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The Southern Ocean provides dominant contributions to global ocean heat and carbon uptake, which is widely interpreted as resulting from its unique upwelling and circulation. Here we show a large asymmetry in these contributions, with the Southern Ocean accounting for 83 ± 33% of global heat uptake versus 43 ± 3% of global ocean carbon uptake over the historical period in state-of-the-art climate models. Using single radiative forcing experiments, we demonstrate that this historical asymmetry is due to suppressed heat uptake by northern oceans from enhanced aerosol forcing. In future projections, such as SSP2-4.5 where greenhouse gases increasingly dominate radiative forcing, the Southern Ocean contributions to global heat and carbon uptake become more comparable, 52 ± 5% and 47 ± 4%, respectively. Hence, the past is not a reliable indicator of the future, with the northern oceans becoming important for heat uptake while the Southern Ocean remains important for both heat and carbon uptake.

Antarctic Bottom Water (AABW) is pivotal for oceanic heat and carbon sequestrations on multidecadal to millennial timescales. The Weddell Sea contributes nearly a half of global AABW through Weddell Sea Deep Water and denser underlying Weddell Sea Bottom Water that form on the continental shelves via sea-ice production. Here we report an observed 30% reduction of Weddell Sea Bottom Water volume since 1992, with the largest decrease in the densest classes. This is probably driven by a multidecadal reduction in dense-water production over southern continental shelf associated with a >40% decline in the sea-ice formation rate. The ice production decrease is driven by northerly wind trend, related to a phase transition of the Interdecadal Pacific Oscillation since the early 1990s, superposed by Amundsen Sea Low intrinsic variability. These results reveal key influences on exported AABW to the Atlantic abyss and their sensitivity to large-scale, multidecadal climate variability.

Subantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current. The Southern Ocean, surrounding the Antarctic continent, plays an important role in the uptake and transport of heat and carbon. Subantarctic mode waters, which are characterized by their low stratification, play an important role in this uptake of heat and carbon, and therefore the factors impacting their properties need to be properly understood. To understand how surface forcing affects Subantarctic mode waters, sensitivity studies are conducted in an ocean state estimate, which consider the relative importance of surface heat flux, freshwater flux, and wind stresses on the stratification of mode waters. Surface heat flux has the largest impact on mode water formation both on seasonal and longer interannual timescales. Initially, surface heat loss leads to a decrease in stratification in the mode waters. However, there is a delayed response where the surface temperature response is effectively damped by the atmosphere and there is an opposing‐signed salinity response advected into the region, leading to a subsequent increase in stratification in the mode waters. The sensitivity of Southern Ocean mode water stratification to surface heat flux changes sign over timeSurface heat loss leads to an initial decrease in stratification in the mode watersSurface heat loss leads to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped The sensitivity of Southern Ocean mode water stratification to surface heat flux changes sign over time Surface heat loss leads to an initial decrease in stratification in the mode waters Surface heat loss leads to a delayed restratification due to a haline contribution after a thermal contribution is effectively damped

The lower limb of the Atlantic overturning circulation is resupplied by the sinking of dense Antarctic Bottom Water (AABW) that forms via intense air–sea–ice interactions next to Antarctica, especially in the Weddell Sea1. In the last three decades, AABW has warmed, freshened and declined in volume across the Atlantic Ocean and elsewhere2–7, suggesting an ongoing major reorganization of oceanic overturning8,9. However, the future contributions of AABW to the Atlantic overturning circulation are unclear. Here, using observations of AABW in the Scotia Sea, the most direct pathway from the Weddell Sea to the Atlantic Ocean, we show a recent cessation in the decline of the AABW supply to the Atlantic overturning circulation. The strongest decline was observed in the volume of the densest layers in the AABW throughflow from the early 1990s to 2014; since then, it has stabilized and partially recovered. We link these changes to variability in the densest classes of abyssal waters upstream. Our findings indicate that the previously observed decline in the supply of dense water to the Atlantic Ocean abyss may be stabilizing or reversing and thus call for a reassessment of Antarctic influences on overturning circulation, sea level, planetary-scale heat distribution and global climate2,3,8.

The Southern Ocean has complex spatial variability, characterized by sharp fronts, steeply tilted isopycnals, and deep seasonal mixed layers. Methods of defining Southern Ocean spatial structures traditionally rely on somewhat ad hoc combinations of physical, chemical, and dynamic properties. As a step toward an alternative approach for describing spatial variability in temperature, here we apply an unsupervised classification technique (i.e., Gaussian mixture modeling or GMM) to Southern Ocean Argo float temperature profiles. GMM, without using any latitude or longitude information, automatically identifies several spatially coherent circumpolar classes influenced by the Antarctic Circumpolar Current. In addition, GMM identifies classes that bear the imprint of mode/intermediate water formation and export, large‐scale gyre circulation, and the Agulhas Current, among others. Because GMM is robust, standardized, and automated, it can potentially be used to identify structures (such as fronts) in both observational and model data sets, possibly making it a useful complement to existing classification techniques. The Southern Ocean is an important part of the climate system, in part because it absorbs a large fraction of the heat and carbon that is added to the atmosphere/ocean system by human‐driven fossil fuel burning. In this work, we use a machine learning technique to automatically sort Southern Ocean temperature measurements into groups based on how those temperature measurements change with depth. Different groups have the fingerprints of different large‐scale circulation patterns, such as the powerful Antarctic Circumpolar Current that flows around Antarctica. The groups that we identify are consistent with our understanding of the Southern Ocean, which gives us confidence that our machine learning technique may be useful for automatically grouping measurements and computer model data in the future. This matters because the climate science community needs a new set of tools, possibly including the machine learning technique that we use in this paper, to deal with a very large, ever‐increasing volume of observational and computer model data. We apply Gaussian mixture modeling (GMM) to Southern Ocean temperature dataGMM identifies spatially coherent profile types without using latitude or longitude informationGMM offers a complementary approach for objectively classifying temperature profiles

In this work, we use realistic isopycnal velocities with a 3‐D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 67°W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20‐fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection‐diffusion model with constant, but different, zonal velocities east and west of 67°W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation. Realistic velocity fields used to advect a model tracer in the Southern Ocean produce lateral tracer distributions similar to an observed tracerDiapycnal diffusivities diagnosed from a best fit of the model tracer distribution to observations are 20 times larger in Drake Passage than in the Southeast PacificResults from models using two different velocity field products are consistent with one another, and with the previous result of Watson et al. (2013)

The Labrador Sea is one of the few regions on the planet where the interior ocean can exchange heat directly with the atmosphere via strong, localized, wintertime convection, with possible implications for the state of North Atlantic climate and global surface warming. Using an observationally constrained ocean adjoint model, we find that annual‐mean Labrador Sea heat content is sensitive to temperature/salinity changes (1) along potential source water pathways (e.g., the subpolar gyre, the North Atlantic Current, the Gulf Stream) and (2) along the West African and European shelves, which are not significant source water regions for the Labrador Sea. The West African coastal/shelf adjustment mechanism, which may be excited by changes in along‐shelf wind stress, involves pressure anomalies that propagate along a coastal waveguide toward Greenland, changing the across‐shelf pressure gradient in the North Atlantic and altering heat convergence in the Labrador Sea. We also find that nonlocal (in space and time) heat fluxes (e.g., in the Irminger Sea, the seas south of Iceland) can have a strong impact on Labrador Sea heat content. Understanding and predicting the state of the Labrador Sea and its potential impacts on North Atlantic climate and global surface warming will require monitoring of oceanic and atmospheric properties at remote sites in the Irminger Sea, the subpolar gyre, and along the West African and European shelf/coast system, among others. There are only a handful of locations on Earth where natural processes can rapidly inject heat and carbon into the interior ocean, where it can remain for decades to centuries, potentially slowing global surface warming. One of these locations is the Labrador Sea, which features strong exchanges of heat with the atmosphere and exceptionally deep mixing between the surface ocean and interior ocean. In this paper, we examine the factors that influence the heat content of the Labrador Sea. Using a numerical model, we find that although the heat content is most sensitive to local exchanges with the atmosphere, there is an unexpected connection between the heat content of the Labrador Sea and wind strength along the coast of West Africa and Europe. Sustained changes in wind strength in those regions can change the large‐scale circulation of the entire North Atlantic, ultimately changing the amount of heat that gets transported into the Labrador Sea and potentially impacting North Atlantic climate and global surface warming. We use adjoint sensitivity fields to quantify possible influences on Labrador Sea heat contentWe identify a basin‐scale adjustment mechanism involving the West African and European shelvesNonlocal heat fluxes can have a considerable impact on Labrador Sea heat content

The waters of the Southern Ocean are projected to warm over the coming century, with potential adverse consequences for native cold-adapted organisms. Warming waters have caused temperate marine species to shift their ranges poleward. The seafloor animals of the Southern Ocean shelf have long been isolated by the deep ocean surrounding Antarctica and the Antarctic Circumpolar Current, with little scope for southward migration. How these largely endemic species will react to future projected warming is unknown. By considering 963 invertebrate species, we show that within the current century, warming temperatures alone are unlikely to result in wholesale extinction or invasion affecting Antarctic seafloor life. However, 79% of Antarctica’s endemic species do face a significant reduction in suitable temperature habitat (an average 12% reduction). Our findings highlight the species and regions most likely to respond significantly (negatively and positively) to warming and have important implications for future management of the region.

In several regions north of the Antarctic Circumpolar Current (ACC), deep wintertime convection refreshes pools of weakly stratified subsurface water collectively referred to as Subantarctic Mode Water (SAMW). SAMW ventilates the subtropical thermocline on decadal timescales, providing nutrients for low‐latitude productivity and potentially trapping anthropogenic carbon in the deep ocean interior for centuries. In this work, we investigate the spatial structure and timescales of mode water export and associated thermocline ventilation. We use passive tracers in an eddy‐permitting, observationally‐informed Southern Ocean model to identify the pathways followed by mode waters between their formation regions and the areas where they first enter the subtropics. We find that the pathways followed by the mode water tracers are largely set by the mean geostrophic circulation. Export from the Indian and Central Pacific mode water pools is primarily driven by large‐scale gyre circulation, whereas export from the Australian and Atlantic pools is heavily influenced by the ACC. Export from the Eastern Pacific mode water pool is driven by a combination of deep boundary currents and subtropical gyre circulation. More than 50% of each mode water tracer reaches the subtropical thermocline within 50 years, with significant variability between pools. The Eastern Pacific pathway is especially efficient, with roughly 80% entering the subtropical thermocline within 50 years. The time required for 50% of the mode water tracers to leave the Southern Ocean domain varies significantly between mode water pools, from 9 years for the Indian mode water pool to roughly 40 years for the Central Pacific mode water pool. The export of Subantarctic Mode Water into the subtropical thermocline displays significant regional variabilityExport pathways display the relative influence of the subtropical gyres and ACCThe Eastern Pacific pathway is an especially efficient mode water export route due to boundary current transport

The Southern Ocean (SO) is a crucial region for the global ocean uptake of heat and carbon. There are large uncertainties in the observations of fluxes of heat and carbon between the atmosphere and the ocean mixed layer, which lead to large uncertainties in the amount entering into the global overturning circulation. In order to better understand where and when fluxes of heat and momentum have the largest impact on near‐surface heat content, we use an adjoint model to calculate the linear sensitivities of heat content in SO mode water formation regions (MWFRs) to surface fluxes. We find that the heat content of these regions is, in all three basins, most sensitive to same‐winter, local heat fluxes, and to local and remote wind one to eight years (the maximum lead‐time of our simulations) previously. This is supported by sensitivities to potential temperature changes, which reveal the sources of the MWFRs as well as dynamic links with boundary current regions and the Antarctic Circumpolar Current. We use the adjoint sensitivity fields to design a set of targeted perturbation experiments, allowing us to examine the linear and non‐linear responses of the heat content to changes in surface forcing. In these targeted experiments, the heat content is sensitive to both temperature changes and mixed layer volume changes in roughly equal magnitude. The Southern Ocean (SO) is of crucial importance to the global ocean's uptake of carbon and heat. However, due to difficulties in making observations in such a remote and hostile environment, we currently don’t know accurately how much heat and carbon enters the SO from the atmosphere. Heat from the SO can get locked away for hundreds to thousands of years in the world's deep oceans, entering through a few key regions. We use a computer model to assess how the heat, fresh water, and wind energy entering through the surface of the SO affects the heat of these key regions. We find that these regions are very sensitive to heat coming in through the surface directly over them, and that winds across a wider area of the SO can affect the heat stored for several years. If we want to estimate the heat stored in these regions more accurately, this information can be used to help us decide where and when it is important to measure the winds and heat entering the ocean better. We perform a comprehensive sensitivity study of the heat distribution in mode water formation regionsSensitivities are highest to local same‐winter heat flux changes, and both local and remote wind stress changes up to at least 8 years pastHigh sensitivity regions reveal kinematic links with source waters and dynamic links with boundary current regions We perform a comprehensive sensitivity study of the heat distribution in mode water formation regions Sensitivities are highest to local same‐winter heat flux changes, and both local and remote wind stress changes up to at least 8 years past High sensitivity regions reveal kinematic links with source waters and dynamic links with boundary current regions