Large-scale circulations forced by localized mixing over a sloping botton

Abstract

A simple, nonlinear, two-layer, planetary geostrophic model of the large-scale circulation forced by localized mixing over a sloping bottom is discussed. The model is forced by parameterized diapycnal mixing at the density interface and/or by a mass flux downward into (unresolved) deep topographic canyons. Two nondimensional parameters are identified: the ratio of the change in Coriolis parameter over the horizontal mixing length scale to the nominal Coriolis parameter and the ratio of the advective speed to the Rossby wave phase speed. The former controls the strength of horizontal recirculation gyres that are forced by spatially variable diapycnal mixing, while the latter is a measure of the importance of nonlinearity in the density equation. When bottom topography is introduced, bottom pressure torque becomes important and the traditional strong horizontal recirculation gyre found for mixing over a flat bottom (beta plume) is gradually replaced by a zonal flow into or out of the mixing region in the deep ocean. Bottom topography becomes important, and the zonal flow emerges when the topographic Rossby wave speed exceeds the baroclinic planetary Rossby wave speed. Nonlinear effects are shown to enhance the upper-layer recirculation for upwelling and to retard the upper-layer circulation for downwelling. The model is finally configured to represent a region of mixing over the western flank of the Mid-Atlantic Ridge in the deep Brazil Basin. The model upper-layer flow is toward the southwest and the deep flow is very weak, zonal, and toward the east, in reasonable agreement with recent observational and inverse model estimates. The bottom pressure torque is shown to be crucial for maintaining this weak, zonal deep flow in the presence of strong turbulent mixing.