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This paper provides an introduction to the special issue of the Philosophical Transactions of the Royal Society of London of papers from the 2022 Royal Society meeting on 'Atlantic overturning: new observations and challenges'. It provides the background and rationale for the meeting, briefly summarizes prior progress on observing the Atlantic overturning circulation and draws out the new challenges that papers presented at the meeting raise, so pointing the way forward for future research. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [ABSTRACT FROM AUTHOR]

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]

Based on modern observations, historical proxy data, and climate model simulations, this paper provides a comprehensive overview of the past, present and future evolution characteristics of the Atlantic Meridional Overturning Circulation (AMOC), as well as its impact on the surface air temperature (SAT) at regional and hemispherical scales. The reconstruction results based on the proxy data indicate that the AMOC has weakened since the late 19th century and experienced overall weakening throughout the 20th century with low confidence. Direct observations show that the AMOC weakened during 2004–2016, but it is not possible to distinguish between its decadal variability and long-term trend. Climate models predict that if greenhouse gas emissions continue to increase, AMOC will weaken in the future, but there will not be a sudden collapse before 2100. For the thermodynamic effects of AMOC, the increased surface heat flux release and meridional heat transport (MHT) over the North Atlantic associated with the strong AMOC cause an increase in the hemispherical SAT. At the millennial scale, climate cooling (warming) periods correspond to a weakened (strengthened) AMOC. The enhanced MHT of a strong AMOC can affect Arctic warming and thus influence regional SAT anomalies and SAT extremes through mutual feedback between Arctic sea ice and AMOC. In terms of dynamic effects, a strong AMOC modulates the Rossby wave trains originating from the North Atlantic and spreading across mid-to-high latitudes in the Northern Hemisphere and causes an increase in the variabilities in the circulation anomalies over the Ural and Siberian regions. Ultimately, a strong AMOC significantly affects the frequencies of extreme cold and warm events in the mid-to-high latitude regions over Eurasia. In addition, AMOC can also influence regional and global SAT anomalies through its dynamic adjustment of planetary-scale circulation. Decadal variation in AMOC is closely related to the Atlantic Multidecadal Oscillation (AMO). During positive phases of AMO and AMOC, enhanced surface heat fluxes over the North Atlantic lead to abnormal warming in the Northern Hemisphere, while during negative phases, the reverse case occurs. Under high emission scenarios in the future, the possibility of AMOC collapse increases due to freshwater forcing. However, most advanced climate models underestimate the strength of the AMOC and its impact on the AMO and relevant climate change, which presents a major challenge for future understanding and prediction of the AMOC and its climate effects. [ABSTRACT FROM AUTHOR]

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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]

Copyright of Advances in Atmospheric Sciences is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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]

This paper studies sensor fault detection using a game theoretic approach. Sensor fault detection is considered as change point analysis in the coefficients of a regression model. A new method for detecting faults, referred to as two-way fault detection, is introduced which defines a game between two players, i.e. the fault detectors. In this new strategic environment, assuming that the independent states of the regression model are known, the test statistics are derived and their finite sample distributions under the null hypothesis of no change are derived. These test statistics are useful for testing the fault existence, as well as, the pure and mixed Nash equilibriums are derived for at-mostone- change and epidemic change models. A differential game is also proposed and solved using the Pontryagin maximum principle. This solution is useful for studying the fault detection problem in unknown state cases. Kalman filter and linear matrix inequality methods are used in finding the Nash equilibrium for the case of unknown states. Illustrative examples are presented to show the existence of the Nash equilibriums. Also, the proposed fault detection scheme is numerically evaluated via its application on a practical system and its performance is compared with the cumulative sum method. [ABSTRACT FROM AUTHOR]

The decline of ancient societies has been often associated with hydroclimate instabilities. In this paper, we report phytolith records from the Yangtze River Delta of China, to reconstruct middle and late Holocene climate dynamics, and understand societal impacts. Our findings reveal that the area experienced a consistent decline in temperature and humidity punctuated by several rapid climate shifts. A distinct dry episode from 7000 to 6000 cal yr BP likely developed in response to weakened Atlantic Meridional Overturning Circulation (AMOC) and a southward shift of the intertropical convergence zone (ITCZ), resulting in a weakened East Asian summer monsoon (EASM) and a strengthened East Asian winter monsoon (EAWM). This dry condition would have favoured the emergence of dryland and facilitated rice cultivation in the lower Yangtze region. Another dry episode at 4500–4000 cal yr BP, which was probably associated with the widespread 4.2 ka climate event, and linked to a further retreat of the ITCZ and highly variable ENSO intensity, led to highly unstable hydroclimate conditions in the region. We argue that this event disrupted social resilience and culminated in the cultural collapse at the end of Liangzhu Culture. • Cooling trend since 8000 cal yr BP in response to decline of summer insolation. • Dry episodes at 7000–6000 and 4500–4000 cal yr BP. • Strength of AMOC and position of ITCZ jointly modulates the EASM. • Neolithic societal resilience shows varied responses to each dry episode. [ABSTRACT FROM AUTHOR]

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

• Vertical grids impact the modeling of baroclinic modes and water mass distribution. • 32-layer HYCOM is adequate to represent first 5 baroclinic modes in mid-latitudes. • A simulation with 16 layers exhibits no dense overflow water and a shallower AMOC. • Simulations with 24 to 96 layers exhibit overall similar North Atlantic circulation. In contrast to the large volume of studies on the impact of horizontal resolution in oceanic general circulation models (OGCMs), the impact of vertical resolution has been largely overlooked and there is no consensus on how one should construct the vertical grid to represent the vertical structure of the baroclinic modes as well as the distribution of distinct water masses throughout the global ocean. In this paper, we document the importance of vertical resolution in the representations of vertical modes and water masses in the North Atlantic and show i) that vertical resolution is unlikely to undermine the resolution capability of the horizontal grid in representing the vertical modes and a 32-layer isopycnal configuration is adequate to represent the first five baroclinic modes in mid-latitudes and ii) that vertical resolution should focus on representing water masses. A coarse vertical resolution (16-layer) simulation exhibits virtually no transport in the dense overflow water which leads to a weaker and significantly shallower Atlantic meridional overturning circulation (AMOC) despite resolving the first baroclinic mode throughout the domain, whereas there are overall very small differences in the subtropical and subpolar North Atlantic circulation in the simulations with finer vertical resolution (24 to 96 layers). We argue that accurately representing the water masses is more important than representing the baroclinic modes for an OGCM in modeling the low-frequency large-scale circulation. [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is a large-scale circulation pattern responsible for northward heat transport in the Atlantic and is associated with climate variations on a wide range of time scales. Observing the time-varying AMOC has fundamentally changed our understanding of the large-scale ocean circulation and its interaction with the climate system, as well as identified shortcomings in numerical simulations. With a wide range of gains already achieved, some now ask whether AMOC observations should continue. A measured approach is required for a future observing system that addresses identified gaps in understanding, accounts for shortcomings in observing methods and maximizes the potential to guide improvements in ocean and climate models. Here, we outline a perspective on future AMOC observing and steps that the community should consider to move forward. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [ABSTRACT FROM AUTHOR]

The latest assessment report from the Intergovernmental Panel on Climate Change concluded that the Atlantic Meridional Overturning Circulation (AMOC) was very likely to decline over the twenty-first century under all emissions scenarios; however, there was low confidence in the magnitude of the decline. Recent research has highlighted that model biases in the mean climate state can affect the AMOC in its mean state, variability and its response to climate change. Hence, understanding and reducing these model biases is critical for reducing uncertainty in the future changes of the AMOC and in its impacts on the wider climate. We discuss how model biases, in particular salinity biases, influence the AMOC and deep convection. We then focus on biases in the UK HadGEM3-GC3-1 climate model and how these biases change with resolution. We also discuss ongoing model development activities that affect these biases, and highlight priorities for improved representation of processes, such as the position of the North Atlantic Current, transports in narrow boundary current, resolution (or improved parameterization) of eddies and spurious numerical mixing in overflows. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [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]

This paper is a review of the recent development of researches on the stability of the Atlantic meridional overturning circulation (AMOC). In particular, we will review recent studies that attempt to best assess the stability of the AMOC in the past, present, and future by using a stability indicator related to the freshwater transport by the AMOC. These studies further illustrate a potentially systematic bias in the state-of-the-art atmosphere-ocean general circulation models (AOGCMs), in which the AMOCs seem to be over-stabilized relative to that in the real world. This common model bias in the AMOC stability is contributed, partly, to a common tropical bias associated with the double intertropical convergence zone (ITCZ) in most state-of-the-art AOGCMs, casting doubts on future projection of abrupt climate changes in these climate models. [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]

Copyright of People & Nature is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Radiocarbon (14C) is a valuable tracer of ocean circulation, owing to its natural decay over thousands of years and to its perturbation by nuclear weapons testing in the 1950s and 1960s. Previous studies have used 14C to evaluate models or to investigate past climate change. However, the relationship between ocean 14C and ocean circulation changes over the past few decades has not been explored. Here we use an Ocean-Sea-ice model (NEMO) forced with transient or fixed atmospheric reanalysis (JRA-55-do) and atmospheric 14C and CO2 boundary conditions to investigate the effect of ocean circulation trends and variability on 14C. We find that 14C/C (∆14C) variability is generally anti-correlated with potential density variability. The areas where the largest variability occurs varies by depth: in upwelling regions at the surface, at the edges of the subtropical gyres at 300 m depth, and in Antarctic Intermediate Water and North Atlantic Deep Water at 1000 m depth. We find that trends in the Atlantic Meridional Overturning Circulation may influence trends in ∆14C in the North Atlantic. In the high-variability regions the simulated variations are larger than typical ocean ∆14C measurement uncertainty of 2–5‰ suggesting that ∆14C data could provide a useful tracer of circulation changes. [ABSTRACT FROM AUTHOR]

The recent Environmental Research Letters article by Caesar, Rahmstorf and Feulner (hereafter CRF) is essentially a Comment on our Nature paper (Chen and Tung 2018 Nature 559 387–91), but without an accompanying rebuttal from us. In this unusual format for the exchange outside Nature , our rebuttal then becomes a Comment here at Environmental Research Letters . Our original proposal that the rate of global warming is enhanced by a weak Atlantic Meridional Overturning Circulation (AMOC) remains valid and is strengthened with this exchange. CRF used “established evidence” to argue against our finding, but such evidence is either misapplied (i.e. applying model results from preindustrial control runs with constant greenhouse gasses to the industrial era with increasing greenhouse gasses), or misinterpreted (i.e. climate model results for the industrial era specifically for the trends interpreted as for the AMOC cycles). While we used the observed energy budget to show that a strong (weak) AMOC transports more (less) heat to below 200 m, CRF replaces the actual budget with a simple energy-balance equation. They used an inappropriate equilibrium approximation to their simple equation to argue that global mean surface temperature (GMST) and AMOC should be in phase. We show here that the exact solution to that same equation actually supports our claim on the relationship between the rate of change of GMST and the AMOC state, which they misunderstood as we claiming a negative correlation between GMST and AMOC themselves. They claimed, incorrectly, that a positive correlation coefficient, no matter how small and even though none of them is statistically significant, is strong evidence that the two time series are in phase. The correlation coefficients that they found using observational data (0.01, 0.28 and 0.45), though positive, correspond to $89^\circ ,74^\circ ,63^\circ$ out of phase, far from being in-phase. Visually they were made to look somewhat in-phase with decadal smoothing and short-period detrending. Both model and observational evidence supports the conclusion of our original paper that the period of AMOC minimum is a period of rapid rate of surface warming.

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]

Climate models are important tools for investigating how the climate might change in the future, however recent observations have suggested that these models are unable to capture the overturning in subpolar North Atlantic correctly, casting doubt on their projections of the Atlantic Meridional Overturning Circulation (AMOC). Here we compare the overturning and surface water mass transformation in a set of CMIP6 models with observational estimates. There is generally a good agreement, particularly in the recent conclusion from observations that the mean overturning in the east (particularly in the Iceland and Irminger seas) is stronger than that in the Labrador Sea. The overturning in the Labrador Sea is mostly found to be small, but has a strong relationship with salinity: fresh models have weak overturning and saline models have stronger mean overturning and stronger relationships of the Labrador Sea overturning variability with the AMOC further south.We also find that the overturning reconstructed from surface flux driven water mass transformation is a good indicator of the actual overturning, though mixing can modify variability and shift signals to different density classes. [ABSTRACT FROM AUTHOR]

The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content (Δ OHC), and the spatial pattern of ocean dynamic sea level ( Δ ζ ). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling CO 2 . Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of Δ OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects Δ ζ in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean Δ ζ and Δ OHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs. [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]