Topographic preconditioning of open ocean deep convection

Abstract

Evidence of enhanced oceanic convection over Maud Rise in the Weddell Sea indicates that bottom topography may play a role in selecting the location and scale of deep convecting oceanic chimneys below large scale atmospheric negative buoyancy forcing. Topographic preconditioning of open ocean deep convection is studied using an idealized, three-dimensional, primitive-equation model. A barotropic mean flow impinges on an isolated Gaussian-shaped seamount in a stratified domain with uniform negative surface buoyancy forcing. A region of topographically trapped flow forms over the topography. When this "Taylor cap" is tall enough to interact with the surface mixed-layer, the local isolation from mean horizontal advection forms a conduit into the deep water. The convective penetration depth within this local region is significantly enhanced relative to ambient levels away from the seamount and to similar runs performed without bottom topography. The parameter dependencies for these preconditioning processes are investigated. With uniform background stratification, the doming of isopycnals does not play a major role in the preconditioning process. However, when a surface intensified stratification is included, domed isopycnals associated with the Taylor cap circulation can also play a preconditioning role. In this case, the pycnocline is first ventilated over the seamount, leading to rapid convective deepening into the weakly stratified deep water. An analytical formula for one-dimensional, non-penetrative convection into an exponential stratification profile is derived and compares well with results from the numerical model. Previous modeling studies have often parameterized the mehanism by which the horizontal scale of oceanographic chimneys is set through the use of disk-shaped surface forcing functions. Unlike in such experiments, topographically preconditioned chimneys are not prone to breakup by the growth of baroclinic instabilities. Instead, convection is generally shut down by horizontal fluxes of heat due to the mean flow across the temperature gradients of the chimney walls. The presence of the mean flow, which is neccessary in order for the topographic preconditioning to work, causes instabilities to be advected downstream faster than they can grow locally. These results suggest that the role of baroclinic eddies in shutting down oceanographic convection is probably misrepresented in studies which parameterize the preconditioning mechanism, particularly if the preconditioning mechanism being parameterized is a topographic one.