and deep limbs of the overturning circulationHigh spatial‐temporal resolution trajectories identify regions where coherent eddies contribute to the overturning circulation [ABSTRACT FROM AUTHOR]

The cycling of climate into and out of glacial states in response to orbital forcing illustrates the important role of feedbacks in the Earth system. In this contribution, the Geophysical Fluid Dynamics Laboratory coupled ocean‐atmosphere model is used to qualitatively assess how changes in the Earth's precessional parameters may have impacted Atlantic Meridional Ocean Circulation and Southern Ocean dynamics. The Southern Ocean region responds to precessional forcing similar to that of the Last Glacial Maximum with a slight shift in maximum average annual wind stress, a reduction in austral winter wind stress, and an increase in austral winter sea ice. During austral summer, stratification and wind stress increase, Changes in the temperature and density of the North Atlantic lead to a decrease in Atlantic Meridional Overturning Circulation. Together these changes contribute to aging of the global deep ocean, except around Antarctica where enhanced local convection decreases the age of upwelling waters. These circulation changes may have implications for a precessionally driven feedback that contributes to enhanced storage of CO2 in the deep ocean and a reduction in atmospheric CO2. Plain Language Summary: The Earth has cycled into and out of ice ages over last 2.6 million years. The precise factors that cause these changes in climate are not fully understood. The correspondence between the Earth's climate and the Earth's position in space suggests that there is a connection between the two. In this work we use a fully coupled global climate model to explore whether the season during which Earth is closest to the Sun has an impact on ocean circulation. We use two model runs, one having the Earth in a northern hemisphere winter perihelion (close pass) position and another placing the Earth in a northern hemisphere summer perihelion. We find that the northern hemisphere winter perihelion position is associated with changes in Southern Ocean circulation that include an increase in upwelling and a decrease in the age of the upwelled water. We also find that deep ocean circulation in the Atlantic Ocean decreases and that global ocean deep waters increase in age. The observed changes are consistent with the idea that a Northern Hemisphere winter perihelion position may contribute to glacial interglacial climate change and a drawdown of atmospheric CO2. Key Points: Precessional forcing that places the Earth at a boreal winter perihelion position causes an increase in wind stress and Ekman upwelling during austral summer/fall and a decrease during austral winter/spring in the high‐latitude Southern OceanGlobal deepwater age responds to a boreal winter perihelion position by increasing ~200 yearsAtlantic Meridional Overturning Circulation decreases in response to a boreal winter perihelion position. Southern Ocean ventilation increases [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken in the 21st century due to increased surface buoyancy. Such AMOC changes in ocean models are often accompanied by a subsurface reduction in density. Here we perform freshwater perturbation experiments with both a 1° coupled model and an idealized zonally averaged ocean‐only model to demonstrate that slow subsurface property changes (1) introduce a negative feedback that erodes the stratification and partially reinvigorates convection and the AMOC and (2) ensure the meridional heat transport weakens less than the AMOC. In the coupled model with a 0.1‐Sv net freshwater flux introduced around Greenland, an initial 22% AMOC reduction over 40 years is followed by a recovery of almost half the lost strength after 400 years. The final heat transport, however, is weakened by only 7%. Similar responses in the idealized model demonstrate that 2‐D ocean‐only dynamics control the changes. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) describes the northward upper‐ocean transport and subsequent deep return flow by the currents in the Atlantic, and its heat transport is an important element of the climate. The AMOC is predicted to weaken over the 21st century, and tentative evidence suggests that it may already be slowing down. Here we use freshwater experiments with both a comprehensive climate model and an idealized numerical model to identify and explain a North Atlantic oceanic mechanism that can induce a partial recovery of the AMOC and its heat transport under climate change forcing. This mechanism operates through gradual subsurface changes in ocean properties that reduce the high‐latitude vertical density gradient and allow a partial reinvigoration of the deep water formation that is necessary to sustain overturning. However, the mechanism requires that the AMOC remain active and therefore that climate change forcing remains limited. Key Points: We isolate a feedback mechanism for a partial AMOC recovery following a simulated AMOC weakeningThe heat transported by the weakening AMOC becomes more efficient as its upper branch warmsSimilar results from a 2‐D model demonstrate the recovery arises through an ocean‐only 2‐D mechanism [ABSTRACT FROM AUTHOR]

Freshwater transports (Fov) by the Atlantic meridional overturning circulation (AMOC) are sensitive to salinity distributions and may determine AMOC stability. However, climate models show large salinity biases, distorting the relation between Fov and the AMOC. Using free‐running models and ocean reanalyses with realistic salinities but quite different AMOCs, we show that the fresh Antarctic Intermediate Water layer eliminates salinity differences across the AMOC branches at ~1,200 m, ∆S1200m, which decouples Fov from the AMOC south of ~10°N. As the Antarctic Intermediate Water disappears north of ~10°N, a large ∆S1200m allows the AMOC to drive substantial southward Fov in the North Atlantic. In the South Atlantic the 0–300 m zonal salinity contrasts control the gyre freshwater transports Fgyre, which also determine the total freshwater transports. This decoupling makes the southern Fov unlikely to play any role in AMOC stability, leaving indirect Fgyre feedbacks or Fov in the north, as more relevant factors. Plain language summary: The Atlantic Ocean has an upper circulation branch transporting warm waters toward the Arctic. These waters sink due to changes in both salinity and temperature, leading to a cold and deep southward circulation branch throughout the Atlantic. This "overturning circulation" moves heat and freshwater over large distances, contributing to regulate Earth's climate, but the circulation strength may also be affected by these transports via feedback effects. It has been proposed that South Atlantic freshwater transports are a sensitive indicator of circulation feedback, which could lead to instability in the climate system. However, models of the ocean and atmospheric circulations used to study climate often show large errors in salinity distributions and freshwater transports and therefore may misrepresent climate stability. We show that with realistic salinities, the overturning circulation produces virtually no freshwater transports throughout the South Atlantic and is unlikely to have any role in feedbacks causing climate instability. Horizontal gyre circulations dominate South Atlantic freshwater transports, which could still have some indirect influence on climate stability. In contrast, the overturning circulation does drive a strong freshwater transport in the North Atlantic, and therefore, salinity feedbacks on the climate stability are much more likely to be important in the north. Key Points: Atlantic overturning freshwater transport is explained by spatial variation in the vertical salinity contrasts between the AMOC branchesNegligible South Atlantic vertical salinity contrasts produce weak freshwater transports, decoupled from the AMOC strengthGyre‐driven freshwater transports depend on 0–300 m zonal salinity contrasts and determine the total South Atlantic freshwater transports [ABSTRACT FROM AUTHOR]