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The 5-year Ocean Regulation of Climate by Heat and Carbon Sequestration and Transports (ORCHESTRA) programme and its 1-year extension ENCORE (ENCORE is the National Capability ORCHESTRA Extension) was an approximately 11-million-pound programme involving seven UK research centres that finished in March 2022. The project sought to radically improve our ability to measure, understand and predict the exchange, storage and export of heat and carbon by the Southern Ocean. It achieved this through a series of milestone observational campaigns in combination with model development and analysis. Twelve cruises in the Weddell Sea and South Atlantic were undertaken, along with mooring, glider and profiler deployments and aircraft missions, all contributing to measurements of internal ocean and air–sea heat and carbon fluxes. Numerous forward and adjoint numerical experiments were developed and supported by the analysis of coupled climate models. The programme has resulted in over 100 peer-reviewed publications to date as well as significant impacts on climate assessments and policy and science coordination groups. Here, we summarize the research highlights of the programme and assess the progress achieved by ORCHESTRA/ENCORE and the questions it raises for the future. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

The Southern Ocean upper-layer freshwater balance exerts a global climatic influence by modulating density stratification and biological productivity, and hence the exchange of heat and carbon between the atmosphere and the ocean interior. It is thus important to understand and quantify the time-varying freshwater inputs, which is challenging from measurements of salinity alone. Here we use seawater oxygen isotopes from samples collected between 2016 and 2021 along a transect spanning the Scotia and northern Weddell Seas to separate the freshwater contributions from sea ice and meteoric sources. The unprecedented retreat of sea ice in 2016 is evidenced as a strong increase in sea ice melt across the northern Weddell Sea, with surface values increasing approximately two percentage points between 2016 and 2018 and column inventories increasing approximately 1 to 2 m. Surface meteoric water concentrations exceeded 4% in early 2021 close to South Georgia due to meltwater from the A68 megaberg; smaller icebergs may influence meteoric water at other times also. Both these inputs highlight the importance of a changing cryosphere for upper-ocean freshening; potential future sea ice retreats and increases in iceberg calving would enhance the impacts of these freshwater sources on the ocean and climate. This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Knowledge of the ocean surface layer beneath Antarctic landfast ice is sparse. In this article surface layer turbulent and fine structure are quantified with and without landfast ice in the same West Antarctic Peninsula location. Landfast ice reduced turbulence levels locally to an order of magnitude less than ice-free values, and near-inertial energy and sub-inertial tidal energy levels to less than half their ice-free values. Vertical turbulent heat and nutrient fluxes were, respectively, 6 and 10 times greater than previously estimated. Under-ice tidal energy dissipation over the entire Antarctic continental shelf due to seasonal landfast ice cover is estimated at 788 MW. The total rate of wind-generated turbulence in the surface ocean is greatly reduced by the presence of seasonal landfast ice to an average of 14% of the ice-free value, but with large sectoral variations. Counter-intuitively, however, tides and wind contribute approximately equally to the turbulent kinetic energy budget of the upper ocean between the Antarctic coastline and the maximal landfast ice extent, with large sectoral variations, attributed to geographic variations in the strength of the barotropic tide.

The extent to which marine species avoid survey vessels is the subject of considerable debate, with stimuli such as noise and pressure waves receiving particular attention. By some estimates, avoidance behaviour is a major bias in the assessment of pelagic species distribution and density. Surveyors of the marine environment are increasingly using autonomous platforms; thus the question of avoidance of these novel platforms must be considered. A long-duration, underwater glider with an integrated echo-sounder was deployed to measure the density and distribution of Antarctic krill. We show that krill reacted to the glider, despite the low-noise profile of the platform. The weighted-mean depth of krill swarms was observed to deepen when a glider came within 30 m. In addition, we observed a near-range decrease in depth-integrated acoustic backscatter and we demonstrate that this effect is not explained by the instrument's acoustic footprint. The potential triggers for these behaviours are discussed. The list of sensors available for use on a glider is increasing, with some producing sound, light or other potential stimuli for behavioural response. Our paper demonstrates that caution should be used when designing mission payloads and in the interpretation of acoustic data for animal density from gliders. A key consideration for surveyors may be the factoring in of an exclusion zone from their analyses.

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Opher, J., Brearley, J., Dye, S., Pickart, R., Renfrew, I., Harden, B., & Meredith, M. The annual salinity cycle of the Denmark Strait Overflow. Journal of Geophysical Research: Oceans, 127(4), (2022): e2021JC018139, https://doi.org/10.1029/2021jc018139., The Denmark Strait Overflow (DSO) is an important source of dense water input to the deep limb of the Atlantic Meridional Overturning Circulation (AMOC). It is fed by separate currents from the north that advect dense water masses formed in the Nordic Seas and Arctic Ocean which then converge at Denmark Strait. Here we identify an annual salinity cycle of the DSO, characterized by freshening in winter and spring. The freshening is linked to freshening of the Shelfbreak East Greenland Current in the Blosseville Basin north of the Denmark Strait. We demonstrate that the East Greenland Current advects fresh pycnocline water above the recirculating Atlantic Water, which forms a low salinity lid for the overflow in Denmark Strait and in the Irminger Basin. This concept is supported by intensified freshening of the DSO in lighter density classes on the Greenland side of the overflow. The salinity of the DSO in the Irminger Basin is significantly correlated with northerly/northeasterly winds in the Blosseville Basin at a lag of 3–4 months, consistent with estimated transit times. This suggests that wind driven variability of DSO source water exerts an important influence on the salinity variability of the downstream DSO, and hence the composition of the deep limb of the AMOC., This research was funded by: NERC EnvEast DTP studentship NE/L002582 (JO) and Cefas Seedcorn DP371 (JO, SRD); as well as by NERC, by AFIS (NE/N009754/1) (IR), JAB is funded by NE/L011166/1, ORCHESTRA (NE/N018095/1) and ENCORE (NE/V013254/1) and RP is funded by the US National Science Foundation grants OCE-1756361 and OCE-1558742. Cefas work on the Angmagssallik array was supported by multiple international partners including NSF, NOAA-CORC-ARCHES, WHOI-OCCI, European Community's fifth & seventh framework programme under grants ASOF-W (contract EVK2-CT-2002-00,149) & No. GA212643 (THOR: “Thermohaline Overturning—at Risk”, 2008–2012) and from UK Department for Environment, Food and Rural Affairs (DEFRA) including A1222, SD0440 & ME5102.

The west Antarctic Peninsula (WAP) is a region of marked climatic variability, exhibiting strong changes in sea ice extent, retreat of most of its glaciers, and shifts in the amount and form of precipitation. These changes can have significant impacts on the oceanic freshwater budget and marine biogeochemical processes; it is thus important to ascertain the relative balance of the drivers, and the spatial scales over which they operate. We present a novel 7‐year summer‐season (October to March; 2011 to 2018) series of oxygen isotopes in seawater (δ18O), augmented with some winter sampling, collected adjacent to Anvers Island at the WAP. These data are used to attribute oceanic freshwater changes to sea ice and meteoric sources, and to deduce information on the spatial scales over which the changes are driven. Sea ice melt shows significant seasonality (∼9% range) and marked interannual changes, with pronounced maxima in seasons 2013/14 and 2016/17. Both of these extrema are driven by anomalous winds, but reflect strongly contrasting dynamic and thermodynamic sea ice responses. Meteoric water also shows seasonality (∼7% range), with interannual variability reflecting changes in the input of accumulated precipitation and glacial melt to the ocean. Unlike sea ice melt, meteoric water extremes are especially pronounced in thin (<10 m) surface layers close to the proximate glacier, associated with enhanced ocean stratification. Isotopic tracers help to deconvolve the complex spatio‐temporal scales inherent in the coastal freshwater budget, and hence improve knowledge of the separate and cumulative physical and ecological impacts.

This chapter gives an overview of recent advances in in situ observations of ocean mixing. It starts by providing a brief history of measuring ocean mixing. It then describes adaptations of traditional measurements and methods to autonomous platforms, including profiling floats and autonomous underwater vehicles. For each approach, the method is described as well as its limitations. Evidence for successful application of each method is provided.

The Greenland Ice Sheet (GrIS) is experiencing significant mass loss and freshwater discharge at glacier fronts. The freshwater input from Greenland will impact the physical properties of adjacent coastal seas, including important regions of deep water formation and contribute to global sea level rise. However, the biogeochemical impact of increasing freshwater discharge from the GrIS is less well constrained. Here, we demonstrate the use of bio-optical sensors on ocean gliders to track biogeochemical properties of meltwaters off southwest Greenland. Our results reveal that fresh, coastal waters, with an oxygen isotopic composition characteristic of glacial meltwater, are distinguished by a high optical backscatter and high levels of fluorescing dissolved organic matter (FDOM), representative of the overall coloured dissolved organic matter pool. Reconstructions of geostrophic velocities are used to show that these particle and FDOM-enriched coastal waters cross the strong boundary currents into the Labrador Sea. Meltwater input into the Labrador Sea is likely driven by mesoscale processes, such as eddy formation and local bathymetric steering, in addition to wind-driven Ekman transport. Ocean gliders housing bio-optical sensors can provide the high-resolution observations of both dissolved and particulate glacially-derived material that are needed to understand meltwater dispersal mechanisms and their sensitivity to future climatic change.

Quantifying and understanding the processes driving turbulent mixing around Antarctica is 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. Twelve 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.

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Cusack, J. M., Brearley, J. A., Garabato, A. C. N., Smeed, D. A., Polzin, K. L., Velzeboer, N., & Shakespeare, C. J. Observed eddy-internal wave interactions in the Southern Ocean. Journal of Physical Oceanography, 50(10), (2020): 3042-3062, doi:10.1175/JPO-D-20-0001.1., The physical mechanisms that remove energy from the Southern Ocean’s vigorous mesoscale eddy field are not well understood. One proposed mechanism is direct energy transfer to the internal wave field in the ocean interior, via eddy-induced straining and shearing of preexisting internal waves. The magnitude, vertical structure, and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14-month deployment of a mooring cluster in the Scotia Sea. Velocity and buoyancy observations are decomposed into wave and eddy components, and the energy transfer is estimated using the Reynolds-averaged energy equation. We find that eddies gain energy from the internal wave field at a rate of −2.2 ± 0.6 mW m−2, integrated from the bottom to 566 m below the surface. This result can be decomposed into a positive (eddy to wave) component, equal to 0.2 ± 0.1 mW m−2, driven by horizontal straining of internal waves, and a negative (wave to eddy) component, equal to −2.5 ± 0.6 mW m−2, driven by vertical shearing of the wave spectrum. Temporal variability of the transfer rate is much greater than the mean value. Close to topography, large energy transfers are associated with low-frequency buoyancy fluxes, the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research. Our work suggests that eddy–internal wave interactions may play a significant role in the energy balance of the Southern Ocean mesoscale eddy and internal wave fields., Funding for DIMES was provided by U.K. Natural Environment Research Council (NERC) Grants NE/E007058/1 and NE/E005667/1. JMC acknowledges the support of a NERC PhD studentship, and ACNG that of the Royal Society and the Wolfson Foundation. NV acknowledges support from the ARC Centre of Excellence for Climate Extremes (CLEX) Honours Scholarship and the ANU PBSA Partnership - Spotless Scholarship. CJS acknowledges support from an ARC Discovery Early Career Researcher Award DE180100087 and an Australian National University Futures Scheme award. Numerical simulations were conducted on the National Computational Infrastructure (NCI) facility, Canberra, Australia. This study has been conducted using E.U. Copernicus Marine Service Information. We thank two anonymous reviewers for their comments which helped to improve the manuscript significantly. Codes and output files are available online at the project repository (https://github.com/jessecusack/DIMES_eddy_wave_interactions).

The physical mechanisms that remove energy from the Southern Ocean’s vigorous mesoscale eddy field are not well understood. One proposed mechanism is direct energy transfer to the internal wave field in the ocean interior, via eddy-induced straining and shearing of preexisting internal waves. The magnitude, vertical structure, and temporal variability of the rate of energy transfer between eddies and internal waves is quantified from a 14-month deployment of a mooring cluster in the Scotia Sea. Velocity and buoyancy observations are decomposed into wave and eddy components, and the energy transfer is estimated using the Reynolds-averaged energy equation. We find that eddies gain energy from the internal wave field at a rate of −2.2 ± 0.6 mW m−2, integrated from the bottom to 566 m below the surface. This result can be decomposed into a positive (eddy to wave) component, equal to 0.2 ± 0.1 mW m−2, driven by horizontal straining of internal waves, and a negative (wave to eddy) component, equal to −2.5 ± 0.6 mW m−2, driven by vertical shearing of the wave spectrum. Temporal variability of the transfer rate is much greater than the mean value. Close to topography, large energy transfers are associated with low-frequency buoyancy fluxes, the underpinning physics of which do not conform to linear wave dynamics and are thereby in need of further research. Our work suggests that eddy–internal wave interactions may play a significant role in the energy balance of the Southern Ocean mesoscale eddy and internal wave fields.

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scott, R. M., Pickart, R. S., Lin, P., Muenchow, A., Li, M., Stockwell, D. A., & Brearley, J. A. Three-dimensional structure of a cold-core Arctic eddy interacting with the Chukchi slope current. Journal of Geophysical Research: Oceans, 124, (2019): 8375-8391, doi: 10.1029/2019JC015523., A rapid, high‐resolution shipboard survey, using a combination of lowered and expendable hydrographic measurements and vessel‐mounted acoustic Doppler current profiler data, provided a unique three‐dimensional view of an Arctic anti‐cyclonic cold‐core eddy. The eddy was situated 50‐km seaward of the Chukchi Sea shelfbreak over the 1,000 m isobath, embedded in the offshore side of the Chukchi slope current. The eddy core, centered near 150‐m depth, consisted of newly ventilated Pacific winter water which was high in nitrate and dissolved oxygen. Its fluorescence signal was due to phaeopigments rather than chlorophyll, indicating that photosynthesis was no longer active, consistent with an eddy age on the order of months. Subtracting out the slope current signal demonstrated that the eddy velocity field was symmetrical with a peak azimuthal speed of order 10 cm s−1. Its Rossby number was ~0.4, consistent with the fact that the measured cyclogeostrophic velocity was dominated by the geostrophic component. Different scenarios are discussed regarding how the eddy became embedded in the slope current, and what the associated ramifications are with respect to eddy spin‐down and ventilation of the Canada Basin halocline., The authors are indebted to the crew of the USCGC Healy for making the eddy survey possible. Funding for the study was provided by the following sources: National Science Foundation Grants OPP‐1702371, OPP‐1733564, and PLR‐1303617 (RS, RP); OPP‐ 0125466 (DS); and OPP‐1604076 (AM). National Oceanic and Atmospheric Administration Grant NA14‐OAR4320158 (PL); National Natural Science Foundation of China Grants 41706025 and 41506018 (ML); Natural Environmental Research Council Grant NE/L011166/1 (AB). RS acknowledges the Challenger Society for helping facilitate the collaboration that resulted in this work. The data used in the study can be found at http://www.eol.ucar.edu/projects/sbi/all_data.shtml.

© The Author(s), 2019. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Testor, P., de Young, B., Rudnick, D. L., Glenn, S., Hayes, D., Lee, C. M., Pattiaratchi, C., Hill, K., Heslop, E., Turpin, V., Alenius, P., Barrera, C., Barth, J. A., Beaird, N., Becu, G., Bosse, A., Bourrin, F., Brearley, J. A., Chao, Y., Chen, S., Chiggiato, J., Coppola, L., Crout, R., Cummings, J., Curry, B., Curry, R., Davis, R., Desai, K., DiMarco, S., Edwards, C., Fielding, S., Fer, I., Frajka-Williams, E., Gildor, H., Goni, G., Gutierrez, D., Haugan, P., Hebert, D., Heiderich, J., Henson, S., Heywood, K., Hogan, P., Houpert, L., Huh, S., Inall, M. E., Ishii, M., Ito, S., Itoh, S., Jan, S., Kaiser, J., Karstensen, J., Kirkpatrick, B., Klymak, J., Kohut, J., Krahmann, G., Krug, M., McClatchie, S., Marin, F., Mauri, E., Mehra, A., Meredith, M. P., Meunier, T., Miles, T., Morell, J. M., Mortier, L., Nicholson, S., O'Callaghan, J., O'Conchubhair, D., Oke, P., Pallas-Sanz, E., Palmer, M., Park, J., Perivoliotis, L., Poulain, P., Perry, R., Queste, B., Rainville, L., Rehm, E., Roughan, M., Rome, N., Ross, T., Ruiz, S., Saba, G., Schaeffer, A., Schonau, M., Schroeder, K., Shimizu, Y., Sloyan, B. M., Smeed, D., Snowden, D., Song, Y., Swart, S., Tenreiro, M., Thompson, A., Tintore, J., Todd, R. E., Toro, C., Venables, H., Wagawa, T., Waterman, S., Watlington, R. A., & Wilson, D. OceanGliders: A component of the integrated GOOS. Frontiers in Marine Science, 6, (2019): 422, doi:10.3389/fmars.2019.00422., The OceanGliders program started in 2016 to support active coordination and enhancement of global glider activity. OceanGliders contributes to the international efforts of the Global Ocean Observation System (GOOS) for Climate, Ocean Health, and Operational Services. It brings together marine scientists and engineers operating gliders around the world: (1) to observe the long-term physical, biogeochemical, and biological ocean processes and phenomena that are relevant for societal applications; and, (2) to contribute to the GOOS through real-time and delayed mode data dissemination. The OceanGliders program is distributed across national and regional observing systems and significantly contributes to integrated, multi-scale and multi-platform sampling strategies. OceanGliders shares best practices, requirements, and scientific knowledge needed for glider operations, data collection and analysis. It also monitors global glider activity and supports the dissemination of glider data through regional and global databases, in real-time and delayed modes, facilitating data access to the wider community. OceanGliders currently supports national, regional and global initiatives to maintain and expand the capabilities and application of gliders to meet key global challenges such as improved measurement of ocean boundary currents, water transformation and storm forecast., The editorial team would like to recognize the support of the global glider community to this paper. Our requests for data and information were met with enthusiasm and welcome contributions from around the globe, clearly demonstrating to us a point made in this paper that there are many active and dedicated teams of glider operators and users. We should also acknowledge the support that OceanGliders has received from the WMO/IOC JCOMM-OCG and JCOMMOPS that have allowed this program to develop, encouraging us to articulate a vision for the role of gliders in the GOOS. We acknowledge support from the EU Horizon 2020 AtlantOS project funded under grant agreement No. 633211 and gratefully acknowledge the many agencies and programs that have supported underwater gliders: AlterEco, ANR, CFI, CIGOM, CLASS Ellet Array, CNES, CNRS/INSU, CONACyT, CSIRO, DEFRA, DFG/SFB-754, DFO, DGA, DSTL, ERC, FCO, FP7, and H2020 Europen Commission, HIMIOFoTS, Ifremer, IMOS, IMS, IOOS, IPEV, IRD, Israel MOST, JSPS, MEOPAR, NASA, NAVOCEANO (Navy), NERC, NFR, NJDEP, NOAA, NRC, NRL, NSF, NSERC, ONR, OSNAP, Taiwan MOST, SANAP-NRF, SENER, SIMS, Shell Exploration and Production Company, Sorbonne Université, SSB, UKRI, UNSW, Vettleson, Wallenberg Academy Fellowship, and WWF.

A rapid, high‐resolution shipboard survey, using a combination of lowered and expendable hydrographic measurements and vessel‐mounted acoustic Doppler current profiler data, provided a unique three‐dimensional view of an Arctic anti‐cyclonic cold‐core eddy. The eddy was situated 50 km seaward of the Chukchi Sea shelfbreak over the 1000 m isobath, embedded in the offshore side of the Chukchi Slope Current. The eddy core, centered near 150 m depth, consisted of newly ventilated Pacific winter water which was high in nitrate and dissolved oxygen. Its fluorescence signal was due to phaeopigments rather than chlorophyll, indicating that photosynthesis was no longer active, consistent with an eddy age on the order of months. Subtracting out the slope current signal demonstrated that the eddy velocity field was symmetrical with a peak azimuthal speed of order 10 cm s‐1. Its Rossby number was ~0.4, consistent with the fact that the measured cyclogeostrophic velocity was dominated by the geostrophic component. Different scenarios are discussed regarding how the eddy became embedded in the slope current and what the associated ramifications are with respect to eddy spin‐down and ventilation of the Canada Basin halocline.

Biogeochemical cycling in high-latitude regions has a disproportionate impact on global nutrient budgets. Here, we introduce a holistic, multi-disciplinary framework for elucidating the influence of glacial meltwaters, shelf currents, and biological production on biogeochemical cycling in high-latitude continental margins, with a focus on the silica cycle. Our findings highlight the impact of significant glacial discharge on nutrient supply to shelf and slope waters, as well as surface and benthic production in these regions, over a range of timescales from days to thousands of years. Whilst biological uptake in fjords and strong diatom activity in coastal waters maintains low dissolved silicon concentrations in surface waters, we find important but spatially heterogeneous additions of particulates into the system, which are transported rapidly away from the shore. We expect the glacially-derived particles – together with biogenic silica tests – to be cycled rapidly through shallow sediments, resulting in a strong benthic flux of dissolved silicon. Entrainment of this benthic silicon into boundary currents may supply an important source of this key nutrient into the Labrador Sea, and is also likely to recirculate back into the deep fjords inshore. This study illustrates how geochemical and oceanographic analyses can be used together to probe further into modern nutrient cycling in this region, as well as the palaeoclimatological approaches to investigating changes in glacial meltwater discharge through time, especially during periods of rapid climatic change in the Late Quaternary.

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 123 (2018): 2615-2634, doi:10.1002/2017JC013536., 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 678W 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 678W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation., UK Natural Environment Research Council Grant Numbers: NE/F020252/1 , NE/N018028/1 , NE/J007757/1; US National Science Foundation Grant Numbers: OCE 0622825 , OCE 1232962, 2018-09-26

Circumpolar Deep Water (CDW) intrudes from the mid-layers of the Antarctic Circumpolar Current onto the shelf of the western Antarctic Peninsula, providing a source of heat and nutrients to the regional ocean. It is well known that CDW is modified as it flows across the shelf, but the mechanisms responsible for this are not fully known. Here, data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events. Clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges. The ridges block the densest waters, with overflowing water descending several hundred metres to fill subsequent basins. This vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events. Initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths. After several ridges, however, the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature, to approximately the same as in the source region of Marguerite Trough. Similarly, it is shown that deep waters of Palmer Deep are warmer than at the same depth at the shelf break. The exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure, but glacially-carved troughs and shallow sills are a common feature of the bathymetry of polar shelves, and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area.

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 3450–3457, doi:10.1002/2015GL063216., 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., Natural Environment Research Council (NERC). Grant Numbers: NE/E007058/1, NE/E005667/1; U.S. National Science Foundation. Grant Numbers: OCE-1231803, OCE-0927583, OCE-1030309; NERC, 2015-11-07

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.