A Budget for Mixing and Transformation in a Partially Enclosed Deep Basin

Water-mass transformation by mixing in the deep ocean has been shown to be vital in closing both regional and global circulation budgets. In many cases, these large-scale budgets do not match well with observed rates of mixing. Here we consider the transformation rate within a small, partially enclosed deep basin, Orkney Passage, to investigate this mismatch. Orkney Passage, a 3650 m deep and 45 km wide gap in the South Scotia Ridge, is a key bottle-neck in the transport of dense water from Antarctica to the global ocean. We collected a series CTD/LADCP sections and VMP microstructure profiles within Orkney Deep. These sections show a focussing of the transport in density space as the water flows northwards. Applying the Walin Framework shows a lightening of the densest waters and a densification of lighter waters. These transformations are consistent with a maximum buoyancy flux of 1.5x10^-8W kg^-1 at a neutral density of 28.32 kg m^-3. Comparing this buoyancy flux with the microstructure estimates of turbulent kinetic energy dissipation rate implies a dissipation ratio of O(1), much larger than the commonly used 0.2. This result is supported by estimates of dissipation ratio taken from the combination of turbulent kinetic energy and temperature variance dissipation rates. The transformation can be decomposed into three terms which broadly depend on the gradients in density space of: the diffusivity, the isopycnal area, and the stratification.We combine the density field from a high resolution simulation of the region with a single realistic mixing profile applied throughout the region to investigate these contributions. The analysis shows that the reduced stratification near the boundary is the key process in driving the lightening of the densest water masses in Orkney Deep, highlighting the joint importance of both enhanced mixing and re-stratification on sloping boundaries.