SEPARATION AND RECIRCULATION OF THE NORTH BRAZIL CURRENT

The North Brazil Current separates from the French Guiana coast in the western tropical North Atlantic and a portion of the flow retroflects to form a recirculation zone known as the Demerara Anticyclone. The remainder of the North Brazil Current continues northwestward to join the Guiana Current. This part of the flow may be particularly significant, since it carries a cross-gyre transport of heat and mass from the tropical Atlantic into the subtropical gyre. An extension of the recirculation model of Cessi (1988), in which a quasi-geostrophic flow was driven by potential vorticity anomalies along a boundary, has been used to investigate dynamics relevant to the separation and recirculation of the North Brazil Current. Unlike that of Cessi, the present model is two dimensional and consists of two recirculating gyres driven only by the western boundary current potential vorticity distribution. The recirculation zones were much smaller than those predicted by one dimensional models and since most of the streamlines passed through a diffusive boundary layer there was no evidence of homogenization of the potential vorticity field. The influence of diffusivity, boundary forcing and boundary orientation were considered. The western boundary layer was similar to a Munk layer over most of the parameter range investigated. Since the boundary forcing diffused into the interior, the meridional and zonal dimensions of the inertial part of the recirculation both had a weak power law dependency on the boundary forcing and diffusivity. When the western boundary was not aligned with the planetary vorticity gradient, westward drift distorted the shape of the recirculation.