Evolution of bottom boundary layers on three dimensional topography --
Buoyancy adjustment and instabilities
Abstract
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 \lq arrest
\rq \, , 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
counter. As the current advects along the ridge slopes, the BBL deepens
and evolves toward thermal wind balance. The onset of negative potential
vorticity (NPV) modes 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 occurred. 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.