Dynamical regimes of a fully nonlinear stratified model of the Atlantic equatorial undercurrent

Abstract

The dynamical regimes governing various regions of a nonlinear stratified mean equatorial undercurrent are investigated by computing zonal momentum balances in Philander and Pacanowski's (1984) numerical simulation of the equatorial Atlantic. Only in a narrow region of the simulation next to the western boundary does a subsurface regime exist where the positive zonal pressure gradient force overcomes the negative zonal frictional forces, resulting in a net inertial acceleration to the east and in the formation of the eastward undercurrent below westward surface flow. Elsewhere, the net force is westward and the undercurrent, experiencing a net loss of eastward momentum, weakens from west to east. For the parameterizations of frictional processes adopted in the model, zonal momentum is dissipated mostly by vertical friction in the upper thermocline and above, and by lateral friction below. Because the net retarding effect of vertical friction on the upper layers of the undercurrent is larger than that of lateral friction on the lower layers, there is on average an apparent migration of the location of the undercurrent core velocity from above the thermocline in the west of the Atlantic basin to below in the east. The bulk of the mean midbasin model undercurrent can be described as terminating in the overlying westward flow; only a small fraction (the deeper more inertial layers) terminates at the eastern coast. It is argued that the mechanisms at work in the model are plausible ones to explain the zonal evolution of the observed undercurrent, which should motivate a reexamination of existing data. Better agreement between simulated and observed eastern undercurrent speeds might be obtained by reducing the eddy coefficient of lateral mixing used in the simulation.