Despite numerous model-based analyses indicating a notable decline in the Atlantic Meridional Overturning Circulation (AMOC) in recent decades, robust, long-term evidence from multilatitudinal in situ observations remains limited. This study uses observational data from four mooring arrays, positioned along the western boundary of the North Atlantic (from 16.5°N to 42.5°N), to examine trends in the deep western overturning transports, derived from the cross-slope gradient in ocean bottom pressure or its equivalent, below and relative to 1000 meters that are linked to changing conditions at the western boundary. We identify a meridionally consistent decline in deep western overturning transport across these latitudes over the past two decades. This decline, observed at the western boundary, may serve as an effective indicator of AMOC weakening, despite the partial compensatory effect of overturn strengthening at the eastern boundary.

Observations from the RAPID array at 26.5^∘N indicate a linear decline in the Atlantic Meridional Overturning Circulation (AMOC) over the past two decades, linked to contrasting boundary changes: a weakening western-boundary contribution that is partly compensated by strengthening at the eastern boundary. Yet it remains unclear whether this partial compensation reflects a basin-wide adjustment or a regional feature, and what processes drive it. Here we use a high-resolution ocean model to investigate the spatial structure and underlying mechanisms of the AMOC change across the mid-latitude North Atlantic. The model reproduces a meridionally coherent decline in western-boundary deep overturning transport together with a partially compensating strengthening at the eastern boundary, consistent with observations at 26.5^∘N. These opposing trends arise from a vertically coherent ocean bottom pressure trend shaped by two competing drivers: rising coastal sea level and decreasing interior density. Through geostrophic balance, this mechanism produces partial boundary compensation across latitudes, yielding a basin-wide AMOC decline throughout the mid-latitude North Atlantic.

The RAPID-MOCHA-WBTS (hereafter RAPID) array is an observing system designed to study the Atlantic Meridional Overturning Circulation (AMOC). It is an international collaboration between the National Oceanography Centre, University of Miami, and NOAA. The primary goals of the RAPID array are to observe and understand changes in the AMOC over time, and improve our understanding of how changes in the ocean circulation system may influence regional and global climate patterns. The array consists of a network of moored instruments, which measure ocean temperature, salinity, dissolved oxygen, and flow velocities. The AMOC at 26◦N has now been continuously measured by the RAPID array over the period April 2004 to present (20 years of observing). This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transport from hydrographic ship sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind forcing, contrasting with previous expectations about a slowly varying buoyancy-forced overturning circulation. We will present the history of the RAPID observational array and its contribution to AMOC science.

The AMOC is usually defined as the maximum of the overturning streamfunction. The time series produced by the RAPID-MOCHA-WBTS observing array uses a streamfunction calculated in depth space. Using data from the RAPID-MOCHA-WBTS array along with additional data from the WBTS sections in the Florida Straits and other hydrographic data, we have made a time series of the overturning streamfunction calculated in density space. The streamfunction in density space reveals the shallow overturning cell associated with subtropical mode waters (STMW) that is obscured in the depth-space streamfunction. The time series of the data also reveal that inter-annual variability in the amount of STMW in the Florida Straits is linked to changes in meridional heat transport.

The Florida Current (FC) provides the majority of the northward volume and heat transports for both the meridional overturning and the horizontal gyre circulations in the subtropical North Atlantic. A unique, sustained observing system in the Florida Straits at about 27°N, consisting of voltage measurements recorded from a submarine telecommunication cable installed between Florida and Grand Bahama Island, paired with regular calibration and validation cruises, was established in 1982. Since then, the recorded cable voltage time series has enabled over 40 years of quasi-continuous, daily estimates of the FC volume transport. The cable data constitutes the longest observational record of any boundary current and a key component of the Atlantic Meridional Overturning Circulation (AMOC) in existence. By this measure, it can be representative of the AMOC weakening, suggested by climate models and proxy-based reconstructions.Here, we reassess the record-long change in the FC strength by revising the processing of voltages measured on the submarine cable. With the increased length of the cable record, we show that it has become necessary to account for the secular change in the Earth’s geomagnetic field, especially when studying processes on decadal and longer time scales. We calculate the corrected estimates of the FC volume transport and show that (i) the FC strength has not declined as reported recently, but has remained remarkably stable since 1982, and (ii) with the corrected FC record, the AMOC at ~26.5°N exhibits a decadal-scale variability rather than a long-term decline.The results of this study indicate that, if climate models are correct that the AMOC is slowing or will soon slow down, this slowdown has not yet been reflected in the FC, or the observational record is still too short to detect it with confidence. The existing records are just starting to resolve decadal-scale signals relevant to climate variability. Continued observations are thus necessary for detection and mechanistic understanding of climate-related changes and for validating and improving ocean and climate models.

The North Atlantic Current (NAC) is a major source of heat toward the subpolar gyre and northern seas. However, its variability and drivers are not well understood. Here, we evaluated 8 years of continuous daily measurements as part of the international program Overturning in the Subpolar North Atlantic Program to investigate the NAC in the Iceland Basin. We found that the NAC volume and freshwater anomaly transport and heat content (HC) were highly variable with significant variability at timescales of 16–120 days to annual. Intraseasonal to short interannual variability was associated with mesoscale and intermittent mesoscale features abundant in the region. Composites analysis revealed that strong NAC periods were associated with less eddy kinetic energy in the Iceland Basin, which was consistent with the presence of frontal‐like structures instead of eddy‐like structures. On longer timescales, the westward migration of the eastern boundary of the subpolar North Atlantic (SPNA) gyre favors a stronger NAC volume transport and HC in the region. Stronger zonal wind stress triggers a fast response that piles water up between the SPNA and subtropical gyres, which increases the sea surface height gradient and drives the acceleration of the NAC. The strengthening of the NAC increases the heat and salt transport northward. During our study period, both heat and salt increased across the moorings. These observations are important for understanding the heat and freshwater variability in the SPNA, which ultimately impacts the Atlantic meridional overturning circulation.

Deep water masses of the North Atlantic Ocean are formed in the subpolar regions through ocean-atmosphere interactions, imprinting unique climate signals like anomalous temperature and salinity on the deep waters that are exported to the Subtropical North Atlantic. Sustained hydrographic, mooring, and seafloor temperature surveys off Abaco Island, Bahamas at 26.5 degrees N have illuminated significant cooling and freshening of the deep ocean (>2000 m) over the past four decades, challenging the paradigm of anticipated deep ocean warming in the Subtropical North Atlantic. Here, we discuss a linkage between the deep Subtropics and Subpolar North Atlantic, finding synchronicity between the observed freshening at 26.5 degrees N and a multi-decadal freshening event in the subpolar basins occurring more than two decades prior. Findings hint at the likely onset of subtropical deep ocean warming and salinification in the near future, which could have notable impacts on deep ocean Atlantic heat content, circulation, and sea level changes.

Direct measurements of the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport (MHT) are necessary to better understand the impact of anthropogenic greenhouse gas emissions for the global climate system. The RAPID-MOCHA-WBTS array at 26°N is the only trans-Atlantic observing system to provide 20 years of continuous measurements of the AMOC and MHT. While the design of the array has continuously evolved as our understanding of the AMOC has advanced and as new technologies have become available, the goal of the RAPID-evolution project is now to design a lower cost and sustainable observing system to continue the measurements at the accuracy required by users. Using the dataset gathered since 2004 and ocean reanalysis, a first objective seeks to evaluate the sensitivity of the AMOC estimate to the choice of methodology and data included in the calculation. The project includes the development of a new high-resolution ocean model to identify the short and longer term impacts of incorporating these datasets in the AMOC estimation. Recent technological developments also enable new approaches that could provide better and more cost-effective calculation of the AMOC. The RAPID-Evolution project investigates these approaches and develops methodologies to make use of them, including a new variation of the stepping method using glider deployments and the telemetry of mooring data via an autonomous vehicle.

Variability of the Atlantic Meridional Overturning Circulation (MOC) has drawn extensive attention due to its impact on the global redistribution of heat and freshwater. Here we present the latest time series (2014-2022) of the Overturning in the Subpolar North Atlantic Program and characterize MOC interannual variability. We find that any single boundary current captures similar to 30% of subpolar MOC interannual variability. However, to fully resolve MOC variability, a wide swath across the eastern subpolar basin is needed; in the Labrador Sea both boundaries are needed. Through a volume budget analysis for the subpolar basins' lower limbs, we estimate the magnitude of unresolved processes (e.g., diapycnal mixing) required to close the mean budget (similar to 2 Sv). We find that in the eastern subpolar basin surface-forced transformation variability is linked to lower limb volume variability, which translates to MOC changes within the same year. In contrast, this linkage is weak in the Labrador Sea.

Of the additional carbon dioxide added to the atmosphere by human activities the ocean absorbs approximately a quarter, with a disproportionate fraction accumulating at depth in the North Atlantic due to the combined action of northward ocean transport (through the meridional overturning circulation) and strong air-sea fluxes. Combining repeat hydrography with circulation estimates from the RAPID mooring array at 26N it was found that between 2004 and 2012 these two processes were roughly equal in magnitude, but decreasing ocean transports were tipping the balance more towards air-sea uptake over time as the AMOC weakened. New observations from 2012 to 2022 show that this process has now reversed - a recovering AMOC combined with increasing loadings of carbon is now transporting substantially greater quantities of anthropogenic carbon northwards into the North Atlantic. Changes in regional air-sea fluxes suggests that the increased northward ocean carbon transport may be affecting CO2 uptake capacity downstream.

Direct measurements of the Atlantic Meridional Overturning Circulation (AMOC) and meridional heat transport (MHT) are necessary to better understand the impact of anthropogenic greenhouse gas emissions for the global climate system. The RAPID-MOCHA-WBTS array at 26°N is the only trans-Atlantic observing system to provide 20 years of continuous measurements of the AMOC and MHT. While the design of the array has continuously evolved as our understanding of the AMOC has advanced and as new technologies have become available, the goal of the RAPID-evolution project is now to design a lower cost and sustainable observing system to continue the measurements at the accuracy required by users. Using the dataset gathered since 2004 and ocean reanalysis, a first objective seeks to evaluate the sensitivity of the AMOC estimate to the choice of methodology and data included in the calculation. The project includes the development of a new high-resolution ocean model to identify the short and longer term impacts of incorporating these datasets in the AMOC estimation. Recent technological developments also enable new approaches that could provide better and more cost-effective calculation of the AMOC. The RAPID-Evolution project investigates these approaches and develops methodologies to make use of them, including a new variation of the stepping method using glider deployments and the telemetry of mooring data via an autonomous vehicle.

Starting in 2012, the eastern subpolar North Atlantic experienced the strongest surface freshening in the past 120 years. It is yet unknown whether this salinity anomaly propagated downward into the water column and affected the properties of the boundary currents of the subpolar gyre, which could slow down the overturning. Here, we investigate the imprint of this salinity anomaly on the warm and saline Irminger Current (IC) in the decade thereafter. Using daily mooring data from the IC covering the period 2014-2022 combined with hydrographic sections across the adjacent basins from 1990, the evolving signal of the salinity anomaly over the water column and its imprint on the transport variability is studied. We find that due to the salinity anomaly, the northward freshwater transport of the IC increased by 10 mSv in summer 2016 compared to summer 2015. In 2018, the salinity anomaly covered the water column down to 1,500 m depth. Hydrographic sections across the basin showed that this recent freshening signal spread across the Irminger Sea. Overall, the freshwater transport of the IC increased by a factor of three between 2014-2015 and 2021-2022. The associated density decrease over the upper 1,500 m of the water column resulted in an increase in the northward transport of waters lighter than sigma 0 = 27.55 kg m-3 from 1.7 to 4.2 Sv. This change in northward IC transport by density class may impact the characteristics of the overturning in the Northeastern Atlantic, its strength and the density at which it peaks.

The potential weakening of the Atlantic Meridional Overturning Circulation (AMOC) in response to anthropogenic forcing, suggested by climate models, is at the forefront of scientific debate. A key AMOC component, the Florida Current (FC), has been measured using submarine cables between Florida and the Bahamas at 27°N nearly continuously since 1982. A decrease in the FC strength could be indicative of the AMOC weakening. Here, we reassess motion-induced voltages measured on a submarine cable and reevaluate the overall trend in the inferred FC transport. We find that the cable record beginning in 2000 requires a correction for the secular change in the geomagnetic field. This correction removes a spurious trend in the record, revealing that the FC has remained remarkably stable. The recomputed AMOC estimates at ~26.5°N result in a significantly weaker negative trend than that which is apparent in the AMOC time series obtained with the uncorrected FC transports.

The North Atlantic subpolar gyre experienced strong freshening in recent years starting around 2012. Here, we investigate the imprint of this freshwater anomaly on the water column hydrography and transport variability of the Irminger Current (IC). The IC transports warm and saline waters northward along the western flank of the Reykjanes Ridge as part of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC). To investigate if the salinity anomaly spread and propagated downward, we used high-resolution mooring data from the IC covering the period 2014 – 2022 combined with hydrographic sections from the Irminger Sea and Iceland Basin. We found that the IC experienced a strong freshening starting in summer 2016. By 2018, this salinity anomaly covers the whole water column down to 1500 m depth and freshened the IC until 2022. In 2022, the IC was at its freshest state observed since the early 1990’s. Hydrographic sections across the adjacent basins showed that the recent freshening spread across the Irminger Sea and was also comparable to its fresh state in the early 1990’s. The salinity anomaly increased the freshwater transport of the IC by a factor of three from 2014-2015 to 2021-2022 and caused a decrease in density over much of the water column. This resulted in an increase in the transport of waters lighter than 27.55 kg m-3, potentially strengthening the upper limb of the AMOC.

The Florida Current (FC) provides the majority of the northward volume and heat transports for both the meridional overturning and the horizontal gyre circulations in the subtropical North Atlantic. A unique, sustained observing system in the Florida Straits at about 27°N, consisting of voltage measurements recorded from a submarine telecommunication cable installed between Florida and Grand Bahama Island, paired with regular calibration and validation cruises, was established in 1982. Since then, the recorded cable voltage time series has enabled over 40 years of quasi-continuous, daily estimates of the FC volume transport. The cable data constitutes the longest observational record of any boundary current and a key component of the Atlantic Meridional Overturning Circulation (AMOC) in existence. By this measure, it can be representative of the AMOC weakening, suggested by climate models and proxy-based reconstructions. Here, we reassess the record-long change in the FC strength by revising the processing of voltages measured on the submarine cable. With the increased length of the cable record, we show that it has become necessary to account for the secular change in the Earth’s geomagnetic field, especially when studying processes on decadal and longer time scales. We calculate the corrected estimates of the FC volume transport and show that (i) the FC strength has not declined as reported recently, but has remained remarkably stable since 1982, and (ii) with the corrected FC record, the AMOC at ~26.5°N exhibits a decadal-scale variability rather than a long-term decline. The results of this study indicate that, if climate models are correct that the AMOC is slowing or will soon slow down, this slowdown has not yet been reflected in the FC, or the observational record is still too short to detect it with confidence. The existing records are just starting to resolve decadal-scale signals relevant to climate variability. Continued observations are thus necessary for detection and mechanistic understanding of climate-related changes and for validating and improving ocean and climate models.

Abstract The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. At 26.5°N, the meridional heat transport has traditionally been partitioned geometrically into vertical and horizontal circulation cells; however, attributing these components to the AMOC and Subtropical Gyre (STG) flow structures remains widely debated. Using water parcel trajectories evaluated within an eddy‐rich ocean hindcast, we present the first Lagrangian decomposition of the meridional heat transport at 26.5°N. We find that water parcels recirculating within the STG account for 37% (0.36 PW) of the total heat transport across 26.5°N, more than twice that of the classical horizontal gyre component (15%). Our findings indicate that STG heat transport cannot be meaningfully distinguished from that of the basin‐scale overturning since water parcels cooled within the gyre subsequently feed the northward, subsurface limb of the AMOC.

The abyssal limb of the global Meridional Overturning Circulation redistributes heat and carbon as it carries Antarctic Bottom Water from the Southern Ocean towards the Northern Hemisphere. Using mooring observations and hydrographic data from multiple sources in the North Atlantic, we show that northward-flowing Antarctic Bottom Water is constrained below 4,500m with a mean volume transport of 2.400.25Sv at 16 degrees N. We find that during 2000-2020, the Antarctic Bottom Water northward transport weakened by approximately 0.35 +/- 0.13Sv, corresponding to a 12 +/- 5% decrease. The weakening of the Atlantic Meridional Overturning Circulation abyssal cell is a probable response to reduced Antarctic Bottom Water formation rates over the past several decades and is associated with abyssal warming observed throughout the western Atlantic Ocean. We estimate that the warming of the Antarctic Bottom Water layer in the subtropical North Atlantic is, on average, 1m degrees C per year in the last two decades due to the downward heaving of abyssal isopycnals, contributing to the increase of abyssal heat content and, hence, sea-level rise in the region (1 m degrees C = 0.001 degrees C). This warming trend is approximately half of the Antarctic Bottom Water warming trend observed in the South Atlantic and parts of the Southern Ocean, indicating a dilution of the signal as the Antarctic Bottom Water crosses the Equator.

The Atlantic Meridional Overturning Circulation (AMOC) plays a critical role in the global climate system through the redistribution of heat, freshwater and carbon. At 26.5N, the meridional heat transport has traditionally been partitioned geometrically into vertical and horizontal circulation contributions; however, attributing these components to the AMOC and Subtropical Gyre (STG) flow structures remains widely debated. Using water parcel trajectories evaluated within an eddy-rich ocean hindcast, we present the first Lagrangian decomposition of the meridional heat transport at 26.5N. We find that water parcels recirculating within the STG account for 37% (0.36 PW) of the total heat transport across 26.5N, more than twice that of the classical horizontal “gyre” component (15%). Rather than being distinct from the overturning circulation, the heat transport associated with the STG is due to the formation of subtropical mode waters via a shallow downward spiral, which ultimately feeds the northward limb of the AMOC.

Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

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