Evolution of Bottom Boundary Layers on Three Dimensional Topography-Buoyancy Adjustment and Instabilities

A current along a sloping bottom gives rise to upwelling, or downwelling Ekman transport within the stratified bottom boundary layer (BBL), also known as the bottom Ekman layer. In 1D models of slope currents, geostrophic vertical shear resulting from horizontal buoyancy gradients within the BBL is predicted to eventually bring the bottom stress to zero, leading to a shutdown, or "arrest," of the BBL. Using 3D ROMS simulations, we explore how the dynamics of buoyancy adjustment in a current-ridge encounter problem differs from 1D and 2D temporal spin up problems. We show that in a downwelling BBL, the destruction of the ageostrophic BBL shear, and hence the bottom stress, is accomplished primarily by nonlinear straining effects during the initial topographic encounter. As the current advects along the ridge slopes, the BBL deepens and evolves toward thermal wind balance. The onset of negative potential vorticitymodes of instability and their subsequent dissipation partially offsets the reduction of the BBL dissipation during the ridge-current interaction. On the upwelling side, although the bottom stress weakens substantially during the encounter, the BBL experiences a horizontal inflectional point instability and separates from the slopes before sustained along-slope stress reduction can occur. In all our solutions, both the upwelling and downwelling BBLs are in a partially arrested state when the current separates from the ridge slope, characterized by a reduced, but non-zero bottom stress on the slopes.Plain Language Summary Surface winds pump mechanical energy into the large-scale circulation of the ocean at an average rate of between 0.8 and 1 TW. This wind-input occurs at large, so-called synoptic scales spanning thousands of kilometers. Absent dissipative pathways, this steady energy input would cause uncontrolled spinup of the ocean gyres. For decades it has been assumed that friction at the seabed has an important role in the eventual turbulent dissipation of the ocean kinetic energy. In the 1990s, theoretical models suggested that turbulence could be wholly suppressed on sloping bottom bathymetry due to the rearrangement of density surfaces within the bottom boundary layer-a mechanism called buoyancy adjustment. Here we revisit this problem using modern 3D simulations of currents encountering a ridge. We find that although the bottom stress can be markedly reduced on topographic slopes, the mechanism through which it occurs is quite different than that in simplified 1D and 2D models. Flow "deformation," or straining effects during the topographic encounter play a more important role in weakening the bottom stress than buoyancy adjustment. Furthermore, geometric effects like curvature, and flow instabilities can partially offset the reduction in dissipation caused by suppression of bottom boundary layer turbulence.