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13 pages, 5 figures, supporting information https://doi.org/10.1029/2022JC018730.-- Data Availability Statement: Microstructure data are available at https://microstructure.ucsd.edu/#/cruise/740H20111224. World Ocean Atlas 2018 climatological data are available from https://www.ncei.noaa.gov/archive/accession/NCEI-WOA18, 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, We acknowledge support from NSF Grant #OCE-1208454 for collection and analysis of the data and the Spanish Government through grants CTM2014-56987-P (VA-DE-RETRO Project) and RTI2018-100844-B-C33 (SAGA project) and through the funding of PhD scholarship for D.O.E. (FPU2013-02884). We also recognize the institutional support of the Spanish Government through the Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S)

Sustaining biological export over the open ocean requires a compensating physical supply of nutrients to the euphotic zone. The relative importance of diapycnal mixing and isopycnal stirring by mesoscale eddies in providing this nutrient supply is explored using a field campaign in some of the most oligotrophic waters in the subtropical North Atlantic, using transects over and off the mid-Atlantic ridge. Eddy stirring rates are estimated, firstly, from the excess temperature variance dissipation relative to the turbulent kinetic energy dissipation and, secondly, using eddy statistics from satellite observations combined with 9-month-long mooring data. The vertical nutrient fluxes by diapycnal mixing and isopycnal mesoscale eddy stirring are assessed using nitrate measurements from observations or a climatology. The diapycnal transfer provides a nutrient supply within the euphotic zone, but induces a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline. The eddy supply of nitrate is relatively small in the centre of the subtropical gyre, reaching up to 0.06 mol N m−2yr−1, but is likely to be enhanced on the flanks of the gyre due to larger isopycnal slopes and lateral nitrate gradients. The nutrient supply to the euphotic zone is achieved via a multi-stage mechanism: a diapycnal transfer of nutrients by small-scale turbulence to the euphotic zone and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline. Plain Language Summary Phytoplankton growth requires a supply of nutrients to the base of the euphotic zone, which is usually provided by a combination of vertical mixing or vertical upwelling of nutrients. However, in the oligotrophic waters of the central North Atlantic, it is unclear how the vertical supply of nutrients is sustained. Here we use field data to explore the roles of diapycnal mixing across density surfaces and mesoscale eddy stirring along density surfaces in supplying nutrients to some of the most nutrient-depleted surface waters in the central North Atlantic. Diapycnal mixing is found to be important in supplying nutrients to the euphotic zone during summer, but at the expense of eroding the nutrients in the upper thermocline. In contrast, mesoscale eddy stirring augments the diapycnal supply of nutrients to the euphotic zone, and replenishes nutrients in the upper thermocline.

Diapycnal mixing of nutrients from the thermocline to the surface sunlit ocean is thought to be relatively weak in the world's subtropical gyres as energy inputs from winds are generally low. The interaction of internal tides with rough topography enhances diapycnal mixing, yet the role of tidally induced diapycnal mixing in sustaining nutrient supply to the surface subtropical ocean remains relatively unexplored. During a field campaign in the North Atlantic subtropical gyre, we tested whether tidal interactions with topography enhance diapycnal nitrate fluxes in the upper ocean. We measured an order of magnitude increase in diapycnal nitrate fluxes to the deep chlorophyll maximum (DCM) over the Mid-Atlantic Ridge compared to the adjacent deep ocean. Internal tides drive this enhancement, with diapycnal nitrate supply to the DCM increasing by a factor of 8 between neap and spring tides. Using a global tidal dissipation database, we find that this spring-neap enhancement in diapycnal nitrate fluxes is widespread over ridges and seamounts. Mid-ocean ridges therefore play an important role in sustaining the nutrient supply to the DCM, and these findings may have important implications in a warming global ocean. Plain Language Summary The subtropical gyres cover an extensive area of the global ocean and account for similar to 30% of carbon export to the deep ocean. The pattern of the winds induces downwelling in these gyres and leads to surface waters being relatively nutrient impoverished. Biological production in the subtropical gyres is primarily limited by the availability of nitrate, which can be increased through mixing in the underlying thermocline. Internal tides can enhance mixing in the ocean interior close to steep sloping topography; deep in the ocean interior, this mixing is a key component of ocean physics. In our field study, we reveal the mixing extending up toward the surface and measured a tenfold increase in nitrate fluxes to phytoplankton in the surface ocean over the Mid-Atlantic Ridge compared to in the surface waters in the adjacent deeper ocean. Importantly, nitrate fluxes over the ridge varied fortnightly with an eightfold increase from neap to spring tides. These inferences of enhanced mixing and nutrient supply along ridges and seamounts are relevant for the rest of the global ocean given the ubiquitous nature of the tides.

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 multi‐year 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 analysed, 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.

In this work, we use realistic isopycnal velocities with a 3D 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 optimise 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.

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, re‐stratification 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 re‐stratification and dissipative effects in climate‐scale ocean models. Plain Language Summary 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, re‐stratification 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.

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.

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 idealised 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 distribution of noble gases and helium isotopes in the dense shelf waters of Antarctica reflect the boundary conditions near the ocean surface: air-sea exchange, sea ice formation and subsurface ice melt. We use a non-linear 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 the contribution from the north Atlantic. The contrast between northern and southern hemisphere recharge properties highlight 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 non-negligible impact on the atmospheric gas content of Antarctic Bottom Water.

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.

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 waters of the Weddell-Scotia Confluence (WSC) lie above the rugged topography of the South Scotia Ridge in the Southern Ocean. Meridional exchanges across the WSC transfer water and tracers between the Antarctic Circumpolar Current (ACC) to the north and the subpolar Weddell Gyre to the south. Here, we examine the role of topographic interactions in mediating these exchanges, and in modifying the waters transferred. A case study is presented using data from a free-drifting, intermediate-depth float, which circulated anticyclonically over Discovery Bank on the South Scotia Ridge for close to 4 years. Dimensional analysis indicates that the local conditions are conducive to the formation of Taylor columns. Contemporaneous ship-derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column, with values of O(1000 m2/s) obtained. Although necessarily coarse, this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the ACC. A picture emerges of the Taylor column acting as a slow, steady blender, retaining the waters in the vicinity of the WSC for lengthy periods during which they can be subject to significant modification. A full regional float data set, bathymetric data, and a Southern Ocean state estimate are used to identify other potential sites for Taylor column formation. We find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the ACC in this sector of the Southern Ocean.

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.

The fate of a deep boundary current that originates in the Southeast Pacific and flows southward along the continental slope of South America is elucidated. The current transports poorly ventilated water of low salinity (a type of Pacific Deep Water, PDW), into Drake Passage. East of Drake Passage, the boundary current breaks into fresh anticyclonic eddies, nine examples of which were observed in mooring data from December 2009 to March 2012. The observed eddies appear to originate mainly from a topographic separation point close to 60°W, have typical diameters of 20–60 km and accompanying Rossby numbers of 0.1–0.3. These features are likely to be responsible for transporting PDW meridionally across the Antarctic Circumpolar Current, explaining the near homogenization of Circumpolar Deep Water properties downstream of Drake Passage. This mechanism of boundary current breakdown may constitute an important process in the Southern Ocean overturning circulation.

[1] This study provides the first physically based mass-balanced transport estimates of dissolved inorganic nutrients (nitrate, phosphate, and silicate) for the Arctic Ocean. Using an inverse model-generated velocity field in combination with a quasi-synoptic assemblage of hydrographic and hydrochemical data, we quantify nutrient transports across the main Arctic Ocean gateways: Davis Strait, Fram Strait, the Barents Sea Opening (BSO), and Bering Strait. We found that the major exports of all three nutrients occur via Davis Strait. Transports associated with the East Greenland Current are almost balanced by transports associated with the West Spitsbergen Current. The most important imports of nitrate and phosphate to the Arctic occur via the BSO, and the most important import of silicate occurs via Bering Strait. Oceanic budgets show that statistically robust net silicate and phosphate exports exist, while the net nitrate flux is zero, within the uncertainty limits. The Arctic Ocean is a net exporter of silicate (−15.7 ± 3.2 kmol s−1) and phosphate (−1.0 ± 0.3 kmol s−1; net ± 1 standard error) to the North Atlantic. The export of excess phosphate (relative to nitrate) from the Arctic, calculated at −1.1 ± 0.3 kmol s−1, is almost twice as large as previously estimated. Net transports of silicate and phosphate from the Arctic Ocean provide 12% and 90%, respectively, of the net southward fluxes estimated at 47°N in the North Atlantic. Additional sources of nutrients that may offset nutrient imbalances are explored, and the relevance and the pathway of nutrient transports to the North Atlantic are discussed.

The time series of high-quality hydrographic measurements of the ACC in Drake Passage is revisited to investigate the extent of baroclinic adjustment in the region on time scales of up to two decades. We find that substantial adjustment of the upper kilometer of northern Drake Passage has occurred on interannual to decadal time scales, driven primarily by tropical Pacific forcing via the poleward propagation of boundary waves. The decadal-scale signal consists of a marked deepening of isopycnals (by ∼200m between the early 1990s and the late 2000s), and reflects ENSO-forced wind-driven baroclinic changes over an extensive region of the eastern South Pacific. © 2012. American Geophysical Union. All Rights Reserved.

The Indian Ocean hosts a vigorous basin-scale overturning that constitutes one of the major deep upwelling branches of the global meridional overturning circulation (MOC). The extent to which the deep Indian Ocean MOC is sustained by breaking internal waves is assessed by quantifying and comparing the energetics of the overturning and those of the regional internal wave field. A range of published inverse estimates of the circulation across 32°S is used to assess the basin average buoyancy fluxes. The turbulent dissipation needed to sustain the MOC ranges between 0.17 ± 0.04 and 1.19 ± 0.17 TW, which is consistent with the estimated 0.35?0.26+1.04 TW dissipated by breaking internal waves, as inferred from observed fine structure. Both estimates of turbulent dissipation are consistent with the total energy input into the regional internal wave field (0.21?0.05+0.08 TW) based on published estimates of energy conversion from winds, tides and geostrophic bottom flows. However, a discrepancy arises when comparing the energetics at different density levels. At mid-ocean density levels (?1000–3000 m) the dissipation of internal wave energy is found to be significantly smaller (factor 5–10) than the dissipation needed to sustain inverse estimates of the MOC. The uncertainty related to undersampling of internal wave breaking hot spots was analyzed and found to be small, which suggests that mixing processes other than wave breaking due to weak wave-wave interactions, may be significant in the deep Indian Ocean.

[1] Antarctic Bottom Water (AABW), the densest water in the global overturning circulation, has warmed in recent decades, most notably in the Atlantic. Time series recorded within the boundary currents immediately upstream and downstream of the most significant outflow of AABW from the Weddell Sea indicate that raised outflow temperatures are synchronous with stronger boundary current flows. These changes occur rapidly in response to changes in wind forcing, suggesting that barotropic dynamics and the response of the bottom Ekman layer are significant. The observed synchronicity indicates that the previously‐detected weakening of the export of the colder forms of AABW from the Weddell Sea need not be associated with a reduction in the total flux of AABW exported via this route. These points need careful consideration when attributing the observed AABW warming in the Atlantic, and when determining its contribution to global heat budgets and sea level rise. Citation: Meredith, M. P., A. L. Gordon, A. C. Naveira Garabato, E. P. Abrahamsen, B. A. Huber, L. Jullion, and H. J. Venables (2011), Synchronous intensification and warming of Antarctic Bottom Water outflow from the Weddell Gyre, Geophys. Res. Lett., 38, L03603, doi:10.1029/2010GL046265.

� 10 � 4 m 2 s � 1 commonly occur in the DWBC. These diffusivities, estimated with an internal wave strain-based parameterisation, are independently confirmed by budgets of density and dissolved O2. ‘‘Moderate’’ mixing rates (� 10 � 4 m 2 s � 1 ) over the DWBC path between Denmark Strait and Cape Farewell (� 1000 km) drive a similar net increase in volume transport by diapycnal entrainment to the more intense (� 10 � 3 m 2 s � 1 ), localised mixing rates found near the Denmark Strait overflow sill. Therefore turbulent mixing associated with the DWBC south of Denmark Strait is instrumental in establishing the magnitude of the North Atlantic Meridional Overturning Circulation. Elevated mixing rates are also seen in the East and West Greenland Currents; this may be significant in preconditioning seasonal restratification of the intermediate layers of the Irminger and Labrador basins. Citation: Lauderdale, J. M., S. Bacon, A. C. Naveira Garabato, and N. P. Holliday (2008), Intensified turbulent mixing in the boundary current system of southern Greenland, Geophys. Res. Lett., 35, L04611

The distribution of turbulent diapycnal mixing in the Nordic seas is mapped from observations of internal wave density and velocity fine structure. The uppermost 500–1500 m host two distinct mixing regimes. In the eastern basins, the diapycnal diffusivity (K) straddles 10^-5 m^2 s^-1, whereas in the weakly stratified Greenland and Boreas basins it is raised by an order of magnitude. Below 2000 m, low stratification is associated with intense turbulent mixing across the Nordic seas, with diffusivities in the range 3 x 10^-4 – 10^-2 m^2 s^-1. These mixing rates agree within uncertainties with three tracer-based diffusivity estimates in the region and are associated with turbulent dissipation rates (E) that are at most moderately enhanced above typical open ocean values. A minimum in both E and K is commonly found at ~1500 m, a depth level that is most efficiently sheltered from shallow and bottom energy sources for the mixing. Available evidence points to wind work on upper ocean inertial motions as a shallow source, with semidiurnal internal tides generated at different levels of the topography contributing to both shallow and deep turbulence. While the closure of the North Atlantic meridional overturning circulation in the Nordic seas appears to be primarily driven by air-sea interaction, turbulent mixing has the potential to play a critical role in shaping the stratification and ventilation of the region via a range of complex interactions with convection.