Abstract

The steady large scale oceanic response to wind stress and a surface buoyancy flux is studied by making use of a numerical model based upon the planetary geostrophic equations for which scales of motions smaller than the internal Rossby radius of deformation have been filtered out. In regions of both heat gain and Ekman convergence, the forcings combine to create a strong subtropical gyre in the upper thermocline. The north-south symmetry imposed by the wind pattern is broken because in a region of heat loss the density field shoals through the action of convection and the result is a broad eastward flow in the top layers. Below the convection zone this flow turns northward in response to increased interior vertical divergence. The lower thermocline circulation separates into two domains: a western domain dominated by the barotropic mode and an eastern domain with cross-gyre baroclinic flow. It is argued that the paths of the characteristics of the second baroclinic Rossby mode are responsible for this partition. The bottom circulation consists of a subpolar cyclonic gyre intensified along the northern and western boundaries with a poleward interior recirculation such as hypothesized by Stommel and Arons (1960). Detailed comparisons of the present solution with recent analytical results have proved difficult but some ubiquitous features, such as the filling of the gyre by low potential vorticity waters originating at northern density outcrops, may be identified. The main difference is the existence of lateral diffusive boundary layers in the present model, which are neglected in the analytical work. Their overall effect is difficult to comprehend because they cannot be easily separated from the interior. Finally, for the heat transport and thermocline depth to be within observational ranges, vertical mixing coefficients between 1 and 5 cm2 s–1 must be used and for such values the buoyancy forcing is as important as the wind forcing for the large scale circulation. Although the bowl structure of the main thermocline favors horizontal recirculations, they are of secondary importance for the meridional heat transport which is carried out mostly by the slow, zonally averaged component of the meridional flow.

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