Topographic preconditioning of open-ocean deep convection

Abstract

Evidence for oceanic convection over Maud Rise in the Weddell Sea suggests that bottom topography may select the location and scale of deep convecting oceanic chimneys forced by seasonal large-scale atmospheric cooling. In this paper, the role of bottom topography in open-ocean deep convection is studied using an idealized three-dimensional primitive equation model. A barotropic mean flow impinges on a Gaussian-shaped seamount in a stratified domain generating a Taylor cap (a region of topographically trapped fluid). Uniform surface cooling is applied throughout the domain. When the Taylor cap is tall enough to interact with the surface mixed layer, the local isolation from the advection of heat by the mean flow forms a conduit into the deep water. Convection within this region is significantly enhanced relative to ambient levels away from the seamount and to similar numerical simulations performed without bottom topography. Given uniform background stratification, domed isopycnals are not important in the preconditioning process. However, when a surface intensification in the stratification exists, 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 analytic formula for one-dimensional nonpenetrative convection into an exponential stratification profile is derived and compares well with results from the numerical model. Parameter dependencies for these topographic preconditioning mechanisms are discussed. These numerical results suggest that bottom topography can play an important role in selecting the location and horizontal scale of deep convection in the ocean.