Impacts of Localized Mixing and Topography on the Stationary Abyssal Circulation

Abstract

Stommel and coworkers calculated the stationary, geostrophic circulation in the abyssal ocean driven by prescribed sources (representing convective downwelling sites) and sinks (slow, widespread upwelling through the thermocline). The applied basin geometries were highly idealized with nearly uniform upwelling and gradual bottom slopes. In this paper, the classical Stommel?Arons theory for the abyssal circulation is extended by introducing pronounced bathymetry in the form of a midocean ridge and strongly enhanced upwelling in the vicinity of this ridge, modeled after direct observations of diapycnal mixing rates in the deep ocean. Locally enhanced upwelling over a midocean ridge drives a ?-plume circulation that is modified by topographic stretching. The dynamics of this abyssal circulation pattern are explained by analyzing the combined impacts of the upwelling pattern and the bathymetry on the stationary circulation, building on their well-known separate impacts. On the western flank of the ridge, the effects of topographic stretching and upwelling oppose, and the direction of the local flow depends on their relative size. In this paper, a simple theoretical estimate is derived that can predict the direction of the flow along the ridge based on the geometry of the basin and the upwelling region. Its applicability is demonstrated for both the idealized model configurations applied in this study and for more realistic model simulations.