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Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation1. However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work2–5has suggested that deep-water upwelling may occur along the ocean’s sloping seafloor; however, evidence has, so far, been indirect. Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling3,6–8. The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 106m3s−1of net upwelling globally9.

Future sea-level rise projections carry large uncertainties, mainly driven by the unknown response of the Antarctic Ice Sheet to climate change. During the past four decades, the contribution of the East Antarctic Ice Sheet to sea-level rise has increased. However, unlike for West Antarctica, the causes of East Antarctic ice-mass loss are largely unexplored. Here, using oceanographic observations off East Antarctica (80–160° E) we show that mid-depth Circumpolar Deep Water has warmed by 0.8–2.0 °C along the continental slope between 1930–1990 and 2010–2018. Our results indicate that this warming may be implicated in East Antarctic ice-mass loss and coastal water-mass reorganization. Further, it is associated with an interdecadal, summer-focused poleward shift of the westerlies over the Southern Ocean. Since this shift is predicted to persist into the twenty-first century, the oceanic heat supply to East Antarctica may continue to intensify, threatening the ice sheet’s future stability.

Small-scale turbulent mixing plays a pivotal role in shaping ocean circulation and a broad range of physical and biogeochemical processes. Despite advances in our understanding of the geophysical processes responsible for this mixing, the nature and importance of biomixing—turbulent mixing caused by marine biota—are still debated. A major source of uncertainty pertains to the efficiency of biomixing (the fraction of the turbulent energy produced through swimming that is spent in mixing the ocean vertically), which the few in situ observations available suggest to be much lower than that of geophysical turbulence. Here we shed light on this problem by analysing 14 days of continuous measurements of centimetre-scale turbulence in an area of coastal upwelling. We show that turbulent dissipation is elevated 10- to 100-fold (reaching 10−6–10−5W kg−1) every night of the survey due to the swimming activity of large aggregations of anchovies that gather regularly over the spawning season. Turbulent mixing is invigorated concurrently with dissipation, and occurs with an efficiency comparable to that of geophysical turbulence. Our results demonstrate that biologically driven turbulence can be a highly effective mixing agent, and call for a re-examination of its impacts on productive upper ocean regions.

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation, ventilates the abyssal ocean and sequesters heat and carbon on multidecadal to millennial timescales. AABW originates on the Antarctic continental shelf, where strong winter cooling and brine released during sea ice formation produce Dense Shelf Water, which sinks to the deep ocean. The salinity, density and volume of AABW have decreased over the last 50 years, with the most marked changes observed in the Ross Sea. These changes have been attributed to increased melting of the Antarctic Ice Sheet. Here we use in situ observations to document a recovery in the salinity, density and thickness (that is, depth range) of AABW formed in the Ross Sea, with properties in 2018–2019 similar to those observed in the 1990s. The recovery was caused by increased sea ice formation on the continental shelf. Increased sea ice formation was triggered by anomalous wind forcing associated with the unusual combination of positive Southern Annular Mode and extreme El Niño conditions between 2015 and 2018. Our study highlights the sensitivity of AABW formation to remote forcing and shows that climate anomalies can drive episodic increases in local sea ice formation that counter the tendency for increased ice-sheet melt to reduce AABW formation.

There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the “efficiency” of diapycnal mixing, that is, the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is nontrivially sensitive to the covariation of these parameters. Varying these parameters one at a time can lead to significant errors in the patterns of diapycnal mixing‐driven upwelling and downwelling and to the over and under estimation of mixing in such a way that the net rate of globally integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both parameters are allowed to covary and be spatially variable. These findings have important implications for the representation of oceanic heat, carbon, nutrients, and other tracer budgets in general circulation models. Deep ocean basins are filled with dense waters that form at high latitudes and sink to the abyss. The overturning circulation of the ocean, a key regulator of the climate system, is only feasible if such dense waters can resurface. The breaking of internal waves makes such resurfacing possible. In the deep ocean, internal waves are largely generated by the flow of tides over topography. Their breaking mixes dense deep waters with lighter waters above them, bringing them upward. Two key parameters in climate models for modeling such mixing are (I) the ratio of energy in the wave field that is spent near rough topography due to breaking as opposed to what is radiated away and (II) the amount of energy from wave breaking that goes to mixing versus what is wasted through dissipation by viscosity of seawater. Both parameters are considered constant in climate models. In this work, we quantify the roles of variations in each of these two parameters in setting the patterns of deep ocean upwelling of dense waters and argue that the two parameters need to be changed realistically and interdependently to avoid significant inaccuracies in the quantification of the mixing‐induced deep branch of ocean circulation. We quantify the sensitivity of the deep ocean circulation to radiation and mixing efficiencies of internal tide‐induced turbulent mixingThe spatial patterns of deep diapycnal upwelling and downwelling are highly sensitive to the two efficiencies and their covariationThe two efficiencies have to be changed in tandem to avoid significant inaccuracy in the quantification of deep branch of ocean circulation

Subantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year. Well‐mixed mode waters that form in the north of the Southern Ocean are particularly important to the ocean absorption of heat and CO2from the atmosphere. Two types of mode water form in the Pacific sector of the Southern Ocean: a Central pool and a Southeast pool. Both have shown significant year‐to‐year variability in recent decades. Variability in the regions where these waters form is shown to be due to changes in air‐sea fluxes, horizontal advection, and the upward transport of deeper water. Transport of freshwater into the mode water formation regions is shown to be correlated with year‐to‐year changes in sea ice area in the Ross Sea and in the Amundsen/Bellingshausen seas. The results suggest that it takes around 6 months for sea ice melt from the Amundsen/Bellingshausen seas to reach the southeast mode water region, and up to 2 years for freshwater from the Ross Sea to reach both mode water formation sites. In 2015, Amundsen/Bellingshausen sea ice was particularly high, leading to more freshwater being transported to the southeast mode water site the following spring/summer. A huge decrease in winter sea ice in 2016 then caused the opposite, and salinity at the formation site was unusually high. Temperature and salinity variability at Pacific Subantarctic Mode Water (SAMW) formation sites is driven by surface fluxes, advection and entrainmentFreshwater anomalies advected from the Ross Sea take up to 2 years to reach the Pacific SAMW formation regionsAnomalies in Amundsen/Bellingshausen sea ice in 2016–2017 drove salinity anomalies at the nearby southeast Pacific SAMW formation site Temperature and salinity variability at Pacific Subantarctic Mode Water (SAMW) formation sites is driven by surface fluxes, advection and entrainment Freshwater anomalies advected from the Ross Sea take up to 2 years to reach the Pacific SAMW formation regions Anomalies in Amundsen/Bellingshausen sea ice in 2016–2017 drove salinity anomalies at the nearby southeast Pacific SAMW formation site

Mooring and glider observations and a high‐resolution satellite sea surface temperature image reveal features of a transient submesoscale front in a typical mid‐ocean region of the Northeast Atlantic. Analysis of the observations suggests that the front is forced by downfront winds and undergoes symmetric instability, resulting in elevated upper‐ocean kinetic energy, restratification, and turbulent dissipation. The instability is triggered as downfront winds act on weak upper‐ocean vertical stratification and strong lateral stratification produced by mesoscale frontogenesis. The instability's estimated rate of kinetic energy extraction from the front accounts for the difference between the measured rate of turbulent dissipation and the predicted contribution from one‐dimensional scalings of buoyancy‐ and wind‐driven turbulence, indicating that the instability underpins the enhanced dissipation. These results provide direct evidence of the occurrence of symmetric instability in a quiescent open‐ocean environment and highlight the need to represent the instability's restratification and dissipative effects in climate‐scale ocean models. Oceanic submesoscale flows, with typical spatial scales of 1 km, are key to providing a dynamical route from energetic mesoscale eddies (10–100 km) to turbulent microscales (∼1 cm). A submesoscale phenomenon thought to draw kinetic energy from mesoscale currents and transfer it to turbulent dissipative processes is symmetric instability. This mechanism has been abundantly documented in strong and persistent ocean fronts such as those associated with western boundary currents, but its occurrence and impacts in the more extensive, quiescent mid‐ocean regions remain little explored. In this work, we present rare observational evidence of symmetric instability at a transient front in a mid‐ocean area of the Northeast Atlantic, founded on high‐resolution mooring and glider measurements. We show that wind‐driven frictional effects at the front trigger a symmetric instability, which leads to elevated upper‐ocean kinetic energy, restratification, and turbulent dissipation. The instability's extraction of kinetic energy from the front quantitatively matches the measured dissipation, which cannot be explained by classical one‐dimensional mixed layer processes. Our findings suggest that submesoscale symmetric instability may occur extensively in the relatively quiescent environment that characterizes the majority of the ocean and point to the need of representing the instability's effects in climate‐scale ocean models. A symmetric instability event is observed in a quiescent mid‐ocean regionSymmetric instability is associated with elevated upper‐ocean kinetic energy, restratification, and turbulent dissipationThe event is triggered by wind forcing of a transient front generated by mesoscale frontogenesis

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 Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state‐of‐the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system's role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures, and presence of sea ice year‐round hamper field and remotely sensed measurements. We highlight the importance of winter and under‐ice conditions in the southern WG, the role that new technology will play to overcome present‐day sampling limitations, the importance of the WG connectivity to the low‐latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East‐West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long‐term data to determine trends and will improve representation of processes for regional, Antarctic‐wide, and global modeling efforts—thereby enhancing predictions of the WG in global ocean circulation and climate. The Weddell Gyre is one of the main oceanographic features in the ocean surrounding Antarctica, the Southern Ocean. Although located far from other continents, this polar region affects the planet through the exchange of gases between frigid ocean waters and the atmosphere, regulating oxygen and carbon dioxide farther north. Studying the Weddell Gyre is challenging, as sea ice covers the ocean surface year around, restricting access by research ships and sensing of ocean surface from satellites. New technology is now available to avoid past limitations, autonomous underwater vehicles, instruments flown by planes, and floats instrumented with sea‐ice detection. Only through international collaboration can we obtain adequate data to populate environmental models and study key areas in the gyre or hot spots. In this review we identify the missing links in our knowledge of the gyre, proposing research to address those questions. Three aspects are critical to understanding the processes that drive the gyre's oceanography, ice, geology, chemistry, and biology: winter and under‐ice conditions that set the stage for the evolution of physics, ice, and biogeochemistry; exchange of water, material, and energy (or heat) with lower latitudes; and intensification of the clockwise circulation of the gyre with changes in winds. Major research priorities to advance understanding of the Weddell Gyre are identified and justified against current knowledgeInterdisciplinary approaches are needed to support system science research of the Weddell Gyre and promote collaborative projectsWinter conditions, connectivity to lower latitudes, and intensification of the gyre are the main interdisciplinary priorities

The response of the subpolar Southern Ocean (sSO) to wind forcing is assessed using satellite radar altimetry. sSO sea level exhibits a phased, zonally coherent, bimodal adjustment to circumpolar wind changes, involving comparable seasonal and interannual variations. The adjustment is effected via a quasi‐instantaneous exchange of mass between the Antarctic continental shelf and the sSO to the north, and a 2‐month‐delayed transfer of mass between the wider Southern Ocean and the subtropics. Both adjustment modes are consistent with an Ekman‐mediated response to variations in surface stress. Only the fast mode projects significantly onto the surface geostrophic flow of the sSO; thus, the regional circulation varies in phase with the leading edge of sSO sea level variability. The surface forcing of changes in the sSO system is partly associated with variations of surface winds linked to the Southern Annular Mode and is modulated by sea ice cover near Antarctica. Variability in subpolar Southern Ocean sea level is documented using satellite radar altimetry, including in ice‐covered areasSubpolar Southern Ocean sea level exhibits a phased, bimodal response to circumpolar wind changes, entailing a strong seasonal cycleThe relevant wind forcing is partially described by the Southern Annular Mode and is modulated by sea ice cover near Antarctica

The role of mesoscale turbulence in maintaining the mean buoyancy structure and overturning circulation of the Southern Ocean is investigated through a 2‐year‐long, single‐mooring record of measurements in Drake Passage. The buoyancy budget of the area is successively assessed within the Eulerian and the Temporal‐Residual‐Mean frameworks. We find that a regime change occurs on time scales of 1 to 100 days, characteristic of mesoscale dynamics, whereby the eddy‐induced turbulent horizontal advection balances the vertical buoyancy advection by the mean flow. We use these diagnostics to reconstruct the region's overturning circulation, which is found to entail an equatorward downwelling of Antarctic Intermediate and Bottom Waters and a poleward upwelling of Circumpolar Deep Water. The estimated eddy‐induced flow can be accurately parameterized via the Gent‐McWilliams closure by adopting a diffusivity of ∼2,000 m2s−1with a middepth increase to 2,500 m2s−1at 2,100 m, immediately underneath the maximum interior stratification. In this study we use a 2‐year‐long, high‐resolution (both temporally and vertically) mooring together with a state‐of‐the‐art method (Temporal‐Residual‐Mean framework) to disentangle the role of mesoscale eddy turbulence for sustaining the Meridional Overturning Circulation leeward of the Drake Passage. Because of its role in carbon sequestration and ocean heat uptake the Meridional Overturning Circulation is a key component of the climate system. However, it is driven by a wide range of physical processes from persistent, large‐scale wind to mesoscale and small‐scale turbulence. Hence, disentangling how these processes contribute to the maintenance of this climatically relevant ocean circulation is one of the current challenges in physical oceanography. Here we apply a state‐of‐the‐art method (Temporal‐Residual‐Mean framework) and show the emergence of a balance between mesoscale eddies and Eulerian‐mean circulation for buoyancy advection. In particular, we demonstrate that the effect mesoscale eddies can be accurately parameterized with a Gent‐McWilliams type diffusivity of ∼2,000 m2s−1. Beyond this result, this study also provides an observational evidence of the typical large‐scale picture of the Meridional Overturning Circulation and shows its relevance at local scales. Horizontal buoyancy advection by mesoscale eddies at a Southern Ocean mooring site balances mean vertical advection on time scale of 100 daysThe overturning circulation of the Southern Ocean emerges on this time scale as a residual between the eddy‐induced and Eulerian‐mean flowsThe eddy‐induced flow can be accurately parameterized with a Gent‐McWilliams diffusivity of ∼2,000 m2s−1

With two 25‐hour series of turbulent microstructure and currents observations carried out in August 2013, during spring (CHAOS 1) and neap tides (CHAOS 2), we investigated the semidiurnal cycle of turbulent dissipation in an embayment affected by coastal upwelling (Ría de Vigo, NW Iberia). At the time of sampling, the bay hosted a net, wind‐driven bi‐directional positive exchange flow and thermal stratification. Turbulent kinetic energy (TKE) dissipation (ɛ) at the interface between upwelled and surface waters was enhanced by two orders of magnitude during the ebbs ( ∼10−6W kg− 1) with respect to the floods ( ∼10−8W kg− 1). This pattern was caused by the constructive interference of the shear associated with the upwelling and tidal currents. The vertical structure of the tidal currents was consistent with a deformation of tidal ellipses by stratification, which was tightly coupled to the intensity of upwelling. This two‐pronged interaction resulted in a modulation of the semidiurnal cycle of turbulent dissipation by coastal upwelling. Thus, as a result of the upwelling relaxation conditions experienced during CHAOS 1, depth‐integrated interior TKE dissipation rates were higher, by a factor of ∼2, compared to CHAOS 2. By using a simple model, we determined that observed variations in turbulent mixing had a limited influence on the tidal variations of stratification, which were dominated by straining and advection. The mixing mechanism described here is potentially relevant for the ecology of upwelling bays, as it can stimulate the transport of nutrients from deep‐upwelled waters to the sun‐lit surface layers where primary production takes place. The semidiurnal cycle of turbulent dissipation in a partially‐mixed coastal embayment results from upwelling‐tide interactionThe overall intensity of turbulent dissipation is modulated by the rate of coastal upwellingThis mechanism acts as a pathway for nutrient supply from deep upwelled to sunlit surface waters

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 region of Maud Rise, a seamount in the Weddell Sea, is known for the occurrence of irregular polynya openings during the winter months. Hydrographic observations have shown the presence of a warmer water mass below the mixed layer along the seamount's flanks, commonly termed the warm‐water Halo, surrounding a colder region above the rise, the Taylor Cap. Here we use two observational data sets, an eddy‐permitting reanalysis product and regional high‐resolution simulations, to investigate the interannual variability of the Halo and Taylor Cap for the period 2007–2022. Observations include novel hydrographic profiles obtained in the Maud Rise area in January 2022, during the first SO‐CHIC cruise. It is demonstrated that the temperature of deep waters around Maud Rise exhibits strong interannual variability within the Halo and Taylor Cap, occasionally to such an extent that the two features become indistinguishable. A warming of deep waters by as much as 0.8°C is observed in the Taylor Cap during the years preceding the opening of a polynya in 2016 and 2017, starting in 2011. By analyzing regional simulations, we show that most of the observed variability in the Halo is forced remotely by advection of deep waters from the Weddell Gyre into the region surrounding Maud Rise. Our highest‐resolution simulation indicates that mesoscale eddies subsequently transfer the properties of the Halo's deep waters onto the Taylor Cap. The eddies responsible for such transfer originate in an abrupt retroflection along the inner flank of the Halo. A polynya is an opening within the sea ice cover during winter. In the Weddell Sea, polynyas occur irregularly and often emerge close to a seamount, Maud Rise. This topographic obstacle disturbs the local circulation of the Weddell Gyre and leads to the formation of two local permanent features: a Taylor Cap on top of the seamount, which is mainly an isolated water mass; and surrounding this, a warm‐water Halo, located along the flanks of Maud Rise. In this study, we use observational and model analyses to show the changes in the temperature of regional water masses on a timescale of years, and to identify the drivers of these changes. The properties of the warm‐water Halo are controlled by the advection of deep waters along the south‐eastern rim of the Weddell Gyre. These waters are partly mixed into the Taylor Cap by eddies, thereby preconditioning the water column for deep convection. In the years preceding the opening of a polynya in 2016–2017, we document a warming trend in the Taylor Cap, starting in 2011. Interannual variability of the subsurface temperature maximum at Maud Rise is documented with observationsAdvection of anomalously cold and fresh deep waters from the Weddell Gyre into the Halo leads to a near‐vanishing of the Taylor Cap in 2014Observed warming of the subsurface layer in the Taylor Cap due to eddy transport preceded the polynya opening in 2016–2017 Interannual variability of the subsurface temperature maximum at Maud Rise is documented with observations Advection of anomalously cold and fresh deep waters from the Weddell Gyre into the Halo leads to a near‐vanishing of the Taylor Cap in 2014 Observed warming of the subsurface layer in the Taylor Cap due to eddy transport preceded the polynya opening in 2016–2017

A high‐resolution satellite image that reveals a train of coherent, submesoscale (6 km) vortices along the edge of an ocean front is examined in concert with hydrographic measurements in an effort to understand formation mechanisms of the submesoscale eddies. The infrared satellite image consists of ocean surface temperatures at ∼390m resolution over the midlatitude North Atlantic (48.69°N, 16.19°W). Concomitant altimetric observations coupled with regular spacing of the eddies suggest the eddies result from mesoscale stirring, filamentation, and subsequent frontal instability. While horizontal shear or barotropic instability (BTI) is one mechanism for generating such eddies (Munk's hypothesis), we conclude from linear theory coupled with the in situ data that mixed layer or submesoscale baroclinic instability (BCI) is a more plausible explanation for the observed submesoscale vortices. Here we assume that the frontal disturbance remains in its linear growth stage and is accurately described by linear dynamics. This result likely has greater applicability to the open ocean, i.e., regions where the gradient Rossby number is reduced relative to its value along coasts and within strong current systems. Given that such waters comprise an appreciable percentage of the ocean surface and that energy and buoyancy fluxes differ under BTI and BCI, this result has wider implications for open‐ocean energy/buoyancy budgets and parameterizations within ocean general circulation models. In summary, this work provides rare observational evidence of submesoscale eddy generation by BCI in the open ocean. Here, we test Munk's theory for small‐scale eddy generation using a unique set of satellite‐ and ship‐based observations. We find that for one particular set of observations in the North Atlantic, the mechanism for eddy generation is not pure horizontal shear, as proposed by Munk et al. ([Munk, W., 2000]) and Munk ([Munk, W., 2001]), but is instead vertical shear, or baroclinic instability. While by itself, this is not a globally important result, taken in the context of mesoscale eddies which are ubiquitous in the World Ocean, this suggests energy exchanges in the more ambient, open ocean are the result of the latter mechanism. In conclusion, submesoscale eddy generation is poorly understood in the ocean and we need to better constrain our geographical and temporal understanding of these processes for representation in coarse‐resolution models. A high‐resolution satellite image reveals an ocean front populated by submesoscale vorticesLinear theory applied to coincident hydrography suggests vortices result not from barotropic but baroclinic instabilityThis study documents rare observational evidence of submesoscale baroclinic instability in the open ocean

The rates of isopycnal stirring and water mass transport by mesoscale eddies, and of diapycnal mixing by small‐scale turbulence, across the Brazil‐Malvinas Confluence (BMC) are assessed from a set of microstructure and hydrographic measurements in the Argentine Basin. This assessment is founded on a theoretical framework that applies a triple decomposition to the temperature variance equation and assumes eddies to transfer potential vorticity downgradient. The BMC is found to host widespread intense isopycnal stirring at rates of O(103–104m2s−1), and generally weak diapycnal mixing at rates of O(10−6–10−5m2s−1). Despite such disparity, both diapycnal mixing and isopycnal stirring play roles of comparable importance in determining regional water mass properties within surface and mode waters. In deeper layers, isopycnal stirring prevails. Eddies are further diagnosed to effect an important cross‐BMC transport, at rates of O(1 m2s−1). When scaled by the along‐stream extent of the BMC, these rates integrate to volume transports that may be as large as O(10 Sverdrups). This suggests that cross‐BMC transfers of waters are substantially effected by eddy‐induced flows. Turbulent mixing plays an important role in large‐scale oceanic processes such as water mass modification, distribution of salinity and temperature properties, and the global overturning circulation. These mixing processes are the direct consequence of different physical phenomena, including mixing across density surfaces (i.e., diapycnal) by small‐scale turbulence and stirring along density surfaces (i.e., isopycnal) by mesoscale eddies. Here, we assess the strength of diapycal mixing and isopycnal stirring across the Brazil‐Malvinas Confluence (BMC). Our results suggest that both processes are key to determine regional water mass properties in the BMC. We find that mixing across density surfaces and stirring along density surfaces are both important in upper layers, while stirring is prevalent in deeper waters. Finally, we estimate the importance of eddies in the exchange of subtropical and subantarctic waters across the BMC, and thus in the Southern Ocean overturning circulation. We find that eddy‐induced flows contribute substantially to the cross‐BMC transfer of water masses. We assess the rates of diapycnal mixing and isopycnal stirring and water mass transports by mesoscale eddies in the Brazil‐Malvinas ConfluenceIsopycnal stirring by eddies dominates water mass transformations across the BMC, especially below 500 mEddy‐indiced flows substantially contribute to the transfer of waters across the Brazil‐Malvinas Confluence We assess the rates of diapycnal mixing and isopycnal stirring and water mass transports by mesoscale eddies in the Brazil‐Malvinas Confluence Isopycnal stirring by eddies dominates water mass transformations across the BMC, especially below 500 m Eddy‐indiced flows substantially contribute to the transfer of waters across the Brazil‐Malvinas Confluence

The ocean's internal pycnocline is a layer of elevated stratification that separates the well‐ventilated upper ocean from the more slowly renewed deep ocean. Despite its pivotal role in organizing ocean circulation, the processes governing the formation of the internal pycnocline remain little understood. Classical theories on pycnocline formation have been couched in terms of temperature and it is not clear how the theory applies in the high‐latitude Southern Ocean, where stratification is dominated by salinity. Here we assess the mechanisms generating the internal pycnocline at southern high latitudes through the analysis of a high‐resolution, realistic, global sea ice–ocean model. We show evidence suggesting that the internal pycnocline's formation is associated with sea ice‐ocean interactions in two distinct ice‐covered regions, fringing the Antarctic continental slope and the winter sea‐ice edge. In both areas, winter‐persistent sea‐ice melt creates strong, salinity‐based stratification at the base of the winter mixed layer. The resulting sheets of high stratification subsequently descend into the ocean interior at fronts of the Antarctic Circumpolar Current, and connect seamlessly to the internal pycnocline in areas further north in which pycnocline stratification is determined by temperature. Our findings thus suggest an important role of localized sea ice‐ocean interactions in configuring the vertical structure of the Southern Ocean. Satellite observations have revealed significant trends in Antarctic sea‐ice concentration over recent decades. While the science community is starting to unravel the causes of the observed changes in sea‐ice extent, our understanding of how these ice changes are influencing ocean circulation remains rudimentary. Here we take a step toward addressing this important gap by analyzing relationships between sea ice and ocean density structure in a state‐of‐the‐art, realistic sea ice‐ocean model. We find that localized sea ice‐ocean interactions in the Southern Ocean, in particular the counter‐intuitive melting of sea ice in winter, contribute to shape the vertical structure of the Southern Hemisphere oceans. The internal pycnocline in the high‐latitude Southern Ocean is generated by winter‐persistent sea ice meltingSea‐ice melt persists in winter due to ice drift and warm‐water entrainment, thus maintaining salinity‐based stratification at the base of the winter mixed layerThe subpolar internal pycnocline descends into the ocean interior at fronts of the Antarctic Circumpolar Current The internal pycnocline in the high‐latitude Southern Ocean is generated by winter‐persistent sea ice melting Sea‐ice melt persists in winter due to ice drift and warm‐water entrainment, thus maintaining salinity‐based stratification at the base of the winter mixed layer The subpolar internal pycnocline descends into the ocean interior at fronts of the Antarctic Circumpolar Current

Ice shelves around Antarctica are vulnerable to an increase in ocean‐driven melting, with the melt rate depending on ocean temperature and the strength of flow inside the ice‐shelf cavities. We present measurements of velocity, temperature, salinity, turbulent kinetic energy dissipation rate, and thermal variance dissipation rate beneath Pine Island Glacier ice shelf, West Antarctica. These measurements were obtained by CTD, ADCP, and turbulence sensors mounted on an Autonomous Underwater Vehicle (AUV). The highest turbulent kinetic energy dissipation rate is found near the grounding line. The thermal variance dissipation rate increases closer to the ice‐shelf base, with a maximum value found ∼0.5 m away from the ice. The measurements of turbulent kinetic energy dissipation rate near the ice are used to estimate basal melting of the ice shelf. The dissipation‐rate‐based melt rate estimates is sensitive to the stability correction parameter in the linear approximation of universal function of the Monin‐Obukhov similarity theory for stratified boundary layers. We argue that our estimates of basal melting from dissipation rates are within a range of previous estimates of basal melting. We present the turbulent kinetic energy and thermal variance dissipation rates beneath an ice shelf for the first timeThe highest thermal variance dissipation rate is found near the ice/ocean interfaceThe highest turbulent kinetic energy dissipation rate is found near the grounding line

Warm ocean waters drive rapid ice‐shelf melting in the Amundsen Sea. The ocean heat transport toward the ice shelves is associated with the Amundsen Undercurrent, a near‐bottom current that flows eastward along the shelf break and transports warm waters onto the continental shelf via troughs. Here we use a regional ice‐ocean model to show that, on decadal time scales, the undercurrent's variability is baroclinic (depth‐dependent). Decadal ocean surface cooling in the tropical Pacific results in cyclonic wind anomalies over the Amundsen Sea. These wind anomalies drive a westward perturbation of the shelf‐break surface flow and an eastward anomaly (strengthening) of the undercurrent, leading to increased ice‐shelf melting. This contrasts with shorter time scales, for which surface current and undercurrent covary, a barotropic (depth‐independent) behavior previously assumed to apply at all time scales. This suggests that interior ocean processes mediate the decadal ice‐shelf response in the Amundsen Sea to climate forcing. The West Antarctic Ice Sheet is losing mass, causing sea level rise. Most of this loss occurs in the Amundsen Sea Embayment, due to melting of coastal glaciers by warm ocean waters. These warm waters are transported toward the glaciers by the Amundsen Undercurrent, a near‐seafloor eastward‐flowing current located at the boundary between the deep ocean and the shallower seas around Antarctica. Changes in the undercurrent thus regulate the amount of heat available to melt the glaciers. Here, we use a model to assess the undercurrent's variability on time scales of decades, as decadal ocean forcing drives periods of enhanced ice‐sheet retreat. Contrary to previous work, our model shows that wind fluctuations, associated with surface temperature changes in the tropical Pacific, lead to changes in the interior ocean density field on decadal time scales. Decadal anomalous cyclonic atmospheric circulation over the Amundsen Sea, associated with cooling in the tropical Pacific, accelerates the near‐surface ocean flow westward, but also accelerates the eastward‐flowing undercurrent and enhances glacial melting. Our work suggests that previous assumptions about the decadal oceanic response of the Amundsen Sea to wind variability might need to be reconsidered, with implications for melting of West Antarctic glaciers. Our modeled decadal strengthening of the Amundsen Undercurrent corresponds with increase in ice‐shelf basal meltingUndercurrent variability on decadal time scales is predominantly baroclinic (i.e., anticorrelated with the surface flow variability)At decadal timescales, this model shows enhanced ice‐shelf melting under westward wind anomalies, a reverse relationship to previous studies Our modeled decadal strengthening of the Amundsen Undercurrent corresponds with increase in ice‐shelf basal melting Undercurrent variability on decadal time scales is predominantly baroclinic (i.e., anticorrelated with the surface flow variability) At decadal timescales, this model shows enhanced ice‐shelf melting under westward wind anomalies, a reverse relationship to previous studies

The export of waters from the Weddell Gyre to lower latitudes is an integral component of the southern subpolar contribution to the three‐dimensional oceanic circulation. Here we use more than 20 years of repeat hydrographic data on the continental slope on the northern tip of the Antarctic Peninsula and 5 years of bottom lander data on the slope at 1000 m to show the intermittent presence of a relatively cold, fresh, westward flowing current. This is often bottom‐intensified between 600 and 2000 dbar with velocities of over 20 cm s−1, transporting an average of 1.5 ± 1.5 Sv. By comparison with hydrography on the continental slope within the Weddell Sea and modeled tracer release experiments we show that this slope current is an extension of the Antarctic Slope Current that has crossed the South Scotia Ridge west of Orkney Plateau. On monthly to interannual time scales the density of the slope current is negatively correlated (r> 0.6 with a significance of over 95%) with eastward wind stress over the northern Weddell Sea, but lagging it by 6–13 months. This relationship holds in both the high temporal resolution bottom lander time series and the 20+ year annual hydrographic occupations and agrees with Weddell Sea export variability observed further east. We compare several alternative hypotheses for this wind stress/export relationship and find that it is most consistent with wind‐driven acceleration of the gyre boundary current, possibly modulated by eddy dynamics, and represents a mechanism by which climatic perturbations can be rapidly transmitted as fluctuations in the supply of intermediate‐level waters to lower latitudes. High temporal resolution bottom observations of Antarctic Slope Current extension are madeExport properties from the Weddell Sea are well correlated with changes of wind stress curl over the Weddell GyreExport time scales indicate a 1–5 month lag between changes in winds and boundary current response in the Weddell Gyre

The hypothesis that the impingement of mesoscale eddy flows on small‐scale topography regulates diapycnal mixing and meridional overturning across the deep Southern Ocean is assessed in an idealized model. The model simulates an eddying circumpolar current coupled to a double‐celled meridional overturning with properties broadly resembling those of the Southern Ocean circulation and represents lee wave‐induced diapycnal mixing using an online formulation grounded on wave radiation theory. The diapycnal mixing generated by the simulated eddy field is found to play a major role in sustaining the lower overturning cell in the model, and to underpin a significant sensitivity of this cell to wind forcing. The vertical structure of lower overturning is set by mesoscale eddies, which propagate the effects of near‐bottom diapycnal mixing by displacing isopycnals vertically. The breaking of topographically generated internal lee waves enhances diapycnal diffusivityThe diapycnal mixing generated by the simulated eddy field regulates the deep overturningMesoscale eddies displace isopycnals vertically and set the vertical structure of the deep MOC

The distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflects the boundary conditions near the ocean surface: air‐sea exchange, sea ice formation, and subsurface ice melt. We use a nonlinear least squares solution to determine the value of the recharge temperature and salinity, as well as the excess air injection and glacial meltwater content throughout the water column and in the precursor to Antarctic Bottom Water. The noble gas‐derived recharge temperature and salinity in the Weddell Gyre are −1.95°C and 34.95 psu near 5500 m; these cold, salty recharge values are a result of surface cooling as well as brine rejection during sea ice formation in Antarctic polynyas. In comparison, the global value for deep water recharge temperature is −0.44°C at 5500 m, which is 1.5°C warmer than the southern hemisphere deep water recharge temperature, reflecting a distinct contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlights the impact of sea ice formation on setting the gas properties in southern sourced deep water. Below 1000 m, glacial meltwater averages 3.5‰ by volume and represents greater than 50% of the excess neon and argon found in the water column. These results indicate glacial melt has a nonnegligible impact on the atmospheric gas content of Antarctic Bottom Water. We adapt the noble gas paleothermometer to quantify recharge properties of the deep ocean, including sea ice and glacial meltwater fractionGlacial meltwater is responsible for a significant fraction of dissolved noble gases in deep waterBrine and gas rejection during sea ice formation alters the dissolved gas budget of deep water

A signature of submesoscale flows in the upper ocean is skewness in the distribution of relative vorticity. Expected to result for high Rossby number flows, such skewness has implications for mixing, dissipation, and stratification within the upper ocean. An array of moorings deployed in the Northeast Atlantic for 1 year as part of the experiment of the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) reveals that relative vorticity is positively skewed during winter even though the scale of the Rossby number is less than 0.5. Furthermore, this skewness is reduced to zero during spring and autumn. There is also evidence of modest seasonal variations in the gradient Rossby number. The proposed mechanism by which relative vorticity is skewed is that the ratio of lateral to vertical buoyancy gradients, as summarized by the inverse gradient Richardson number, restricts its range during winter but less so at other times of the year. These results support recent observations and model simulations suggesting that the upper ocean is host to a seasonal cycle in submesoscale turbulence. Submesoscale relative vorticity (RV) is estimated for approximately 1 year in the Northeast AtlanticVariance and skewness of RV within the upper ocean are enhanced during winterObservations are consistent with seasonally varying submesoscale turbulence

The thermodynamic balance of the Weddell Gyre is assessed from an inverse estimate of the circulation across the gyre's rim. The gyre experiences a weak net buoyancy gain that arises from a leading‐order cancelation between two opposing contributions, linked to two cells of water mass transformation and diapycnal overturning. The lower cell involves a cooling‐driven densification of 8.4 ± 2.0 Sv of Circumpolar Deep Water and Antarctic Bottom Water near the gyre's southern and western margins. The upper cell entails a freshening‐driven conversion of 4.9 ± 2.0 Sv of Circumpolar Deep Water into lighter upper ocean waters within the gyre interior. The distinct role of salinity between the two cells stems from opposing salinity changes induced by sea ice production, meteoric sources, and admixture of fresh upper ocean waters in the lower cell, which contrasts with coherent reductions in salinity associated with sea ice melting and meteoric sources in the upper cell. The Weddell Gyre hosts two overturning cells with contrasting buoyancy forcingsLower cell involves cooling‐driven densification of deep and bottom watersUpper cell entails freshening‐driven lightening of deep water

This paper examines two internal lee wave closures that have been used together with ocean models to predict the time‐averaged global energy conversion rate into lee waves and dissipation rate associated with lee waves and topographic blocking: the Garner (2005) scheme and the Bell (1975) theory. The closure predictions in two Southern Ocean regions where geostrophic flows dominate over tides are examined and compared to microstructure profiler observations of the turbulent kinetic energy dissipation rate, where the latter are assumed to reflect the dissipation associated with topographic blocking and generated lee wave energy. It is shown that when applied to these Southern Ocean regions, the two closures differ most in their treatment of topographic blocking. For several reasons, pointwise validation of the closures is not possible using existing observations, but horizontally averaged comparisons between closure predictions and observations are made. When anisotropy of the underlying topography is accounted for, the two horizontally averaged closure predictions near the seafloor are approximately equal. The dissipation associated with topographic blocking is predicted by the Garner (2005) scheme to account for the majority of the depth‐integrated dissipation over the bottom 1000 m of the water column, where the horizontally averaged predictions lie well within the spatial variability of the horizontally averaged observations. Simplifications made by the Garner (2005) scheme that are inappropriate for the oceanic context, together with imperfect observational information, can partially account for the prediction‐observation disagreement, particularly in the upper water column. Average abyssal closure predictions within factor of 2 of each other, observationsClosures differ most in their treatment of topographic blocking, which cannot be validated yetBottom enhancement of dissipation is mostly due to topographic blocking

The West Spitsbergen Current, which flows northward along the western Svalbard continental slope, transports warm and saline Atlantic water (AW) into the Arctic Ocean. A combined analysis of high‐resolution seismic images and hydrographic sections across this current has uncovered the oceanographic processes involved in horizontal and vertical mixing of AW. At the shelf break, where a strong horizontal temperature gradient exists east of the warmest AW, isopycnal interleaving of warm AW and surrounding colder waters is observed. Strong seismic reflections characterize these interleaving features, with a negative polarity reflection arising from an interface of warm water overlying colder water. A seismic‐derived sound speed image reveals the extent and lateral continuity of such interleaving layers. There is evidence of obliquely aligned internal waves emanating from the slope at 450–500 m. They follow the predicted trajectory of internal S2 tidal waves and can promote vertical mixing between Atlantic and Arctic‐origin waters. Seismic imaging is used to reveal oceanic fine structure to the west of SvalbardIsopycnal interleaving between warm Atlantic and colder Polar water on the shelfOn the upper slope, semidiurnal internal tidal beams are seismically imaged

The impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows. Middepth Southern Ocean eddies impact local dissipation of internal wave energyTurbulent dissipation is enhanced on eddy periphery and suppressed in eddy coreRay tracing elucidates mechanisms by which internal waves evolve in ACC eddy

The impact on the upper ocean of the passage of a short, intense storm over a Southern Ocean site, in proximity to an Antarctic Circumpolar Current front, is characterized. The storm causes a wind‐induced deepening of the mixed layer and generates an inertial current. Immediate poststorm observations indicate a mixed layer extending to approximately 50 m depth. Subsequent measurements show the upper ocean to have restratified, injecting near‐inertial shear in stratified waters within 1 day of the storm's passage. This time scale for the development of near‐inertial shear is 1 order of magnitude shorter than that predicted by the βdispersion paradigm. The observed rapid changes in upper ocean stratification point to the existence of an as yet undocumented, efficient mechanism for injection of near‐inertial shear into the stratified ocean that is in turn associated with enhanced turbulence and mixing. Rapid restratification of upper ocean following a storm close to an ocean frontDevelopment of near‐inertial shear deviates from beta dispersion paradigm

The propagation characteristics of near‐inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves' vertical penetration are investigated using observations from a mooring array located in the northeast Atlantic. The year‐long observations show that near‐inertial motions are mainly generated by local wind forcing, and that they radiate equatorward and downward following several strong wind events (wind stress ≳0.5 N m−2). Observational estimates of horizontal group speed typically exceed those of vertical group speed by two orders of magnitude, consistent with predictions from the dispersion relation. Enhanced near‐inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales, shorter time scales, and confined vertical extent compared to mesoscale eddies. Near‐inertial waves (NIWs) are excited mainly by variable winds at the ocean surface and can carry their energy into the ocean interior, thus playing an important role in mixing the deep ocean. However, the propagation behaviors of NIWs, and how such waves are affected by mesoscale and submesoscale processes, are still understudied, especially over periods of months to years. In this study, we examine an annual cycle of wind‐generated NIWs based on moored observations in a typical open‐ocean region of the northeast Atlantic. Our results show that NIWs propagate downward and equatorward following several strong wind events. Enhanced near‐inertial kinetic energy and vertical shear are found preferentially in regions of anticyclonic vorticity with Rossby number of O(0.1). By contrast, submesoscale anticyclones with Rossby number of O(1) are ineffective at trapping and accelerating near‐inertial motions into the ocean interior. This is due to the smaller horizontal scales, shorter time scales, and confined vertical extent of submesoscale motions compared to mesoscale eddies. Our findings highlight the major role of mesoscale anticyclones in draining NIWs from the upper ocean to the ocean interior, and have implications for detecting regions of active turbulent mixing driven by NIWs in the deep ocean. We provide observational evidence of downward‐ and equatorward‐propagating near‐inertial waves over a full annual cycleEnhanced near‐inertial kinetic energy and vertical shear are found preferentially in regions of anticyclonic vorticityThe chimney effect for near‐inertial waves is very likely controlled by mesoscale, rather than submesoscale, anticyclones We provide observational evidence of downward‐ and equatorward‐propagating near‐inertial waves over a full annual cycle Enhanced near‐inertial kinetic energy and vertical shear are found preferentially in regions of anticyclonic vorticity The chimney effect for near‐inertial waves is very likely controlled by mesoscale, rather than submesoscale, anticyclones

Sub‐Antarctic Mode Waters (SAMW) form to the north of the Antarctic Circumpolar Current (ACC) through deep winter mixing. SAMW connect the atmosphere with the oceanic pycnocline, transferring heat and carbon into the ocean interior and supplying nutrients to the northern ocean basins. The processes controlling SAMW ventilation and properties remain poorly understood. Here, we investigate the significance and origin of a ubiquitous feature of SAMW formation regions: The seasonal build‐up of a subsurface salinity maximum. With biogeochemical Argo floats, we show that this feature influences SAMW mixed‐layer dynamics, and that its formation is associated with a decline in preformed nutrients comparable to biological drawdown in surface waters (∼0.15 mol m−2y−1). Our analysis reveals that these features are driven by advection of warm, salty, nutrient‐poor waters of subtropical origin along the ACC. This influx represents a leading‐order term in the SAMW physical and biogeochemical budgets, and can impact large‐scale nutrient distributions. Sub‐Antarctic Mode Waters (SAMW) form during vigorous, deep winter mixing at the northern edge of the Antarctic Circumpolar Current (ACC). SAMW constitute an important portion of the upper ocean volume, and slow down climate change by storing vast amounts of anthropogenic heat and carbon. SAMW also transport Southern Ocean nutrients toward lower latitudes, sustaining primary productivity around and north of the tropics. Using data from biogeochemical Argo floats, we investigate the seasonal dynamics of SAMW formation, which are hitherto poorly understood due to the region's remoteness and harsh winter conditions. We show that SAMW formation regions feature the seasonal build‐up of a subsurface salinity maximum along with a decrease in nutrient levels, which are the imprint of the along‐ACC transport of salty, nutrient‐poor waters of subtropical origin. These findings highlight the important contribution of subtropical waters to SAMW heat, salt and nutrient budgets, which contrasts with the widespread view that SAMW form primarily through atmospheric lightening of cold, fresh Antarctic waters. These diverse views involve different feedback mechanisms between climate and SAMW formation and have divergent implications for understanding the impacts of climate change on heat and carbon sequestration, and nutrient redistribution, by SAMW. A subsurface salinity maximum develops every spring‐summer in the formation regions of Sub‐Antarctic Mode Waters (SAMW)The salinity maximum is formed by advection of warm, salty, nutrient‐poor subtropical waters via the northern Antarctic Circumpolar CurrentSubtropical waters contribute importantly to SAMW heat, salt and nutrient budgets and condition mixed‐layer seasonal dynamics A subsurface salinity maximum develops every spring‐summer in the formation regions of Sub‐Antarctic Mode Waters (SAMW) The salinity maximum is formed by advection of warm, salty, nutrient‐poor subtropical waters via the northern Antarctic Circumpolar Current Subtropical waters contribute importantly to SAMW heat, salt and nutrient budgets and condition mixed‐layer seasonal dynamics

A novel multi‐satellite product is used to shed light on the sea surface height seasonal cycle and associated geostrophic circulation in the subpolar Southern Ocean. We find three main modes of variability governing the Sea Level Anomaly seasonal cycle, all of them primarily governed by wind forcing. The main mode of seasonal variability is associated with the seasonality of the main subpolar gyres governed by large‐scale wind stress curl, qualitatively consistent with Sverdrup dynamics. The second seasonal mode is related to the Antarctic Slope Current (ASC), governed by the coastal easterlies, with a rapid circumpolar propagation of anomalous sea‐level along the continental slope that is dynamically consistent with the so‐called Southern Mode. The first two modes induce an acceleration of the gyres and the ASC in winter. The third seasonal mode appears to be driven by sea ice‐modulated surface stress and induces an offshore extension of the ASC from autumn to winter. The Southern Ocean circulation has a strong impact on the global climate. Yet, the variability of the subpolar circulation features is poorly known due to the large sea ice cover, especially in winter. Here, we benefit from new satellite measurements, enabling us to measure ocean circulation in both the ice‐covered and open regions of the Southern Ocean. These measurements are used to describe the seasonal cycle of the ocean circulation in the subpolar Southern Ocean, south of the Antarctic Circumpolar Current. Three processes are identified as being the main drivers of the seasonal variation of the circulation: the large scale wind variations over the whole subpolar Southern Ocean, the seasonal changes in the amplitude of the coastal winds around Antarctica, and the seasonal cycle of the sea ice, locally modifying the wind influence on the ocean surface. This study identifies the effect of these three drivers on the seasonal cycle of the subpolar Southern Ocean circulation, by highlighting their distinct seasonal variations and the regions where they are dominant. A novel satellite altimetry data set is used to describe the subpolar Southern Ocean circulation seasonal cycleLarge scale variations of surface winds are the main drivers of the Southern Ocean circulation seasonal variabilitySea ice modulates locally the surface stress and induces an unphased seasonal mode of variability A novel satellite altimetry data set is used to describe the subpolar Southern Ocean circulation seasonal cycle Large scale variations of surface winds are the main drivers of the Southern Ocean circulation seasonal variability Sea ice modulates locally the surface stress and induces an unphased seasonal mode of variability

The Antarctic shelf seas are a climatically and ecologically important region, and are at present receiving increasing amounts of freshwater from the melting of the Antarctic Ice Sheet and its fringing ice shelves, primarily around the Antarctic Peninsula and the Amudsen Sea. In response, the surface ocean salinity in this region has declined in past decades. Here, we assess the effects of the freshwater input on regional sea level using satellite measurements of sea surface height (for months with no sea-ice cover) and a global ocean circulation model. We find that from 1992 to 2011, sea-level rise along the Antarctic coast is at least 2  ±  0.8 mm yr-1greater than the regional mean for the Southern Ocean south of 50° S. On the basis of the model simulations, we conclude that this sea-level rise is almost entirely related to steric adjustment, rather than changes in local ocean mass, with a halosteric rise in the upper ocean and thermosteric contributions at depth. We estimate that an excess freshwater input of 430 ± 230 Gt yr-1is required to explain the observed sea-level rise. We conclude that accelerating discharge from the Antarctic Ice Sheet has had a pronounced and widespread impact on the adjacent subpolar seas over the past two decades.

The Southern Ocean plays a pivotal role in the global ocean circulation and climate. There, the deep water masses of the world ocean upwell to the surface and subsequently sink to intermediate and abyssal depths, forming two overturning cells that exchange substantial quantities of heat and carbon with the atmosphere. The sensitivity of the upper cell to climatic changes in forcing is relatively well established. However, little is known about how the lower cell responds, and in particular whether small-scale mixing in the abyssal Southern Ocean, an important controlling process of the lower cell, is influenced by atmospheric forcing. Here, we present observational evidence that relates changes in abyssal mixing to oceanic eddy variability on timescales of months to decades. Observational estimates of mixing rates, obtained along a repeat hydrographic transect across Drake Passage, are shown to be dependent on local oceanic eddy energy, derived from moored current meter and altimetric measurements. As the intensity of the regional eddy field is regulated by the Southern Hemisphere westerly winds, our findings suggest that Southern Ocean abyssal mixing and overturning are sensitive to climatic perturbations in wind forcing.

This article (1) reviews and clarifies the basic physics underpinning finescale parameterizations of turbulent dissipation due to internal wave breaking and (2) provides advice on the implementation of the parameterizations in a way that is most consistent with the underlying physics, with due consideration given to common instrumental issues. Potential biases in the parameterization results are discussed in light of both (1) and (2), and illustrated with examples in the literature. The value of finescale parameterizations for studies of the large‐scale ocean circulation in the presence of common biases is assessed. We conclude that the parameterizations can contribute significantly to the resolution of large‐scale circulation problems associated with plausible ranges in the rates of turbulent dissipation and diapycnal mixing spanning an order of magnitude or more. Basic physics of internal wave interactions clarifiedAdvice given on internally consistent estimatesExamples of biases discussed

Quantifying and understanding the processes driving turbulent mixing around Antarctica are key to closing the Southern Ocean's heat budget, an essential component of the global climate system. In 2016, a glider deployed in Ryder Bay, West Antarctic Peninsula, collected hydrographic and microstructure data, obtaining some of the first direct measurements of turbulent kinetic energy dissipation off West Antarctica. Elevated dissipation O(10−8) W kg−1is found above a topographic ridge separating the 520‐m‐deep bay, where values are O(10−10) W kg−1, from a deep fjord of the continental shelf, suggesting the ridge is important in driving upward mixing of warm Circumpolar Deep Water. The 12 glider transects reveal significant temporal variability in hydrographic and dissipation conditions. Mooring‐based current and nearby meteorological data are used to attribute thermocline shoaling (deepening) to Ekman upwelling (downwelling) at Ryder Bay's southern boundary, driven by ∼3‐day‐long south‐westward (north‐westward) wind events. Anticyclonic winds generated near‐inertial shear in the bay's upper layers, causing elevated bay‐wide shear and dissipation ∼1.7 days later. High dissipation over the ridge appears to be controlled hydraulically, being co‐located (and moving) with steeply sloping isopycnals. These are observed in ∼60% of the transects, with a corresponding mean upward heat flux of ∼2.4 W m−2. The ridge, therefore, provides sustained heat to the base of the thermocline, which can be released into overlying waters during the bay‐wide, thermocline‐focused dissipation events (mean heat flux of ∼1.3 W m−2). This highlights the role of ridges, which are widespread across the West Antarctic Peninsula, in the regional heat budget. Glacial ice flowing off the Antarctic continent either terminates directly into the ocean or forms floating tongues called ice shelves. Marine‐terminating glaciers and ice shelves in West Antarctica are melting rapidly, contributing significantly to global sea‐level rise. The melting has largely been attributed to the presence of warm deep waters, which travel at depth from the shelf break toward the coast, before mixing upwards and coming into contact with ice. However, we lack an understanding of the vertical mixing processes. In 2016, an autonomous underwater robot was used to measure the vertical mixing in Ryder Bay, off the West Antarctic Peninsula. Increased vertical mixing was consistently observed above a ridge on the seafloor at the bay's entrance, suggesting that the ridge is a hotspot for bringing sustained deep‐ocean heat to shallower depths. The heat rises until it reaches a strong temperature gradient in the ocean, with colder, less‐dense water above. This gradient typically prevents rapid mixing; however, strong winds are found to cause bay‐wide intense mixing events that release heat across the gradient into overlying waters. Similar ridges are widespread off the West Antarctic Peninsula, suggesting that they are important regionally for bringing heat toward glaciers and ice shelves. Underwater glider time series reveals spatiotemporally varying turbulent dissipation and heat flux in Ryder BayBasin‐wide thermocline dissipation is associated with near‐inertial shear that occurs ∼1.7 days after strong wind eventsElevated dissipation at depth is generally confined to a topographic ridge and is likely controlled hydraulically Underwater glider time series reveals spatiotemporally varying turbulent dissipation and heat flux in Ryder Bay Basin‐wide thermocline dissipation is associated with near‐inertial shear that occurs ∼1.7 days after strong wind events Elevated dissipation at depth is generally confined to a topographic ridge and is likely controlled hydraulically

Major gaps exist in our understanding of the pathways between internal wave generation and breaking in the Southern Ocean, with important implications for the distribution of internal wave‐driven mixing, the sensitivity of ocean mixing rates and patterns to changes in the ocean environment, and the necessary ingredients of mixing parameterizations. Here we assess the dominant processes in internal wave evolution by characterizing wave and mesoscale flow scales based on full‐depth in situ measurements in a Southern Ocean mixing hot spot and a ray tracing calculation. The exercise highlights the importance of Antarctic Circumpolar Current jets as a dominant influence on internal wave life cycles through advection, the modification of wave characteristics via wave‐mean flow interactions, and the set‐up of critical layers for both upward‐ and downward‐propagating waves. Our findings suggest that it is important to represent mesoscale flow impacts in parameterizations of internal wave‐driven mixing in the Southern Ocean. In situ observations show internal wave‐like coherent features in the Antarctic Circumpolar CurrentWave and background flow scales suggest that horizontal advection and wave‐mean flow interactions are leading‐order terms in wave evolutionFeatures are detected where the background flow shear is large and where ray tracing calculations suggest a critical layer scenario In situ observations show internal wave‐like coherent features in the Antarctic Circumpolar Current Wave and background flow scales suggest that horizontal advection and wave‐mean flow interactions are leading‐order terms in wave evolution Features are detected where the background flow shear is large and where ray tracing calculations suggest a critical layer scenario

Major deep‐convection activity in the northwestern Mediterranean during winter 2005 triggered the formation of a complex anomalous deep‐water structure that substantially modified the properties of the Western Mediterranean deep layers. Since then, evolution of this thermohaline structure, the so‐called Western Mediterranean Transition (WMT), has been traced through a regularly sampled hydrographic deep station located on the outer continental slope of Minorca Island. A rapid erosion of the WMT's near‐bottom thermohaline signal was observed during 2005–2007. The plausible interpretation of this as local bottom‐intensified mixing motivates this study. Here, the evolution of the WMT structure through 2005–2007 is reproduced by means of a one‐dimensional diffusion model including double‐diffusive mixing that allows vertical variation of the background mixing coefficient and includes a source term to represent the lateral advection of deep‐water injections from the convection area. Using an optimization algorithm, a best guess for the depth‐dependent background mixing coefficient is obtained for the study period. WMT evolution during its initial stages is satisfactorily reproduced using this simple conceptual model, indicating that strong depth‐intensified mixing (K∞(z) ≈ 22 × 10−4m2s−1; z⪆ 1,400 dbar) is a valid explanation for the observations. Extensive hydrographic and current observations gathered over the continental slope of Minorca during winter 2018, the first deep‐convective winter intensively sampled in the region, provide evidence of topographically localized enhanced mixing concurrent with newly formed dense waters flowing along‐slope toward the Algerian sub‐basin. This transport‐related boundary mixing mechanism is suggested to be a plausible source of the water‐mass transformations observed during the initial stages of the WMT off Minorca. In winter 2005, an exceptional production of deep waters with anomalous temperature and salinity induced a series of drastic changes in the deep waters of the Western Mediterranean. The evolution of these new deep waters was traced up to the present day through hydrographic measurements in the outer continental slope of Minorca. During 2005–2007, a rapid erosion of the bottom water characteristics was observed. This may be indicative of enhanced mixing near the seafloor. By means of an idealized numerical model, the evolution of the deep waters off Minorca during that period is satisfactorily reproduced, indicating that the deep waters experienced persistent, strongly depth‐intensified mixing. More than a decade later, extensive hydrographic and current observations gathered above the continental slope of Minorca evidence enhanced bottom mixing associated with the along‐slope flow of newly formed deep waters during winter of 2018, a much less relevant winter in terms of deep‐water formation albeit intensively sampled. This boundary mixing mechanism is suggested to be a plausible source of the strong mixing necessary to reproduce the observed evolution of the deep waters during the 2005–2007 period, as indicated by our modeling approach. A rapid erosion of bottom water thermohaline signatures was observed off Minorca Island during the Western Mediterranean Transition's (WMT) initial stagesLocal evolution of the WMT deep hydrographic structure during 2005–2007 is consistent with persistent intense mixingEvidence that intense mixing occurs at the continental slope and mixed waters are exported offshore is found A rapid erosion of bottom water thermohaline signatures was observed off Minorca Island during the Western Mediterranean Transition's (WMT) initial stages Local evolution of the WMT deep hydrographic structure during 2005–2007 is consistent with persistent intense mixing Evidence that intense mixing occurs at the continental slope and mixed waters are exported offshore is found

Mesoscale eddies are ubiquitous dynamical features that tend to propagate westward and disappear along ocean western boundaries. Using a multiscale observational study, we assess the extent to which eddies dissipate via a direct cascade of energy at a western boundary. We analyze data from a ship‐based microstructure and velocity survey, and an 18‐month mooring deployment, to document the dissipation of energy in anticyclonic and cyclonic eddies impinging on the topographic slope east of the Bahamas, in the North Atlantic Ocean. These observations reveal high levels of turbulence where the steep and rough topographic slope modified the intensified northward flow associated with, in particular, anticyclonic eddies. Elevated dissipation was observed both near‐bottom and at mid depths (200–800 m). Near‐bottom turbulence occurred in the lee of a protruding escarpment, where elevated Froude numbers suggest hydraulic control. Energy was also radiated in the form of upward‐propagating internal waves. Elevated dissipation at mid depths occurred in regions of strong vertical shear, where the topographic slope modified the vertical structure of the northward eddy flow. Here, low Richardson numbers and a local change in the isopycnal gradient of potential vorticity (PV) suggest that the elevated dissipation was associated with horizontal shear instability. Elevated mid‐depth dissipation was also induced by topographic steering of the flow. This led to large anticyclonic vorticity and negative PV adjacent to the topographic slope, suggesting that centrifugal instability underpinned the local enhancement in dissipation. Our results provide a mechanistic benchmark for the realistic representation of eddy dissipation in ocean models. Turbulent dissipation at mid depth linked to sheared flow generated by eddy‐topography interactionElevated dissipation associated with centrifugal instability where eddy flow was blocked by topographyDissipation in the lee of an escarpment with internal wave generation and possible hydraulic jump Turbulent dissipation at mid depth linked to sheared flow generated by eddy‐topography interaction Elevated dissipation associated with centrifugal instability where eddy flow was blocked by topography Dissipation in the lee of an escarpment with internal wave generation and possible hydraulic jump

The Amundsen Sea has the highest thinning rates of ice shelves in Antarctica. This imbalance is caused by changes in ocean melting induced by warm Circumpolar Deep Water (CDW) intrusions. The resulting changing freshwater balance could affect the on‐shelf currents and mixing. However, a clear understanding of the sources and sinks of freshwater in the region is lacking. Here we use a model of the Amundsen Sea, with passive freshwater tracers, to investigate the relative magnitudes and spatial distributions of the different freshwater components. In the surface layer and as a depth average, all freshwater tracer concentrations are of comparable magnitude, though on a depth average, sea ice and ice shelf are largest. The total freshwater tracer distribution is similar to that of the ice‐shelf tracer field. This implies a potential for ice‐shelf meltwater feedbacks, whereby abundant ice‐shelf meltwater alters the ocean circulation and stratification, affecting melting. Ice‐shelf and sea‐ice freshwater fluxes have the largest interannual variability. The effect of including grounded icebergs and iceberg freshwater flux are studied in detail. The presence of icebergs increases CDW intrusions that reach the base of ice shelves. This suggests another possible feedback mechanism, whereby more icebergs induce greater ice‐shelf melting and hence more icebergs. However, the strength of this potential feedback is dependent on poorly constrained sea‐ice model parameters. These results imply that poorly constrained parameters relating to the ocean freshwater balance, such as those relating to icebergs and sea ice, impact predictions for melting of the West Antarctic Ice Sheet. The Amundsen Sea in Antarctica is an important region for understanding future sea‐level rise from the West Antarctic Ice Sheet, as the region's ice shelves (the floating extension of ocean terminating glaciers) are thinning due to changes in ocean melting. This could affect the amount of freshwater in the Amundsen Sea, possibly affecting ocean currents. In order to understand the distribution of freshwater from different sources (snowfall, ice‐shelf melting, iceberg melting, and sea‐ice melting) we use a specialized ocean model. This model shows that freshwater from all different sources have comparable magnitudes, though ice‐shelf and sea‐ice sources are largest. The fact that ice‐shelf meltwater is an important component implies a potential feedback in the system, whereby more ice‐shelf melting could alter ocean currents, and thereby cause more melting. We also investigated the effect of icebergs that ground on a seabed ridge. Icebergs increase the amount of warm water that reaches ice shelves. This provides a potential for another feedback, where more icebergs increase ice‐shelf melting, thereby producing more icebergs. These results imply that poorly known aspects of freshwater sources can impact predictions for future sea‐level rise contributions from the West Antarctic. Sea ice and ice shelves provide the strongest freshwater fluxes in the Amundsen Sea, with the largest interannual variabilityParameterized icebergs affect oceanic heat content at ice shelves, with a strong potential for ice‐shelf meltwater and iceberg feedbacksPoorly constrained parameters relating to the freshwater balance may impact predictions for melting of the West Antarctic Ice Sheet

The changing supply of warm Circumpolar Deep Water (CDW) to the West Antarctic continental shelf is responsible for the basal melting and thinning of the West Antarctic ice shelves that has occurred in recent decades. Here we assess the variability in CDW supply, and its drivers, from a multiyear mooring deployed in, and a regional ocean model spanning, the Getz‐Dotson Trough, Amundsen Sea. Between 2010 to 2015, the CDW within the trough underwent a pronounced cooling and freshening, associated with changes in thermohaline properties on isopycnals. Variability in the rate of CDW inflow is controlled by local wind forcing of a shelf break undercurrent, which determines the hydrographic properties of inflowing CDW via tilting of density surfaces above the continental slope. Local wind is coupled to the Amundsen Sea Low (ASL) low‐pressure system, which is modulated by large‐scale climatic modes via atmospheric teleconnections. For the period analyzed, a deeper ASL was associated with westward wind anomaly at the shelf break. Changes in the sea surface slope decelerated the shelf break undercurrent, resulting in less heat accessing the continental shelf and, consequently, a cooling of the Getz‐Dotson Trough. Therefore, the present work suggests that the fate of the West Antarctic ice shelves is closely tied to the future evolution of the ASL. The heat available for the melting of the ice shelves of the Amundsen Sea is supplied by the warm waters flooding the continental shelf. Here we show that the waters flowing into the Getz‐Dotson Trough underwent a pronounced cooling between 2010 and 2015, associated with a weakening of the bottom‐intensified currents within the trough. This process was controlled by changes of shelf break winds. A weakening of the eastward wind impacts the middepth undercurrent system above the continental slope, which controls the inflow of warm water via tilting of density surfaces above the slope. The local winds are modulated by the low‐pressure system located offshore of the Amundsen Sea, known as the Amundsen Sea Low. The Amundsen Sea circulation is controlled by this low‐pressure system, and changes in its configuration can impact the warmer waters flooding the West Antarctic continental shelf, with consequences for the melting of the region's ice shelves. Circumpolar Deep Water inflow to the Getz‐Dotson Trough, Amundsen Sea, is investigated with a multiyear mooringOn‐shelf heat content decreased between 2010 and 2015 in response to weaker Circumpolar Deep Water inflowGetz‐Dotson Trough cooling is driven by a westward wind anomaly at the shelf break, associated with a deeper Amundsen Sea Low

The upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time and zonal mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here an observationally constrained ocean model and an observation‐based seasonal climatology are analyzed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two‐stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near‐surface winter water via wintertime mixing. Second, winter water warms to form intermediate water through summertime surface heat fluxes. The mixing‐driven pathway from deep water to winter water follows mixing lines in thermohaline coordinates indicative of nonlinear processes. Deep, cold and salty water is drawn to the surface ocean around Antarctica. This is controlled by the combined action of winds blowing over the sea surface and the stirring by large 100‐km eddies. Studying the ocean around Antarctica is notoriously challenging. We therefore have sparse ocean measurements in this region. This limits our understanding of some of the processes involved in the circulation of deep water, notably, a stage involving the warming and freshening of deep water to form intermediate water. Intermediate water plays a key role in drawing excess anthropogenic heat and carbon dioxide from the sea surface into the interior ocean. Our results show that the conversion of deep to intermediate water involves a two‐stage “cold transit” that takes place over the course of the austral season. First, during the winter, deep water sits below the very cold and fresh surface water. Ice formation around Antarctica makes this surface water slightly saltier, decreasing the density difference between the deep and surface water. This drives a mixing which cools and freshens the deep water while drawing it to the surface. Second, during summer surface warming and melting sea ice warm and freshen this deep/surface water mixture forming intermediate water. Volumetric distribution reveals new thermohaline pathway for transformation of Circumpolar Deep Water to intermediate waterTransformation pathway relies on seasonal cycle of air‐ice‐sea buoyancy fluxesUpwelling of deep water is sensitive to nonlinearities in the equation of state for sea water

The rate of dissipation of turbulent kinetic energy is estimated using Seaglider observations of vertical water velocity in the midlatitude North Atlantic. This estimate is based on the large‐eddy method, allowing the use of measurements of turbulent energy at large scales O(1–10 m) to diagnose the rate of energy dissipated through viscous processes at scales O(1 mm). The Seaglider data considered here were obtained in a region of high stratification (1 × 10−4

The first quasi‐synoptic estimates of Arctic Ocean and sea ice net fluxes of volume, heat and freshwater are calculated by application of an inverse model to data around the ocean boundary. Hydrographic measurements from four gateways to the Arctic (Bering, Davis, and Fram Straits and the Barents Sea Opening) completely enclose the ocean, and were made within the same 32‐day period in summer 2005. The inverse model is formulated as a set of full‐depth and density‐layer‐specific volume and salinity transport conservation equations, with conservation constraints also applied to temperature, but only in non‐outcropping layers. The model includes representations of Fram Strait sea ice export and of interior Arctic Ocean diapycnal fluxes. The results show that in summer 2005 the transport‐weighted mean properties are, for water entering the Arctic: potential temperature 4.49°C, salinity 34.50 and potential density (σ0) 27.34 kg m−3; and for water leaving the Arctic, including sea ice: 0.25°C, 33.81, and 27.13 kg m−3, respectively. The net effect of the Arctic in summer is to freshen and cool the inflows by 0.69 in salinity and 4.23°C, respectively, and to decrease density by 0.21 kg m−3. The volume transport into the Arctic of waters above ∼1000 m depth is 9.2 Sv (1 Sv = 106m3s−1), and the export (similarly) is 9.3 Sv. The net oceanic and sea ice freshwater flux is 187 ± 48 mSv. The net heat flux (including sea ice) is 189 ± 37 TW, representing loss from the ocean to the atmosphere. A synoptic summer pan‐Arctic heat and freshwater boundary fluxes