Topographic control of mixing in a dense boundary current in the Orkney Passage

The Dynamics of Orkney Passage Overflow field program followed the path of dense water flowing between the Weddell Sea and the Scotia Sea, through the complicated topography of the Orkney Passage. Several intensively sampled sections through the dense boundary current revealed regions adjacent to the boundary where density overturns and horizontal density gradients give rise to conditions conducive to a combination of convective and symmetric instability. These instabilities could mix the water masses near the boundary and export them into the stratified ocean interior, a scenario supported by high levels of turbulence near the topography deduced from microstructure profiles. High-resolution numerical simulations of this region allow us to isolate the causes of these instabilities and quantify their effect on turbulence generation and mixing. In particular, the simulations show that the generation of instability and turbulence is strongly tied to the details of the topography. Whereas smooth sloping topography leads to thin layers of overturned fluid and little extension of turbulence into the interior, the ridges and spurs of the actual topography lead to boundary layer separation, which carries the fluid mixed at the boundary into the interior, and generates a more efficient connection with the interior fluid. With the simulations, we can explore the sensitivity of these boundary mixing processes to changes in dense flow transports, and estimate the net water mass transformation resulting from the passage of the dense water through the Orkney Passage region.