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Sea surface temperature (SST) has been increasing since industrialization with rising greenhouse gases. However, a warming hole exists in the North Atlantic where SST has cooled by 0.4 K/century during 1900–2017. It has been argued that this cooling is due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), and subpolar North Atlantic SST has thus been utilized to estimate AMOC variability. We assess the robustness of subpolar North Atlantic SST as a proxy for AMOC strength under historical forcing, abrupt quadrupling of CO2, and a medium future emissions pathway, finding that AMOC's fingerprint on SST depends upon forcing scenarios. AMOC is important in warming hole development during significant warming periods, although SST may introduce uncertainties for AMOC reconstruction in stabilized regimes due to diverse forcing mechanisms and decadal variability. Our results caution against using SST alone as a proxy for AMOC variability—both on paleoclimatic and contemporary time scales. Plain Language Summary: As greenhouse gas concentrations increase, the global average sea surface temperature (SST) has risen in response at a rate of 1.4 K/Century since industrialization; however, there exists an area in the North Atlantic decreasing in SST, the North Atlantic Warming Hole (NAWH). Some have argued that the cooling is due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a global circulation current that transports warm equatorial surface water northward. We analyze forcing scenarios using a global climate model under three differing forcing mechanisms, one similar to the real world, one where CO2 initially starts at four times pre‐industrial levels and then held constant, and one based on a medium level future emissions scenario. From this, we study the quality of North Atlantic SST as a proxy for AMOC strength. Our study finds that the impact of the AMOC on SST changes between scenarios, and thus we believe that North Atlantic SST may introduce uncertainty into paleoclimatic studies attempting to reconstruct AMOC change. Additionally, we found that a slowdown of the AMOC plays an important role in the cooling of the North Atlantic when AMOC is slowing rapidly (or "shutting down"), although has a more minimal impact in less extreme scenarios. Key Points: Sea surface temperature based fingerprint of Atlantic Meridional Overturning Circulation (AMOC) changes under different forcing scenariosAMOC in steep decline significantly contributes to North Atlantic warming hole formation, largely due to cloud radiative forcingAMOC plays a smaller role in North Atlantic SST variability in historical period and after CO2 stabilization than during significant warming [ABSTRACT FROM AUTHOR]

The faster warming for Arctic Ocean surface air temperature (SAT) relative to that at lower latitude is connected with various processes, including local radiation feedback, poleward oceanic and atmospheric heat transport. It is unclear how combinations of different low‐frequency internal climate modes influence Arctic amplification on the decadal timescale. Here, the decadal Arctic SAT variation, its connection with the Atlantic meridional overturning circulation (AMOC) and possible underlying mechanisms, are investigated based on several independent observational proxies, pre‐industrial experiments, and historical large ensembles of two CMIP6 models. Our study suggests that AMOC and Arctic SAT vary in phase on the decadal timescale, whereas this relationship is insignificant at the interannual timescale. Further analysis shows that the AMOC accompanied with cross‐basin oceanic water/heat transport between Atlantic and Arctic would alter air–sea interface exchange over the melting ice regions, and then amplified poleward atmospheric heat and moisture transports. The resulting enhanced downward longwave radiation ultimately warms the Arctic SAT. Additionally, the decadal‐scale North Pacific Oscillation (NPO) can modulate the relationship between AMOC and Arctic SAT by influencing poleward moisture transport and cross‐basin circulation. Specifically, the phase shift of combined NPO and AMOC can contribute 14%–41% covariance relationship between AMOC and Arctic SAT. Our study provides potential sources for predicting the Arctic climate and constraining its uncertainty in future projections. Plain Language Summary: The Arctic has warmed faster than lower latitudes and is capable of influencing weather and climate. Previous studies suggest that Atlantic meridional overturning circulation (AMOC) can induce warming over the Arctic region, however, the underlying mechanism for AMOC influencing Arctic SAT and whether their relationship is stable are still unclear. In this study, we have identified the relationship between the AMOC and Arctic SAT during 1950–2014 on the decadal scale. Our results demonstrate that AMOC changes in phase with the Arctic SAT, which is dominated by surface downward longwave radiation resulting from poleward moisture and heat transport, and air‐ice‐sea interaction associated with AMOC. Additionally, the North Pacific Oscillation (NPO) can modulate the correlation between AMOC and Arctic SAT by influencing the meridional eddy advection of water vapor and cross‐basin processes. Specifically, the phase shift of combined NPO and AMOC can contribute 14%–41% covariance relationship between AMOC and Arctic SAT. Key Points: The Atlantic meridional overturning circulation (AMOC) change in phase with the Arctic surface air temperature (SAT) during 1950–2014 on a decadal timescale in multiple observationsDecadal Arctic SAT change associated with AMOC is dominated by surface downward longwave radiation, evaporation and poleward moisture transportThe North Pacific Oscillation accompanied with the cross‐basin processes modulates the relationship between AMOC and Arctic SAT [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial system influencing regional and even global climate changes, with the Subpolar North Atlantic (SPNA) being a key region affecting this system. In the context of global climate change, the impact and mechanisms of aerosols on the AMOC remain unclear. This study, based on state-of-the-art ensemble model simulations paired with latest observations, reveals that aerosols in the SPNA region significantly influence sea surface temperature anomalies by affecting cloud cover, which in turn affects ocean stratification, thus impacting the strength of the AMOC. Based on this, we present the trends and spatiotemporal characteristics of future AMOC under different forcings. Our research contributes to a deeper understanding and prediction of the AMOC. [ABSTRACT FROM AUTHOR]

Enhanced warming of the Arctic region relative to the rest of the globe, known as Arctic amplification, is caused by a variety of diverse factors, many of which are influenced by the Atlantic meridional overturning circulation (AMOC). Here, we quantify the role of AMOC changes in Arctic amplification throughout the twenty-first century by comparing two suites of climate model simulations under the same climate change scenario but with two different AMOC states: one with a weakened AMOC and another with a steady AMOC. We find that a weakened AMOC can reduce annual mean Arctic warming by 2 °C by the end of the century. A primary contributor to this reduction in warming is surface albedo feedback, related to a smaller sea ice loss due to AMOC slowdown. Another major contributor is the changes in ocean heat uptake. The weakened AMOC and its associated anomalous ocean heat transport divergence lead to increased ocean heat uptake and surface cooling. These two factors are inextricably linked on seasonal timescales, and their relative importance for Arctic amplification can vary by season. The weakened AMOC can also abate Arctic warming via lapse rate feedback, creating marked cooling from the surface to lower-to-mid troposphere while resulting in relatively weaker cooling in the upper troposphere. Additionally, the weakened AMOC increases the low-level cloud fraction over the North Atlantic warming hole, causing significant cooling there via shortwave (sw) cloud feedback despite the overall effect of sw cloud feedback being a slight warming of the average temperature over the Arctic. [ABSTRACT FROM AUTHOR]

Most of net energy captured by the increase in greenhouse gasses are subsequently absorbed by the oceans, with the majority being taken up by the subpolar North Atlantic and the Southern Ocean, in the form of ocean heat uptake (OHU). In this study, we examine the temporal evolution of OHU from the Coupled Model Intercomparison Project Phase 5&6 (CMIP 5&6) data, with a focus on the time period that would follow reaching CO2 concentration maximum. Our results show that the Southern Ocean and global mean OHU would show a negative trend following abrupt CO2 quadrupling. In contrast, the OHU in the subpolar North Atlantic would still rise and then remain at a high level for a century or longer. The SSP experiments show the same phenomenon when CO2 concentration starts decreasing. This opposite trend of OHU between the Southern Ocean and subpolar North Atlantic is mainly driven by the surface latent heat flux. Further analysis and a freshwater perturbation experiment using a coupled ocean–atmosphere model suggest that the prolonged period of strong OHU over the North Atlantic in the post-CO2 concentration maximum era would be the result of an overshoot of the Atlantic Meridional Overturning Circulation (AMOC) in response to the increase of CO2 concentration. [ABSTRACT FROM AUTHOR]

The mid-Piacenzian Warm Period (MPWP, ~ 3.264–3.025 Ma) is the most recent example of a persistently warmer climate in equilibrium with atmospheric CO2 concentrations similar to today. Towards studying patterns and dynamics of a warming climate the MPWP is often compared to today. Following the Pliocene Model Intercomparison Project, Phase 2 (PlioMIP2) protocol we prepare a water isotope-enabled Community Earth System Model (iCESM1.2) simulation that is warmer and wetter than the PlioMIP2 multi-model ensemble (MME). While our simulation resembles PlioMIP2 MME in many aspects we find added insights. (1) Considerable warmth at high latitudes exceeds previous simulations. Polar amplification (PA) is comparable to proxies, enabled by iCESM1.2's high climate sensitivity and a distinct method of ocean initialization. (2) Major driver of warmth is the downward component of clear-sky surface long-wave radiation ( Δ T rlds _ clearsky ). (3) In iCESM1.2 modulated dominance of dynamic (δDY) processes causes different low-latitude (~ 30 S°–10°N) precipitation response than the PlioMIP2 MME, where thermodynamic processes (δTH) dominate. (4) Modulated local condensation leads to lower δ18Op across tropical Indian Ocean and surrounding Asian-African-Australian monsoon regions. (5) We find contrasting changes in tropical atmospheric circulations (Hadley and Walker cells). Anomalous regional meridional (zonal) circulation, forced by changes in tropical-subtropical (tropical) diabatic processes, presents a more comprehensive perspective than explaining weakened and expanded Hadley circulation (strengthened and westward-shifted Walker circulation) via static stability. (6) Enhanced Atlantic meridional overturning circulation owes to a closed Bering Strait. [ABSTRACT FROM AUTHOR]

The Iceland Scotland Overflow Water (ISOW) plume supplies approximately a third of the production of North Atlantic Deep Water and is a key component of the meridional overturning circulation (MOC). The Overturning in the Subpolar North Atlantic Program (OSNAP) mooring array in the Iceland Basin has provided high‐resolution observations of ISOW from 2014 to 2020. The ISOW plume forms a deep western boundary current along the eastern flank of Reykjanes Ridge, and its total transport varies by greater than a factor of two on intra‐seasonal timescales. EOF analysis of moored current meter records reveal two dominant modes of velocity variance. The first mode explains roughly 20% of the variance and shows a bottom intensified structure concentrated in the rift valley that runs parallel to the ridge axis. The transport anomaly reconstructed from the first mode explains nearly 80% of the total ISOW plume transport variance. The second mode accounts for 15% of velocity variance, but only 5% of the transport variance. The geostrophically estimated transport (2.9 Sv) recovers only 70% of the total ISOW transport along the ridge flank estimated from the direct current meter observations (4.2 Sv), implying a significant ageostrophic component of ISOW mean transport and variability. Ageostrophic flow is strongly linked to the leading mode of velocity variability within the rift valley. The ISOW transport variability along the upper and middle part of the ridge is further shown to correlate with changes in the strength of deep MOC limb across the basin‐wide OSNAP array. Plain Language Summary: The Iceland Scotland Overflow (ISOW) as a major component of deep ocean circulation in the North Atlantic. Newly available mooring observations from the Overturning in the Subpolar North Atlantic Program show that this current has significant intra‐seasonal variability. This study shows that the flow is strongly affected by the topography of the Reykjanes Ridge which splits the current into multiple branches on the ridge slope and in the Iceland Basin interior. Most of the ISOW transport occurs along the ridge while the basin interior flow dominates the net variability. The results show that a major portion of the flow variability along the ridge is driven by flow funneled through a rift valley along the ridge slope. Geostrophic estimates of transport (the classic method of estimating large scale transport in the ocean) in this region fail to recover the flow within the rift valley. Key Points: Iceland Scotland Overflow transport variability is governed by a combination of barotropic and topographic effectsTransport variability along the Reykjanes Ridge slope is dominated by deep ageostrophic flow within an axial rift valleyAgeostrophic transport accounts for ∼30% of Iceland Scotland Overflow Water (ISOW) transport along the Reykjanes Ridge flank and is a significant component of net ISOW export [ABSTRACT FROM AUTHOR]

The acceleration of Greenland ice sheet melting over the past decades is raising concern regarding the impacts on ocean circulation in the North Atlantic and Arctic regions. Global climate models struggle to assess these impacts as they do not include a realistic amount of meltwater from the Greenland ice sheet. Using an extended observation-based dataset of runoff and solid ice discharge for the recent historical period (1920–2019), we force the EC-Earth3 climate model following the ensemble approach and protocol of a previous study using a different model. We observe a slight increase of the ensemble mean AMOC with a large spread in the response: 0.20 ± 0.81 Sv for the maximum AMOC at 45 ∘ N. We notice that members with a strong initial AMOC state (18 Sv) show a strengthening of the AMOC, while members with intermediate AMOC strength between 16 and 18 Sv are not affected by the freshwater forcing. Weaker initial AMOC members respond with a mean weakening of - 0.21 ± 0.58 Sv of the AMOC. The AMOC ensemble spread is reduced by half in the freshwater forced ensemble at the end of the experiment and the negative trend is stronger compared to historical simulations without the forcing. This suggests the possible ability of the freshwater to constrain AMOC variability on multi-decadal time-scales. Comparison of the two ten member ensembles with ORAS5 reanalysis shows a reduction of the surface temperature and salinity biases in the freshwater forced ensemble in key regions such as the North Atlantic subpolar gyre and the Beaufort Gyre relative to the historical simulations. Recent trends are also more aligned with reanalysis in those regions. Arctic Ocean Atlantic Water subsurface core temperature is also closer to reanalysis but still strongly biassed. Members with a high AMOC initial state display an unrealistic Atlantic water layer core depth. [ABSTRACT FROM AUTHOR]

The freshwater transport (Mov) by the Atlantic Meridional Overturning Circulation (AMOC) across 34.5°S is computed using observations from 49 eXpendable BathyThermograph (XBT) transects between 2002 and 2019. The Mov at 34.5°S serves as a possible indicator of the AMOC stability, with a negative (southward) freshwater transport indicating a possible bistable AMOC regime and positive (northward) transport indicating a monostable regime. A negative Mov mean of −0.15 ± 0.09 Sv is estimated from the repeated XBT transects, suggesting a bistable AMOC regime. Results are complemented with two data sets derived from Argo float observations, numerical ocean models, and coupled climate models. More than half of the coupled models examined, 20 out of 32, present positive Mov mean values. To investigate the causes of the differing signs of the Mov across the models, we examine the salinity vertical structure in models with positive and negative Mov, indicating fresher upper and saltier deep waters in models with positive Mov. The South Atlantic meridional fluxes show linear relationships, with a negative slope (positively correlated in magnitude) between Mov/MOC and Mov/MHT, and a positive slope (positively correlated) between MHT/MOC. Seasonally, the South Atlantic meridional fluxes from most of the data sets considered here, show a more negative Mov and a more positive MOC and MHT in austral fall and winter from April to August across 34.5°S. Plain Language Summary: It is well known that the meridional (north‐south) overturning circulation, a large system of ocean currents driven by winds, buoyancy (density) differences, mixing, and eddies, has a significant impact on the world's climate system. Based on observations and numerical model data, this study presents a multi‐data set analysis of the freshwater transported by this circulation system across the nominal latitude of 34.5°S in the South Atlantic. The observed southward meridional freshwater transport (out of the South Atlantic) derived from all the observational data sets considered indicates a bistable regime of the meridional overturning circulation. Some coupled models that suggest a mono‐stable (one stable state) regime have fresher upper and saltier deep waters than the models that indicate a bistable regime (two stable states). We confirm that there is a linear relationship between mass transport by the meridional overturning circulation with the meridional freshwater and heat transports. Finally, we determine the seasonal variability of these meridional fluxes, with a more negative meridional freshwater transport, as well as a more positive overturning circulation volume and heat transports from April to August in the South Atlantic Ocean across 34.5°S. Key Points: Observational‐based estimates of negative freshwater transports by the AMOC (Mov) at 34.5°S, suggest a bistable AMOC regimeMany coupled models demonstrate positive Mov values, attributed to biased salinity vertical profile patternsThe Mov is positively correlated in magnitude with mass and heat transport by the meridional overturning circulation [ABSTRACT FROM AUTHOR]

Abstract As exceeding the 1.5°C level of global warming is likely to happen in the near future, understanding the response of the ocean‐climate system to temporarily overshooting this warming level is of critical importance. Here, we apply the Adaptive Emissions Reduction Approach to the Earth System Model GFDL‐ESM2M to conduct novel overshoot scenarios that reach 2.0, 2.5 and 3.0°C of global warming before returning to 1.5°C over the time period of 1861–2500. We also perform a complementary scenario that stabilizes global temperature at 1.5°C, allowing to isolate impacts caused by the temperature overshoots alone, both during their peaks and after their reversals. The simulations indicate that substantial residual ocean surface warming persists in the high latitudes after the overshoots, with most notable regional anomalies occurring in the North Atlantic (up to +3.1°C in the 3°C overshoot scenario compared to the 1.5°C stabilization scenario) and the Southern Ocean (+1.2°C). The residual warming is primarily driven by the recoveries of the Atlantic and Southern Ocean meridional overturning circulation and associated increases in ocean heat transport. Excess subsurface heat storage in low and mid‐latitudes prevents steric sea level rise (SLR) from reverting to 1.5°C stabilization levels in any overshoot scenario, with steric sea level remaining up to 32% higher in the 3°C overshoot scenario on centennial time scales. Both peak impacts and persistent changes after overshoot reversal bear significant implications for future assessments of coastlines, regional climates, marine ecosystems, and ice sheets.

Abstract Sea surface temperature (SST) has been increasing since industrialization with rising greenhouse gases. However, a warming hole exists in the North Atlantic where SST has cooled by 0.4 K/century during 1900–2017. It has been argued that this cooling is due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), and subpolar North Atlantic SST has thus been utilized to estimate AMOC variability. We assess the robustness of subpolar North Atlantic SST as a proxy for AMOC strength under historical forcing, abrupt quadrupling of CO2, and a medium future emissions pathway, finding that AMOC's fingerprint on SST depends upon forcing scenarios. AMOC is important in warming hole development during significant warming periods, although SST may introduce uncertainties for AMOC reconstruction in stabilized regimes due to diverse forcing mechanisms and decadal variability. Our results caution against using SST alone as a proxy for AMOC variability—both on paleoclimatic and contemporary time scales.

All climate models project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) strength in response to greenhouse gas forcing. However, the climate impacts of the AMOC decline alone cannot be isolated from other drivers of climate change using existing Coupled Model Intercomparison Project simulations. To address this issue, we conduct idealized experiments using the EC‐Earth3 climate model. We compare an abrupt 4×CO2 simulation with the same experiment, except we artificially fix the AMOC strength at preindustrial levels. With this design, we can formally attribute differences in climate change impacts between these two experiments to the AMOC decline. In addition, we quantify the state‐dependence of AMOC impacts by comparing the aforementioned experiments with a preindustrial simulation in which we artificially reduce the AMOC strength. Our findings demonstrate that AMOC decline impacts are state‐dependent, thus understanding AMOC impacts on future climate change requires targeted model experiments. Plain Language Summary: Climate models predict that the Atlantic Ocean's major circulation system, known as the Atlantic Meridional Overturning Circulation (AMOC), will weaken during the 21st century. This weakening could have significant impacts on the climate. However, it is challenging to isolate the AMOC's effects because other factors, such as rising greenhouse gas levels, also affect the climate. To better understand the AMOC's role, in this study we use a climate model to conduct numerical experiments. We compare a simulation of the preindustrial climate with one in which we artificially decrease the strength of the AMOC. Then, we compare the preindustrial climate with two forced simulations: one with a fourfold increase in atmospheric carbon dioxide, where the AMOC weakens as expected, and another where we keep the AMOC at its preindustrial strength despite higher CO2 levels. By comparing these experiments, we determine that the impacts of an AMOC decline depend on the background climate state. This research demonstrates that ad‐hoc model experiments are needed to understand the impacts of a weakened AMOC in a changing climate. Key Points: We present new idealized experiments to assess the influence of a weakened Atlantic Meridional Overturning Circulation (AMOC) on future climate changeWe use the EC‐Earth3 climate model to carry out experiments imposing abrupt 4×CO2 forcing but fixing the AMOC strength, and then we compare them with preindustrial water hosing experimentsWe find that AMOC impacts on temperature and precipitation depend on the background climate state [ABSTRACT FROM AUTHOR]

Antarctic meltwater discharge has been largely emphasized for its potential role in climate change mitigation, not only by reducing global warming, but also by stabilizing the Atlantic Meridional Overturning Circulation (AMOC). Despite the tremendous impact of the AMOC on the climate system, its temporal evolution in response to the meltwater remains poorly understood. Here, we investigate the meltwater impacts on the AMOC based on the GFDL CM2.1 experiments and discover its fast weakening and slow strengthening to the Antarctic meltwater discharge. Cold ocean surface caused by meltwater spread throughout the globe and eventually strengthened the AMOC. However, in the early stages, the tropical temperature response could stimulate the Rossby wave teleconnection, modulating atmospheric circulation in the North Atlantic, and weakening convection and even the AMOC. This counterintuitive evolution implies a potential destabilizing effect of Antarctic meltwater, underscoring the importance of the atmospheric dynamics in the interaction between the two poles. Plain Language Summary: The climate system is made up of interactions between different subsystems, so that regional climate changes can have global effects. The freshwater discharge from Antarctica would increase in the future and result in regional cooling. Atmospheric and oceanic dynamics extend this local effect to the globe, reducing global surface temperature and strengthening the large‐scale ocean circulation in the Atlantic. These mitigating effects naturally put the spotlight on Antarctic meltwater. Our study however suggests that the mitigation effect depends on the time scale. Although the global mean temperature is always reduced, the ocean circulation in the Atlantic surprisingly slows down as an early response to Antarctic meltwater; fast atmospheric teleconnection enables it. This non‐monotonic evolution emphasizes the importance of the atmospheric teleconnection between the two poles, which should be carefully considered to understand the polar climate. Key Points: The response of the Atlantic Meridional Overturning Circulation to perennial meltwater‐induced cooling is investigated by GFDL CM2.1 experimentsBefore the cooling spreads out the Atlantic, tropical atmospheric teleconnection could weaken the Greenland ocean convectionThe fast and slow responses imply the importance of the atmospheric teleconnection to regulate polar climate [ABSTRACT FROM AUTHOR]

Understanding the response of the Earth system to varying concentrations of carbon dioxide (CO2) is critical for projecting possible future climate change and for providing insight into mitigation and adaptation strategies in the near future. In this study, we generate a dataset by conducting an experiment involving carbon dioxide removal (CDR)—a potential way to suppress global warming—using the Chinese Academy of Sciences Earth System Model version 2.0 (CAS-ESM2.0). A preliminary evaluation is provided. The model is integrated from 200–340 years as a 1% yr−1 CO2 concentration increase experiment, and then to ~478 years as a carbon dioxide removal experiment until CO2 returns to its original value. Finally, another 80 years is integrated in which CO2 is kept constant. Changes in the 2-m temperature, precipitation, sea surface temperature, ocean temperature, Atlantic meridional overturning circulation (AMOC), and sea surface height are all analyzed. In the ramp-up period, the global mean 2-m temperature and precipitation both increase while the AMOC weakens. Values of all the above variables change in the opposite direction in the ramp-down period, with a delayed peak relative to the CO2 peak. After CO2 returns to its original value, the global mean 2-m temperature is still ~1 K higher than in the original state, and precipitation is ~0.07 mm d−1 higher. At the end of the simulation, there is a ~0.5°C increase in ocean temperature and a 1 Sv weakening of the AMOC. Our model simulation produces similar results to those of comparable experiments previously reported in the literature. [ABSTRACT FROM AUTHOR]

This study investigates the representation of ocean convection in the Labrador Sea in seven Earth System Models (ESMs) from the Coupled Model Intercomparison Project Phase 5 and 6 datasets. The relative role of the oceanic and atmospheric biases in the subpolar North Atlantic gyre are explored using regional ocean simulations where the atmospheric forcing or the ocean initial and boundary conditions are replaced by reanalysis data in the absence of interactive air-sea coupling. Commonalities and differences among model behaviors are discussed with the objective of finding a pathway forward to improve the representation of the ocean mean state and variability in a region of fundamental importance for climate variability and change. Results highlight that an improved representation of ocean stratification in the North Atlantic subpolar gyre is urgently needed to constrain future climate change projections. While improving the ocean model resolution in the North Atlantic alone may contribute a better representation of both boundary currents and propagation of heat and freshwater anomalies into the Labrador Sea, it may not be sufficient. Addressing the atmospheric heat flux bias with better resolution in the atmosphere and land topography may allow for deep convection to occur in the Labrador Sea in some of the models that miss it entirely, but the greatest priority remains improving the representation of ocean stratification. [ABSTRACT FROM AUTHOR]

Global surface temperature short‐term trends fluctuate between cooling and fast‐warming under the combined action of external forcing and internal variability, significantly influencing the detectability of near‐term climate change. A key driver of these variations is anthropogenic aerosols (AAs), which have undergone a non‐monotonic evolution with rapid reduction in recent decades. However, their reduction is projected to decelerate under a high carbon emission scenario, yet the impact on surface temperature trends remains unknown. Here, using initial‐perturbation large ensembles, we find that future slowdown in AA reduction over Europe and North America expedites the subpolar North Atlantic surface warming by intensifying the Atlantic meridional overturning circulation. Further, it accelerates the South Indian Ocean and Southern Ocean surface warming through positive low‐cloud feedback and oceanic dynamical adjustment, triggered by the poleward migration of westerlies under interhemispheric energy constraint. These AA‐driven warmings exacerbate greenhouse warming, significantly enhancing the detectability of local decadal warming trends. Plain Language Summary: Global surface temperature rises non‐monotonically, exhibiting phases of accelerated warming, warming slowdown, and transitions between warming and cooling. Anthropogenic aerosols (AAs) are a key driver of historical trend variations, making human‐induced trends more detectable. However, AAs follow a nonlinear trajectory with a rapid decline over Europe and North America in recent decades, and such reduction is projected to slow down in the future. The saturation of AA reduction signifies a diminishing proportion of anthropogenic forcings, which were previously considered unrelated to future global warming. Nevertheless, based on large ensembles of climate change simulations, we reveal that this projected slowdown in AA reduction could significantly accelerate surface warming in the subpolar North Atlantic, South Indian and Southern Oceans. This is achieved by the strengthening of Atlantic meridional overturning circulation and the coupled oceanic‐atmospheric adjustments triggered by the southward shift of southern westerlies. Those AA‐driven ocean surface warmings exacerbate greenhouse warming, significantly enhancing the predominance of external signals over strong internal noise and thus the detectability of regional human‐induced decadal warming trends. It suggests that upholding stringent AA mitigation policies is imperative for effectively mitigating the risk of severe ocean warming. Key Points: Recent rapid reduction of anthropogenic aerosols (AA) over the northern extratropics would slow down under a high carbon emission scenarioSlower AA reduction accelerates subpolar North Atlantic surface warming by intensifying Atlantic meridional overturning circulationSlower AA reduction accelerates South Indian and Southern Ocean warming via positive low‐cloud feedback and oceanic dynamical adjustment [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]

Suspension is the key mechanism by which fine‐grained sediment (≤125 μm) is winnowed and transported across shallow‐water carbonate platforms into adjacent deep waters. Unlike sliding and saltation, which deliver sedimentary structures via bedload, the sedimentological signature of suspended sediment is more cryptic. This study focuses on suspended sediment, and its drivers – wind, waves and tides – to better constrain the processes responsible for the transport of fine‐grained sediments. By building forward from remote sensing algorithms developed for deep‐waters, sediment suspension in the shallow water column can be mapped from satellite. By applying machine learning to Moderate Resolution Imaging Spectroradiometer data for Great Bahama Bank, this study demonstrates how the drivers of sediment suspension change over 18 years across this 100 000 km2 carbonate platform. Through time, both seasonal patterns of suspension, as well as those induced by El Niño‐Southern Oscillation and, more subtly, the Atlantic Meridional Overturning Circulation were tracked. El Niño‐Southern Oscillation modulates wind‐induced currents, while Atlantic Meridional Overturning Circulation affects local sea level. Across space, this study shows how the eastern margin of Great Bahama Bank, which is traditionally considered to be wind‐dominated, primarily owes its suspended sediment to tidal currents focused between islands. Sediment suspension across the leeward margin of Great Bahama Bank, meanwhile, can be attributed to wind‐induced currents and waves. The authors consider this work a step towards delivering a quantitative, reproducible, process‐based understanding of sediment suspension atop carbonate platforms using Earth observation data. [ABSTRACT FROM AUTHOR]

Recent studies found that on long time scales there are often unexplained opposite trends in sea level variability between the upper and lower Chesapeake Bay (CB). Therefore, daily sea level and temperature records were analyzed in two locations, Norfolk in the southern CB and Baltimore in the northern CB; surface currents from Coastal Ocean Dynamics Application Radar (CODAR) near the mouth of CB were also analyzed to examine connections between the CB and the Atlantic Ocean. The observations in the bay were compared with daily Atlantic Meridional Overturning Circulation (AMOC) observations during 2005–2021. Empirical Mode Decomposition (EMD) analysis was used to show that variations of sea level and temperature in the upper and lower CB are positively correlated with each other for short time scales of months to few years, but anticorrelated on low frequency modes representing decadal variability and long-term nonlinear trends. The long-term CB modes seem to be linked with AMOC variability through variations in the Gulf Stream and the wind-driven Ekman transports over the North Atlantic Ocean. AMOC variability correlates more strongly with variability in the southern CB near the mouth of the bay, where surface currents indicate potential links with AMOC variability. For example, when AMOC and the Gulf Stream were especially weak during 2009–2010, sea level in the southern bay was abnormally high, temperatures were colder than normal and outflow through the mouth of CB was especially high. Sea level in the upper bay responded to this change only 1–2 years later, which partly explains phase differences within the bay. A persistent trend of 0.22 cm/s per year of increased outflow from the CB, may be a sign of a climate-related trend associated with combination of weakening AMOC and increased precipitation and river discharge into the CB. [ABSTRACT FROM AUTHOR]

It has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2‐CMIP5). This AMOC response is then compared to the CMIP5‐generation CESM1 large ensemble (CESM1‐LE) and the CMIP6‐generation CESM2 large ensemble (CESM2‐LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol‐indirect effects cannot explain differences in AMOC response between CESM1‐LE and CESM2‐LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC. Plain Language Summary: The Atlantic meridional overturning circulation (AMOC) is important for the wider climate because it transports a large amount of warm water northward away from the equator. The most recent generation of climate models disagree with the observed behavior of the AMOC over the twentieth century, and it has been suggested that this is due to the inclusion of aerosol‐cloud interactions in many of the newest models. Here we look at model simulations of the AMOC in several configurations to show that the disagreement in the past AMOC behavior is instead primarily due to changes in the inputs given to the models, rather than to changes in the models themselves. Key Points: The CESM2 Atlantic meridional overturning circulation (AMOC) response to aerosols depends on if CMIP5 or CMIP6 emissions are appliedLarge interannual variability in CMIP6 emissions appears to enhance the interannual variability of north Atlantic turbulent heat fluxesThis heat flux variability may drive a non‐linear AMOC response to aerosols [ABSTRACT FROM AUTHOR]

The interannual variability of deep convection in the Irminger Sea is considered essential in controlling the intensity of the Atlantic Meridional Overturning Circulation (AMOC). Based on the ARMOR-3D oceanic dataset and the Era-Interim atmospheric reanalysis, we suggest that the oceanic heat advection in the sea of the Irminger Current governs the interdecadal variability of deep convection of the Irminger Sea by modulating the upper ocean heat content in the basin. This forms a negative feedback that stabilizes the AMOC: an increase in the advection of oceanic heat into the Irminger Sea leads to a decrease in the convection depth, which further reduces the northward heat transport by the AMOC. We suggest that, on interdecadal time scales, the effect of ocean–atmosphere heat exchange on deep convection is relatively small due to a much lower interannual variability of the former compared to that of the oceanic heat convergence. During the positive phase of the North Atlantic Oscillation (NAO), the Irminger Current becomes colder, which allows an alternative explanation for the intermittently observed intensification of convection in the Irminger Sea with an increase in the NAO index. Thus, the long-period variability of the convection intensity in the Irminger Sea is associated not with local atmospheric influence, but rather with remote atmospheric forcing. [ABSTRACT FROM AUTHOR]

A speleothem collected from Palco Cave (Puerto Rico) spans the 8.2 ka event, a time interval associated with fluctuations of Atlantic Ocean circulation and possible drying in the Caribbean region. While stalagmite δ18O, δ13C, and Mg/Ca data do not show a sustained change in mean state over the 8.2 ka event, the proxies provide robust evidence for three abrupt fluctuations toward drier conditions in rapid succession, each lasting less than two decades, occurring at 8.20, 8.14, and 8.02 ka BP. A cave monitoring program at Palco Cave supports the interpretation of the speleothem proxy records. Because changes in the position of the Intertropical Convergence Zone (ITCZ) are directly coupled to sea-surface temperature variations in the North Atlantic, we hypothesize that cold events in the North Atlantic temporarily limited the northward migration of the ITCZ and tropical rain belt in boreal summer during these abrupt drying periods. The speleothem record suggests that the 8.2 ka event was associated with rapid rainfall fluctuations in the northern Caribbean followed by a comparably warm and wet phase after the 8.2 ka event. This enhanced variability during the transitional period of the deglaciation appears to be linked to a fast coupling between interacting oceanic and atmospheric processes. This involves, in particular, the Atlantic Meridional Overturning Circulation in modulating interhemispheric heat transport. [ABSTRACT FROM AUTHOR]

Antarctic ice cores reveal a glacial climate state during Marine Isotope Stage 3 (MIS‐3; 57–29 ka) punctuated by millennial‐scale warming events and pulses of CO2. This study further explores how changes in Southern Ocean carbon cycling contributed to these millennial‐scale fluctuations in climate. Evidence from South Atlantic sediment cores suggests that warming events were associated with decreased dust‐borne iron flux, reduced export production, and increased upwelling from the deep Southern Ocean (SO). These processes are considered to have contributed to rising atmospheric CO2 during periods of rapid warming. Here we investigate whether the same processes occurred in the southwest Pacific sector of the SO at TAN1106‐28. We show that reduced New Zealand glaciation and localized iron limitation in the southwest Pacific led to reduced export production during millennial‐scale warming events. Decreases in foraminifera‐bound δ15N during all MIS‐3 warming events may reflect increased nutrient supply by upwelling. Increased calcium carbonate flux during MIS‐3 warming events likely reflects coccolithophore production in response to sea surface temperatures, which, would increase carbonate counter pump strength and reduce CO2 sequestration. Concomitant decreases in bottom water oxygen, inferred from redox‐sensitive U and Mn sediment concentrations, and increases in the 14C age of deep waters, suggest that old, nutrient‐rich waters influenced southwest Pacific middepth waters during warming events. This signature may reflect an expansion of Pacific Deep Water into the SO during warming. Taken together, our multi‐proxy data set reveals that the southwest subantarctic Pacific acted as a source of CO2 during millennial‐scale warming events of MIS‐3. Plain Language Summary: Antarctic ice cores have provided evidence of episodes of warming and cooling of the Southern Hemisphere that took place over hundreds to thousands of years between 57,000 and 29,000 years ago. During warming events, the Southern Ocean is thought to have played a role in the increasing concentration of CO2 in the atmosphere. Here we use a sediment core from the southwest Pacific Southern Ocean to show that decreased iron input to the surface ocean reduced biological productivity during warming events, which hampered CO2 sequestration in the region. While in the deep ocean, proxies for the concentration of oxygen and the relative age of deep waters suggest that conditions may have become favorable for a release of CO2 from the ocean interior to the atmosphere during episodic warming. These findings support the theory that the Southern Ocean played an important role in the past carbon cycle by modulating the exchange of CO2 between the deep ocean and the atmosphere during periods of rapid climate change. Key Points: Decreased iron from New Zealand limits subantarctic Pacific export production during MIS‐3 warming eventsDecreases in foraminifera bound‐δ15N during MIS‐3 warming events are likely due to increased nutrient supply and reduced utilisationDecreases in bottom water O2 during MIS‐3 warming events may result from increased Pacific Deep Water influence [ABSTRACT FROM AUTHOR]

The Eocene‐Oligocene transition (EOT) (∼34 Ma) is marked by the rapid development of a semi‐permanent Antarctic ice‐sheet, as indicated by ice‐rafted debris and a 1–1.5‰ increase in deep sea δ18O. Proxy reconstructions indicate a drop in atmospheric CO2 and global cooling. How these changes affected surface ocean temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped‐isotope thermometry to well‐preserved planktonic foraminifera, that are associated with lower mixed‐layer to subthermocline dwelling depths from the drift sediments at international ocean discovery program Site 1411, Newfoundland, across four intervals bracketing the EOT. The thermocline/lower mixed‐layer dwelling foraminifera record a cooling of 1.9 ± 3.5 K (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous sea surface temperature (SST) reconstructions, absolute temperatures (Eocene 20.0 ± 2.9°C, Oligocene 18.0 ± 2.2°C) appear colder than previous organic proxy reconstructions for the northernmost Atlantic extrapolated to this location. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Our subthermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.5°C, Oligocene 13.0 ± 3.2°C, cooling of 5.5 ± 4.6 K) than foraminifera from the thermocline/lower mixed‐layer, consistent with global cooling and an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation. Plain Language Summary: During the Eocene temperatures on Earth were much warmer than today. It is generally believed that the Antarctic ice‐sheet first developed around 34 million years ago, during the Eocene‐Oligocene transition (EOT). How this change occurred is still widely debated, but it was probably caused by decreased CO2 levels and changes to heat distribution through ocean currents. Here, we study how water temperatures at the dwelling depths of planktonic microfossils in the Atlantic Ocean changed across this event. We use clumped isotopes—a way of reconstructing the temperature from fossil shells. We measured shells of planktonic foraminifera, single‐celled microscopic plankton, that lived at two different depth levels in the upper ocean. We find that the foraminifera with a shallower (thermocline/lower mixed‐layer) water dwelling depth in the North Atlantic Ocean cooled by about ∼1.9 K, while those that dwelled deeper cooled by ∼5.5 K across the EOT. The thermocline/lower mixed‐layer cooling is similar in magnitude to reconstructions from organic biomarkers. However, our reconstructed absolute temperatures are colder than previous estimates. We think that our deeper water temperature reconstructions reflect global cooling, while thermocline/lower mixed‐layer temperatures did not cool as much because a warm water current developed, similar to the Gulf Stream. Key Points: Clumped isotopes from well‐preserved planktonic foraminifera imply 1.9 K lower mixed‐layer/thermocline cooling across Eocene‐Oligocene transition (EOT) in North AtlanticOur thermocline temperatures for Eocene (20°C) and Oligocene (18°C) are cooler than organic proxy sea surface temperature estimatesIncreased subthermocline cooling compared to the thermocline indicates increased stratification across the EOT, hinting at intensified Atlantic meridional overturning circulation [ABSTRACT FROM AUTHOR]

Marine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short‐lived nature and dependence on air‐sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3–5 years. The pattern has a significant in‐phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward‐propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 3–9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic. Plain Language Summary: Here we show that ocean dynamics can affect marine heatwaves (MHWs) and cold spells (CSs) in the western subtropical South Atlantic. We focused our analysis on the sea level anomaly features that propagate westward, crossing the basin in 3–5 years near 30°S. As they propagate, the sea level anomalies drive clockwise (for negative anomalies) or anticlockwise (for positive anomalies) ocean circulation around them. The circulation transports either tropical or subpolar waters into the subtropical region, warming and cooling the subtropical region, respectively, and influencing MHWs and CSs. Since the anomalies influence meridional ocean transport, we analyzed their link to the basin integrated meridional heat transport associated with the Atlantic Meridional Overturning Circulation. We show that there is a good correlation between the phase of the sea level and circulation anomaly propagation (either centered in the eastern, western or interior of the basin) and the meridional heat transport. Therefore, the South Atlantic Meridional Overturning Circulation index can serve as an early warning of MHWs and CSs in the subtropical western South Atlantic. Key Points: Marine heatwaves and cold spells are in phase with westward propagating sea level anomaly featuresMeridional advection is important for the interannual mixed layer heat budget in the western South AtlanticThe South Atlantic Meridional Overturning Circulation leads the beginning of the westward‐propagating sea level mode by 3–9 months [ABSTRACT FROM AUTHOR]

Changes in the Atlantic Meridional Overturning Circulation (AMOC) in the quadrupled CO2 experiments conducted under the sixth Coupled Model Intercomparison Project (CMIP6) are examined. Increased CO2 triggers extensive Arctic warming, causing widespread melting of sea ice. The resulting freshwater spreads southward, first from the Labrador Sea and then the Nordic Seas, and proceeds along the eastern coast of North America. The freshwater enters the subpolar gyre north of the separated Gulf Stream, the North Atlantic Current. This decreases the density gradient across the current and the current weakens in response, reducing the inflow to the deepwater production regions. The AMOC cell weakens in tandem, first near the North Atlantic Current and then spreading to higher and lower latitudes. This contrasts with the common perception that freshwater caps the convection regions, stifling deepwater production; rather, it is the inflow to the subpolar gyre that is suppressed. Changes in surface temperature have a much weaker effect, and there are no consistent changes in local or remote wind forcing among the models. Thus an increase in freshwater discharge, primarily from the Labrador Sea, is the precursor to AMOC weakening in these simulations. [ABSTRACT FROM AUTHOR]

Despite decades of research, the cause of deglaciations is not fully understood, leaving a critical gap in our understanding of Earth's climate system. During the most recent deglaciation (Termination I (T I)), abrupt declines in the stable carbon isotope ratio (δ13C) of benthic foraminifera occurred throughout the mid‐depth (1,500–2,500 m) Atlantic. The spatial pattern in δ13C anomalies was likely due to Atlantic Meridional Overturning Circulation (AMOC) weakening and the accumulation of respired carbon, which also yields negative excursions in carbonate ion concentration (CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$). To investigate whether a similar pattern occurred during prior deglaciations, we developed δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records from 1,800 and 2,300 m water depth in the Southwest Atlantic spanning the last 150 ka. The new records reveal negative δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ anomalies during Termination II (TII) and the smaller deglaciations of Marine Isotope Stages (MIS) 4/3, 5b/a, and 5d/c, suggesting AMOC weakening is a common feature of deglaciation. The anomalies are more pronounced in the shallower core following MIS 2, 4, and 6 and in the deeper core following MIS 5b and 5d. The depth‐dependent pattern is most likely due to shoaling of Northern Source Water during glacial maxima and deepening during interglacial intervals. Comparison of CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records from TI and TII suggests similar levels of carbon accumulation in the mid‐depth Atlantic. The Brazil Margin δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ results indicate the AMOC plays a key role in the series of events causing deglaciation, regardless of differences in orbital configuration, ice volume, and mean global temperature. Key Points: Brazil Margin δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ records suggest the Atlantic Meridional Overturning Circulation weakened during each deglaciation of the last 150,000 yearsMovement of northern source water yields larger δ13C and CO32− $\left[{{\text{CO}}_{3}}^{2-}\right]$ anomalies at 1,800 m during MIS 2, 4, and 6 and at 2,300 m during MIS 5The dynamic nature of Atlantic watermass structure complicates the diagnosis of overturning patterns on glacial‐interglacial timescales [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]

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 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]

Atlantic multidecadal variability (AMV) has long been thought to be an expression of low-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). However, alternative hypotheses have been forwarded, including that AMV is primarily externally forced. Here, we review the current state of play by assessing historical simulations made for the sixth coupled model intercomparison project (CMIP6). Overall, the importance of external forcing is sensitive to the type of AMV index used, due to the importance of globally coherent externally forced signals in the models. There are also significant contrasts between the processes that drive internally and externally forced AMV, but these processes can be isolated by exploring the multivariate expression of AMV. Specifically, internal variability in CMIP6 models is consistent with an important role of ocean circulation and AMOC and the externally forced AMV is largely a surface-flux forced mechanism with little role for the ocean. Overall, the internal multivariate fingerprint of AMV is similar to the observed, but the externally forced fingerprint appears inconsistent with observations. Therefore, climate models still suggest a key role for ocean dynamics, and specifically AMOC, in observed AMV. Nevertheless, models remain deficient in a number of areas, and a stronger role for externally forced dynamical changes cannot be ruled out. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'. [ABSTRACT FROM AUTHOR]

The interannual variability of deep convection in the Irminger Sea is considered essential in controlling the intensity of the Atlantic Meridional Overturning Circulation (AMOC). Based on the ARMOR-3D oceanic dataset and the Era-Interim atmospheric reanalysis, we suggest that the oceanic heat advection in the sea of the Irminger Current governs the interdecadal variability of deep convection of the Irminger Sea by modulating the upper ocean heat content in the basin. This forms a negative feedback that stabilizes the AMOC: an increase in the advection of oceanic heat into the Irminger Sea leads to a decrease in the convection depth, which further reduces the northward heat transport by the AMOC. We suggest that, on interdecadal time scales, the effect of ocean–atmosphere heat exchange on deep convection is relatively small due to a much lower interannual variability of the former compared to that of the oceanic heat convergence. During the positive phase of the North Atlantic Oscillation (NAO), the Irminger Current becomes colder, which allows an alternative explanation for the intermittently observed intensification of convection in the Irminger Sea with an increase in the NAO index. Thus, the long-period variability of the convection intensity in the Irminger Sea is associated not with local atmospheric influence, but rather with remote atmospheric forcing. [ABSTRACT FROM AUTHOR]

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Abstract All climate models project a weakening of the Atlantic Meridional Overturning Circulation (AMOC) strength in response to greenhouse gas forcing. However, the climate impacts of the AMOC decline alone cannot be isolated from other drivers of climate change using existing Coupled Model Intercomparison Project simulations. To address this issue, we conduct idealized experiments using the EC‐Earth3 climate model. We compare an abrupt 4×CO2 simulation with the same experiment, except we artificially fix the AMOC strength at preindustrial levels. With this design, we can formally attribute differences in climate change impacts between these two experiments to the AMOC decline. In addition, we quantify the state‐dependence of AMOC impacts by comparing the aforementioned experiments with a preindustrial simulation in which we artificially reduce the AMOC strength. Our findings demonstrate that AMOC decline impacts are state‐dependent, thus understanding AMOC impacts on future climate change requires targeted model experiments.

Abstract Antarctic meltwater discharge has been largely emphasized for its potential role in climate change mitigation, not only by reducing global warming, but also by stabilizing the Atlantic Meridional Overturning Circulation (AMOC). Despite the tremendous impact of the AMOC on the climate system, its temporal evolution in response to the meltwater remains poorly understood. Here, we investigate the meltwater impacts on the AMOC based on the GFDL CM2.1 experiments and discover its fast weakening and slow strengthening to the Antarctic meltwater discharge. Cold ocean surface caused by meltwater spread throughout the globe and eventually strengthened the AMOC. However, in the early stages, the tropical temperature response could stimulate the Rossby wave teleconnection, modulating atmospheric circulation in the North Atlantic, and weakening convection and even the AMOC. This counterintuitive evolution implies a potential destabilizing effect of Antarctic meltwater, underscoring the importance of the atmospheric dynamics in the interaction between the two poles.

Abstract Global surface temperature short‐term trends fluctuate between cooling and fast‐warming under the combined action of external forcing and internal variability, significantly influencing the detectability of near‐term climate change. A key driver of these variations is anthropogenic aerosols (AAs), which have undergone a non‐monotonic evolution with rapid reduction in recent decades. However, their reduction is projected to decelerate under a high carbon emission scenario, yet the impact on surface temperature trends remains unknown. Here, using initial‐perturbation large ensembles, we find that future slowdown in AA reduction over Europe and North America expedites the subpolar North Atlantic surface warming by intensifying the Atlantic meridional overturning circulation. Further, it accelerates the South Indian Ocean and Southern Ocean surface warming through positive low‐cloud feedback and oceanic dynamical adjustment, triggered by the poleward migration of westerlies under interhemispheric energy constraint. These AA‐driven warmings exacerbate greenhouse warming, significantly enhancing the detectability of local decadal warming trends.

Abstract It has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2‐CMIP5). This AMOC response is then compared to the CMIP5‐generation CESM1 large ensemble (CESM1‐LE) and the CMIP6‐generation CESM2 large ensemble (CESM2‐LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol‐indirect effects cannot explain differences in AMOC response between CESM1‐LE and CESM2‐LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC.

Abstract To effectively navigate out of the climate crisis, a new interdisciplinary approach is needed to guide and facilitate research that integrates diverse understandings of how transitions evolve in intertwined social–environmental systems. The concept of tipping points, frequently used in the natural sciences and increasingly in the social sciences, can help elucidate processes underlying major social–environmental transitions. We develop the notion of interlinked ‘social–climatic tipping points’ in which desirability and intentionality are key constitutive features alongside stable states, feedbacks, reversibility and abruptness. We demonstrate the new insights that our interdisciplinary framework can provide by analysing the slowdown of the Atlantic Meridional Overturning Circulation and associated flooding of the Ahr Valley in Germany as a social–climatic tipping point. This framework can enable more sustainable and equitable futures by prioritising social–climatic tipping points for interdisciplinary research, identifying opportunities for action, and evaluating the nuanced desirability and acceptability of proposed solutions. Read the free Plain Language Summary for this article on the Journal blog.

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]

Monsoon rainfall proxy records show clear millennial variations corresponding to abrupt climate events in Greenland ice cores during Marine Isotope Stage 3 (MIS3). The occurrence of these abrupt climate changes is associated with Atlantic Meridional Overturning Circulation (AMOC) strength variations which greatly impact the global oceanic energy transport. Hence, the AMOC most likely plays a key role in modulating the global monsoon rainfall at millennial time scale. No modeling work has hitherto investigated the global monsoon system response to AMOC changes under a MIS3 background climate. Using the coupled climate model CCSM3, we simulated MIS3 climate using full 38 ka before present boundary conditions and performed a set of freshwater hosing/extraction experiments. We show not only agreement between modeling results and proxies of monsoon rainfall within the global monsoon domain but also highlight a nonlinear relationship between AMOC strength and annual mean global monsoon precipitation related to oceanic heat transport constraints. During MIS3, a weakened AMOC induces a decrease in annual mean global monsoon rainfall dominated by the northern hemisphere, whereas southern hemisphere monsoon rainfall increases. Above about 16 Sverdrups a further strengthening of the AMOC has no significant impact on hemispheric and global monsoon domain annual mean rainfall. The seasonal monsoon rainfall shows the same nonlinear response like annual mean both hemispherically and globally. Plain Language Summary: Approximately from 57 to 29 thousand years before present, fast warming events of around 10°C in Greenland temperature occurred on time scales of 1,000 years. The Atlantic Ocean circulation which transport heat northward could greatly impact global energy balance and a change in its strength is one plausible explanation for the occurrence of the warming events. The rainfall in monsoon regions also showed variations on the same time scale. So far, no study has investigated the linkage between monsoon rainfall and climate shift in high northern latitudes on such time scale with a specific background setup in a climate model. We show that ocean circulation change during the warming events could significantly impact global monsoon rainfall. When the ocean circulation slows down, northern/southern hemisphere monsoon rainfall decreases/increases linearly to circulation strength change. Interestingly, when the ocean circulation strengthens, monsoon rainfall in both hemispheres does not show obvious change. This nonlinear behavior is controlled by northward heat transport associated with the Atlantic Ocean circulation. Key Points: Nonlinear response of global monsoon rainfall to Atlantic Ocean circulation strength change in Marine Isotope Stage 3Simulated rainfall changes are consistent with reconstructed precipitation variations during millennial abrupt climate eventsGlobal monsoon rainfall is constrained by ocean heat transport [ABSTRACT FROM AUTHOR]

We aimed to analyze the learning process of the uniportal video-assisted thoracoscopic surgery (uVATS), the most up-to-date approach in VATS, to determine the learning curve cut-off value and to share the technical details in our application. Fifty patients who underwent anatomical pulmonary resection with the uVATS technique in our institute between May 2020 and June 2021 were retrospectively analyzed. The cut-off point in the learning curve (patient 26) was determined with the activity monitor operating characteristic (AMOC) method, which was developed for monitoring the most sensitive changes in the timeline. Patients were divided into two groups based on this cut-off value (group 1: first 25 patients vs. group 2: established phase, 25 patients) and compared. Comparison of groups (group 1 vs. 2) revealed a median resection time of 200 vs. 110 min (p < 0.001) and median bleeding amount of 50 vs. 30 cc (p = 0.033). Duration of pulmonary resection was statistically significantly correlated with lymph node dissection time (r = 0.698; p < 0.001) and hospital stay (r = 0.437; p = 0.029) in group 1. The most common complication was prolonged air leak (16%). The uVATS technique has video features very different from the tri-port and bi-port VATS methods. Our study using the AMOC method to compare two groups of uVATS in a continuous timeline over approximately 1 year (May 2020–June 2021) showed that, with increasing experience, resection times and bleeding amounts decreased significantly between the groups. Thus emphasizing the learning curve required for uVATS, anatomical pulmonary resection could be adequately and safely performed in all patients in this study. [ABSTRACT FROM AUTHOR]

Given paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) may affect the global climate system, we conduct model experiments with EC-Earth3, a state-of-the-art GCM, to specifically investigate, for the first time, mechanisms of precipitation change over the Euro-Atlantic sector induced by a weakened AMOC. We artificially weaken the strength of the AMOC in the model through the release of a freshwater anomaly into the Northern Hemisphere high latitude ocean, thereby obtaining a ~ 57% weaker AMOC with respect to its preindustrial strength for 60 model years. Similar to prior studies, we find that Northern Hemisphere precipitation decreases in response to a weakened AMOC. However, we also find that the frequency of wet days increases in some regions. By computing the atmospheric moisture budget, we find that intensified but drier storms cause less precipitation over land. Nevertheless, changes in the jet stream tend to enhance precipitation over northwestern Europe. We further investigate the association of precipitation anomalies with large-scale atmospheric circulations by computing weather regimes through clustering of geopotential height daily anomalies. We find an increase in the frequency of the positive phase of the North Atlantic Oscillation (NAO+), which is associated with an increase in the occurrence of wet days over northern Europe and drier conditions over southern Europe. Since a ~ 57% reduction in the AMOC strength is within the inter-model range of projected AMOC declines by the end of the twenty-first century, our results have implications for understanding the role of AMOC in future hydrological changes. [ABSTRACT FROM AUTHOR]

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]

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Abstract 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.

Abstract 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.