Mixing in the Orkney Passage Overflow

The Orkney Passage (OP), the deepest trench in the South Scotia Ridge, plays an important role in the flow of Antarctic Bottom Water from the Weddell Sea to the Scotia Sea. Observations from the Dynamics of Orkney Passage Overflow field program suggest that intense mixing occurs in this region, but its extent and causes are unknown. This Southern Ocean water enters Atlantic circulation, so water mass transformations near the OP can affect properties that propagate throughout other oceans. I conducted numerical simulations to identify mixing processes and their impact on water mass transformations, with the goal of providing information to aid observationalists in reconciling processes they see in the field data. The first part of this project focused on bottom topography. To test the hypothesis that the roughness of the topography impacts mixing, I ran a simulation using "smoothed" topography that preserved general features but leveled out roughness. I compared this simulation to an existing simulation with realistic topography by calculating potential vorticity (PV). When PV has the opposite sign of planetary vorticity, it indicates gravitational, centrifugal, or symmetric instability. With rough topography, unstable areas extended kilometers from the bottom, especially around topographic peaks. With smoothed topography, unstable areas were smaller and closer to the boundary. The latter part of this project investigated how variable heat and salt diffusivity affects transports throughout the OP region. The hypothesis was that the distribution of diffusivity is important - not just the mean value of the diffusivity profile. I ran 1 simulation with variable diffusivity (higher near the bottom) and 3 with different values of constant diffusivity, then compared transports of neutral density (γⁿ) classes at observational sections and a closed box section. Variable and high diffusivity values increased the peak transport of the γⁿ=28.34 kg/m^3 class through the OP. A volume balance calculation for each γⁿ class showed destruction of γⁿ=28.36 kg/m^3 water and creation of γⁿ=28.34 kg/m^3 water in the complicated topography region around the Orkney Deep. The importance of diffusivity distribution and locations of PV reversal indicate that copious mixing occurs near bottom topography in the region surrounding the OP.