Alejandra Sanchez-Franks

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). This study investigates the latitudinal coherence of this AMOC weakening and how it differs across depth- and density-space, future forcing scenarios and individual models. Percentage AMOC weakening is presented as a function of latitude and time between 1900 and 2100. By identifying the first year when specific percentage thresholds of weakening are reached at each latitude, contour lines of weakening are mapped as a function of latitude and time. Propagation of AMOC weakening is inferred from the gradient of these lines (defined as weakening gradients). Percentage weakening is found to originate at high latitudes (45–51° N in depth-space and 51–61° N in density-space) and propagate southwards. Because density-space weakening originates further north than depth-space, it has a longer lead time for propagation and our results suggest it could provide a more effective framework for early warning signal development. South of 40° N, propagation speeds in depth-space and density-space representations are similar. In the most extreme future forcing scenario, the multi-model mean weakening gradient reduces from ∼ 3.0 to ∼ 1.4° yr ⁻¹ , implying that as the AMOC weakens so too does the propagation of AMOC weakening. This behaviour is also seen for the majority of models (67 %). For a small fraction of models (13 %) the weakening gradients increase as the AMOC weakens and for the remainder (20 %) there is no discernable trend. The weakening gradients reduce more steeply in less extreme future forcing scenarios than in more extreme ones. The initial speed of propagation is found to be positively related to historical AMOC strength.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.

Since 2004 the Atlantic meridional overturning circulation (AMOC) has been observed at 26°N by the RAPID project, and in 2014 it was joined by the Overturning in the Sub-Polar North Atlantic Programme (OSNAP) at around 58°N. These two array-based programmes have provided many insights into the AMOC and its variability over a range of timescales, however there is little coherence between the two time series, not helped by OSNAP's relatively short duration.The MEZCAL (Methods for Extending the Horizontal Coverage of the AMOC Latitudinally and retrospectively) project seeks to leverage indirect observations and existing reconstruction methods to create an AMOC framework covering the North Atlantic between these two existing observing systems. The primary reconstruction method was developed at 41°N, a latitude between the subpolar and subtropical gyres, and combines data from Argo floats and satellite altimetry to estimate the AMOC from 2002 onwards.Here we examine how the assumptions of this method hold as we move northwards and southwards from 41°N into the more active gyre regions. We evaluate the resulting AMOC estimates against three 0.25° ocean reanalysis products, the 1° ECCO v4r4 state estimate, and other observation-based AMOC time series, and compare how well each captures ocean processes over varying timescales. We also show at which latitude it becomes important to define the AMOC in density-space rather than depth-space, and how the reconstructions are affected by features like the Gulf Stream extension.The increased spatial coverage these reconstructions provide show AMOC changes and variability at a greater range of latitudes, give insight into the connectivity and coherence between latitudes, and provide a comparison basis for numerical models. The increased temporal coverage increases the likelihood of identifying climate-driven low frequency changes to the Atlantic meridional overturning circulation.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.

The Atlantic Meridional Overturning Circulation (AMOC) is projected to weaken in the latest generation of climate models used in the Coupled Model Intercomparison Project, Phase 6 (CMIP6). There is uncertainty to the extent of weakening predicted, associated with model differences within the CMIP6 ensemble. The focus of this study is the latitudinal coherence of AMOC decline and how it differs between models and future forcing scenarios. AMOC weakening is presented as a function of latitude and time between 1900 and 2100 for depth-space and density-space. Weakening originates at high latitudes (45–47° N) in depth-space and 51–61° N in density-space) and propagates southwards. As density-space weakening originates further north than depth-space, density-space is better suited for early warning signals. For density-space, the 25 % weakening signal has an 11-year lag between 45° N and 26° N and in depth-space there is a 9-year lag. However, density-space has an additional 8-year lag between 57° N and 45° N giving a total lag of 19 years. As the AMOC weakens, the southward propagation of weakening (south of 40° N) slows with speeds ranging from 6.29 degrees per year to 0.40 degrees per year, dependent on model and scenario. Propagation speeds in depth-space and density-space are similar. In the strongest future forcing scenario, the propagation speed at 5 % weakening is 2.78 degrees per year and at 30 % weakening is 1.45 degrees per year. Also, the speed of southward propagation slows more intensely in less extreme climate change scenarios than in more extreme ones. As the AMOC weakens, the propagation speed of weakening is found to slow for some models while it remains constant for others. The CMIP6 models are categorised by whether the propagation speed is constant, declines to a constant, declines and increases again or continuously declines. The speed of propagation does not appear to be related to AMOC strength.

The ocean has absorbed over 90% of the excess heat trapped in the Earth system due to rising greenhouse gas emissions, with upper layers playing a crucial role. This study finds that 35% of the total ocean heat content (OHC) in the western Indian Ocean is stored within the upper 300 m. From 2000 to 2023, this layer shows a significant warming trend of over 24 years, making it the only tropical ocean basin with such a persistent rise. In contrast, the net surface heat flux into the ocean shows a declining trend of over 24 years, suggesting that direct atmospheric forcing is not the primary driver. Instead, seasonal ocean dynamics explains the observed increase in OHC and surface heat loss. During the winter monsoon, enhanced westward heat transport from the eastern equatorial Indian Ocean, driven by strengthened northeast monsoon currents, leads to heat accumulation in the western Indian Ocean. In the summer monsoon, the Great Whirl, a large anticyclonic eddy, plays a central role. Although northward heat transport associated with the Great Whirl has weakened, the southward transport has declined more sharply, resulting in net heat gain. Additionally, a northward shift in monsoon winds displaces the Great Whirl closer to the Socotra Islands, altering upwelling patterns and further redistributing heat. These findings underscore the dominant role of ocean circulation in driving long‐term upper‐ocean warming in the western Indian Ocean, contrasting with the expected influence of surface heat fluxes.

Oxygen minimum zones (OMZs) occupy large areas of the tropical subsurface oceans and substantially alter regional biogeochemical cycles. In particular, the removal rate of bio-available nitrogen (denitrification) from the water column in OMZs is disproportionate to their size. The Bay of Bengal is one of the strongest OMZs in the global oceans; however, variable sources of oxygen prevent the onset of large-scale denitrification. The various oxygen-supply mechanisms that maintain oxygen concentrations in the OMZ above the denitrification threshold are currently unknown. Here, using a combination of multi-platform observations and model simulations, we identify an annual supply of oxygen to the Bay of Bengal in the high-salinity core of the Southwest Monsoon Current, a seasonal circulation feature that flows northward into the bay during the South Asian southwest monsoon (i.e. June–September). Oxygen concentrations within the Southwest Monsoon Current (80–100 µmol kg⁻¹) are higher than those of waters native to the bay (i.e. −1). These high-oxygen waters spread throughout the central and western Bay of Bengal, leading to substantial spatio-temporal variability in observed oxygen concentrations. Moreover, the northward oxygen transport of the Southwest Monsoon Current is a spatially and temporally distinct event that stands out from background oxygen transport. Model results indicate that, interannually, oxygen supply to the bay varies with the strength of the Southwest Monsoon Current more closely than with its oxygen concentration. Consequently, we suggest that predictability of the annual oxygen flux is likely aided by understanding and predicting the physical forcing of the Southwest Monsoon Current. Our results demonstrate that the current, in particular its high-salinity and high-oxygen core, is a feature relevant to the processes and communities that drive denitrification within the Bay of Bengal that has heretofore not been considered.

Oxygen minimum zones occupy large areas of the tropical subsurface oceans and substantially alter regional bio- geochemical cycles. In particular, the removal rate of bio-available nitrogen (de-nitrification) from the water column in oxygen minimum zones is disproportionate to their size. The Bay of Bengal is one of the strongest OMZs in the global oceans; however, variable sources of oxygen prevent the onset of large-scale de-nitrification. The various oxygen-supply mechanisms that maintain oxygen concentrations in the OMZ above the denitrification threshold are currently unknown. Here, using a combination of multi-platform observations and model simulations, we identify an annual supply of oxygen to the Bay of Bengal in the high-salinity core of the Southwest Monsoon Current, a seasonal circulation feature that flows northwards into the Bay during the South Asian southwest monsoon (i.e. June to September). Oxygen concentrations within the Southwest Monsoon Current (80 to 100 μmol kg⁻¹) are higher than those of waters native to the Bay (i.e. < 20 μmol kg⁻¹). These high-oxygen waters spread throughout the central and western Bay of Bengal, leading to substantial spatio-temporal variability in observed oxygen concentrations. Moreover, the northward oxygen transport of the Southwest Monsoon Current is a spatially and temporally distinct event that stands out from background oxygen transport. Models indicate that variability in annually integrated oxygen supply to the BoB varies with the strength of the Southwest Monsoon Current more closely than with its oxygen concentration. Consequently, we suggest that predictability of the annual oxygen flux is likely aided by understanding and predicting the physical forcing of the Southwest Monsoon Current. Our results demonstrate that the current, and in particular its high-salinity, high-oxygen core, is a feature relevant to the processes and communities that drive denitrification within the Bay of Bengal that has heretofore not been considered.

Oxygen minimum zones occupy large areas of the tropical subsurface oceans and substantially alter regional bio- geochemical cycles. In particular, the removal rate of bio-available nitrogen (de-nitrification) from the water column in oxygen minimum zones is disproportionate to their size. The Bay of Bengal is one of the strongest OMZs in the global oceans; however, variable sources of oxygen prevent the onset of large-scale de-nitrification. The various oxygen-supply mechanisms that maintain oxygen concentrations in the OMZ above the denitrification threshold are currently unknown. Here, using a combination of multi-platform observations and model simulations, we identify an annual supply of oxygen to the Bay of Bengal in the high-salinity core of the Southwest Monsoon Current, a seasonal circulation feature that flows northwards into the Bay during the South Asian southwest monsoon (i.e. June to September). Oxygen concentrations within the Southwest Monsoon Current (80 to 100 μmol kg⁻¹) are higher than those of waters native to the Bay (i.e. < 20 μmol kg⁻¹). These high-oxygen waters spread throughout the central and western Bay of Bengal, leading to substantial spatio-temporal variability in observed oxygen concentrations. Moreover, the northward oxygen transport of the Southwest Monsoon Current is a spatially and temporally distinct event that stands out from background oxygen transport. Models indicate that variability in annually integrated oxygen supply to the BoB varies with the strength of the Southwest Monsoon Current more closely than with its oxygen concentration. Consequently, we suggest that predictability of the annual oxygen flux is likely aided by understanding and predicting the physical forcing of the Southwest Monsoon Current. Our results demonstrate that the current, and in particular its high-salinity, high-oxygen core, is a feature relevant to the processes and communities that drive denitrification within the Bay of Bengal that has heretofore not been considered.

The ocean has absorbed over 90% of the excess heat trapped in the Earth system due to rising greenhouse gas emissions, with upper layers playing a crucial role. This study finds that 35% of the total ocean heat content in the western Indian Ocean is stored within the upper 300 meters. From 2000 to 2023, this layer shows a significant warming trend of 0.90 GJ/m^2 over 24 years, making it the only tropical ocean basin with such a persistent rise. In contrast, the net surface heat flux into the ocean shows a declining trend of -11.53 W/m^2 over 24 years, suggesting that direct atmospheric forcing is not the primary driver. Instead, seasonal ocean dynamics explain nearly 87% of the observed increase in ocean heat content and surface heat loss. During the winter monsoon, enhanced westward heat transport from the eastern equatorial Indian Ocean, driven by strengthened northeast monsoon currents, leads to heat accumulation in the western Indian Ocean. In the summer monsoon, the Great Whirl, a large anticyclonic eddy, plays a central role. Although northward heat transport associated with the Great Whirl has weakened, the southward transport has declined more sharply, resulting in net heat gain. Additionally, a northward shift in monsoon winds displaces the Great Whirl closer to the Socotra Islands, altering upwelling patterns and further redistributing heat. These findings underscore the dominant role of ocean circulation in driving long-term upper-ocean warming in the western Indian Ocean, contrasting with the expected influence of surface heat fluxes.