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Control of the abyssal ocean overturning circulation by mixing-driven bottom boundary layers

Henri Drake,Ph.D., 2021
Raffaele Ferrari, Advisor
The abyssal circulation is primarily driven by turbulent mixing, which is generally weak in the ocean interior but is enhanced above rough topography, such as mid-ocean ridges. Abyssal upwelling is the small residual of vigorous upwelling in thin sloping Bottom Boundary Layers (BBLs) and broad downwelling in the Stratified Mixing Layers (SML) above. A multi-scale hierarchy of models reveals their dynamics; the most complex model is consistent with observations. Sloping boundary layer theory predicts that SML downwelling largely compensates for BBL upwelling, resulting in a net overturning much weaker than observed. At equilibrium, however, the increase of stratification with height suppresses much of the SML downwelling and thus supports a realistic finite overturning circulation that scales roughly with the BBL upwelling. The magnitude of BBL upwelling is set by the near-boundary stratification, which is maintained by a variety of processes: a buoyancy-driven cross-slope “secondary” circulation, restratification by submesoscale baroclinic eddies, topographic standing eddies, and inhibition of de-stratifying rotational effects within narrow canyons. Both mixing rates and the diapycnal velocity can be inferred using purposeful Tracer Release Experiments, although estimates of the mixing rates must account for additional diapycnal stretching effects which can be of leading order near rough topography.