Rates and Mechanisms of Turbulent Mixing in a Coastal Embayment of the West Antarctic Peninsula

Ryan Scott1,2, J. Alexander Brearley3, Alberto Naveira Garabato1, Mark Inall4, Hugh J Venables2 and Michael Paul Meredith2, (1)University of Southampton, Southampton, United Kingdom, (2)NERC British Antarctic Survey, Cambridge, United Kingdom, (3)NERC British Antarctic Survey, Polar Oceans, Cambridge, United Kingdom, (4)Scottish Association for Marine Science, Oban, United Kingdom
Quantifying and understanding the processes driving ocean turbulent mixing around Antarctica is fundamental to closing the heat budget of the Southern Ocean and the ocean-to-cryosphere heat flux. Underwater autonomous gliders equipped with microstructure sensors increase the number of available measurements by an order of magnitude compared to more conventional methods such as free-failing profilers. In 2016, a glider deployment in Ryder Bay, a 520 m deep bay adjacent to the British Antarctic Survey station at Rothera, collected both hydrographic and microstructure data, obtaining some of the first direct (not-inferred) measurements of dissipation of turbulent kinetic energy and dissipation of temperature variance in the west Antarctic. Elevated dissipation estimates of O(1x10-8 W kg-1) and heat fluxes up to ∼12.6 W m-2 are found above a topographic sill separating the bay from the open continental shelf. Contrasting values of O(1x10-10 W kg-1) and ∼0.2 W m-2 are observed in the deep basin, suggesting that the topography of the bay significantly modulates both the vertical mixing of Circumpolar Deep Water and its associated upward heat flux. A nearby ADCP mooring is used to investigate the temporal controls on the mixing. Dissipation values above the ridge appear to be controlled by the direction of flow into and out of the basin, with highest values found during periods of inflow. Wavelet analysis of the ADCP data reveals the presence of baroclinic sub-inertial oscillations, with significant peaks at periods of 48-100 hours. Investigation into the nature of these oscillatory signals is ongoing, with the possibility that these are wind-forced coastally trapped waves formed on the Antarctic shelf and advected into the bay.