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

Abstract

AbstractA 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.