On Boundary Mixing

We report here on three different control volume budgets / tracer release experiments combined with fine- and / or microstructure estimates of turbulent mixing. One (BBTRE) is an example of a tidally dominated environment. The other two, concerning the northern deepwater GOM and AABW in the Scotia Sea, appear to have substantial subinertial contributions to near-boundary mixing. We forward the hypothesis that these subinertial contributions are dominated by mixing associated with non-propagating form drag.

Our proposed hypothesis has roots in a discussion between Garrett and Armi (1979, J. Geophys. Res.). Armi interprets detached mixed layers in CTD casts from the western North Atlantic as providing evidence for ventilation of the abyssal ocean at rates of $O(1x10^-4 m² s^-1). Garrett criticizes this on energetic grounds, suggesting that the required energy source is not to be found with a quadratic drag law, U ~ O(0.1 m s^-1) and C_d = 1x10^-3. Garrett also points to an issue of a likely reduction in mixing efficiency with the bottom mixed layer and an issue of transporting mixed fluid away from the boundary.

Our response to this is that Garrett is simply looking at the wrong problem. The issue is one of form drag above rough topography, not shear stresses associated with hydrodynamically smooth surfaces. With form drag, the effective drag coefficients are orders of magnitude larger and mixing is associated with highly sheared regions in well stratified regions of O(U/N) height above the bottom. The 3-dimensional character of topographic roughness likely makes for more efficient off-boundary buoyancy transport.

These issues are explored with preliminary numerical simulations.