Presenter: Sonya Legg (Princeton University)

Description:
An important pathway for dense water from the Weddell sea into the Scotia Sea is through the complicated topography of the Orkney Passage region, where several different processes can lead to mixing, modifying the properties of the dense water feeding into Antarctic Bottom Water. A recent field program, the Dynamics of Orkney Passage Outflow, explored the modification of the dense water following its path from the Weddell Sea, into the Orkney Deep and out through the Orkney Passage into the Scotia Sea. In addition to conventional hydrographic sections across the dense current, numerous microstructure profiles and high-resolution underway CTD transects allowed examination of small-scale processes and turbulence. Since the field program, the observational data has been combined with a numerical model of the Orkney Passage region to explore the processes in greater detail. In particular, the simulations allow the sensitivity of the small scale processes to different barotropic inflow strength to be explored, as well as their sensitivity to the details of the topography. As the dense flow exits the Orkney Passage, the simulations indicate potential for symmetric and centrifugal instabilities driven by frictional downslope flow on the sloping boundary to the left of the dense current, as well as a rotationally-constrained hydraulic jump downstream of the main sill, both of which can lead to mixing. Further topographic spurs lead to flow separation, peeling the boundary layer away from the bottom, and thickening the turbulent region. Energetic analysis confirms that lateral and vertical shear production are both important contributors to turbulent kinetic energy. Stronger barotropic flows lead to thicker boundary layers prone to instability, and more mixing downstream of topography spurs. This connection between barotropic flow speed and deep mixing intensity helps link changing wind forcing to changes in Antarctic Bottom Water properties.