A Study on the Effects of Bottom Topography on Deep Circulation with a Diffusive Reduced-Gravity Model

Abstract

Abyssal circulation around a sea mountain and a depression in a midlatitude basin is investigated with a reduced-gravity model characterized by vertical and horizontal diffusion of both momentum and density. Above either topography a low with the associated cyclonic circulation is induced. The low, however, extends to the west at poleward latitudes for the sea mountain and equatorward at latitudes for the depression. These features are explained well by a perturbation theory to the second order with respect to the height of a sea mountain (depression). Even when topography is steep enough to generate a region called a geostrophic island or a planetary island, the resulting circulation remains qualitatively the same. One difference is that the current is concentrated to make a jet along the border and the tail of the geostrophic island, whore the tail is defined by the branch of the critical geostrophic contour extending westward. Inside the geostrophic island, upwelling becomes quite feeble or even reversed (downwelling) and forms a distinct clockwise (counterclockwise) spiral pattern winding in for the sea mountain (depression). Those peculiar features are argued well dynamically in terms of potential vorticity through analytical solutions obtained for locally simplified equations.