Topographic effects on the bottom water circulation of the western tropical North Atlantic Ocean

Abstract

A simple numerical model of the abyssal circulation of the ocean is used to explore the possibility, first suggested by Warren (MIT Press, pp. 6–41, 1981), that the northward deepening of the sea floor in the western basin of the tropical North Atlantic Ocean is responsible for the observed eastward confinement of the Antartic Bottom Water current, a result contrary to what the Stommel-Arons model predicts. It is shown that (a) a topographic slope of magnitude observed in this region of the Atlantic forces separation of the northward-flowing deep western boundary current at a low latitude, (b) north of this latitude the circulation is dominated by a large-scale cyclonic gyre extending up to where the sea floor becomes flat again, with a northward flow along the eastern boundary as a part of it and a westward flow at about 25°N. Simulated Lagrangian particle trajectories show that new abyssal water from the south is confined to a band next to the eastern boundary in the gyre region as observed. These results are robust under variation of topographic, forcing and frictional parameters, although quantitative aspects and details are sensitive to them. Eastward flow through the Vema Fracture Zone, recently estimated by McCartney et al. ( Journal of Physical Oceanography, 21, 1089–1121, 1991) to be about 2 Sv, diverts some of the northward-flowing eastern boundry current, but otherwise does not affect the circulation. These results compare favorably with recent observations that report southeastward flow along the western boundary at least down to 8°N and suggest a recirculation of abyssal water in this region, as well as with those that suggest a northern limit of the Antarctic Bottom Water flow along the eastern boundary at about 25°N and a possible westward separation there. An analytical solution of the planetary geostrophic equation applied to an abyssal ocean with topography, which captures some features of the numerical model flow, is presented in the Appendix.