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The Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub‐surface mixing. Using microstructure measurements and a small‐scale variance conservation framework, we quantify the diapycnal and isopycnal contributions –by microscale turbulence and mesoscale eddies, respectively– to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400 m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for O $\mathcal{O}$(5–10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water‐mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning. Plain Language Summary: The North Atlantic hosts an ocean circulation system called the Atlantic Meridional Overturning Circulation (AMOC). It is often likened to a giant conveyor belt in the ocean, moving warm, salty waters from south to north and transforming them into cold, fresh waters that flow back southward at greater depth. The AMOC is a crucial element of the Earth's climate, and if it were to slow down, it could lead to major climatic changes. For a long time, scientists thought that the AMOC was mainly driven by cooling in the North Atlantic. But recently, we have discovered that the mixing of different water masses is also important. In our study, we used small‐scale measurements of ocean properties to examine the processes behind this mixing. Our findings show that large swirling flows known as mesoscale eddies, which are tens to hundreds of kilometers wide and hundreds of meters deep, play a dominant role in mixing heat and salt in the North Atlantic. This discovery helps us to better understand the AMOC and its future behavior. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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(103–104 m2 s−1), and generally weak diapycnal mixing at rates of O(10−6–10−5 m2 s−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 m2 s−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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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 largescale 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. [ABSTRACT FROM AUTHOR]

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−1 is 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Abstract: 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 − 6 W kg− 1) with respect to the floods ( ∼ 10 − 8 W 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

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 [ABSTRACT FROM AUTHOR]