THE WOCE MODEL IN THE WESTERN EQUATORIAL ATLANTIC - UPPER LAYER CIRCULATION

A high-resolution general circulation model of the North Atlantic, first developed at the National Center for Atmospheric Research and then run at the Institut fur Meereskunde in Kiel for two different wind climatologies and reduced vertical friction, is evaluated in the upper 500 m for the western tropical Atlantic, 5-degrees-S to 15-degrees-N. Although the general features of the vigorous seasonal circulation changes documented in previous studies and in the earlier high-resolution model of Philander and Pacanowski (1986a) are reproduced, there are some interesting differences. Lack of eastward penetration of the Equatorial Undercurrent (EUC) and a thermocline that is too diffuse are model deficiencies due to the constant vertical eddy diffusion coefficient. In the lower friction case the undercurrent partially surfaces in the west, causing an eastward surface current on the equator, which is not apparent in the earlier model studies. Further, the zonal currents, in the low-friction version, have high-velocity bands, resulting, e.g., in two separate current cores in the North Equatorial Countercurrent (NECC) region; and an eastward surface core just south of the equator, connected to the EUC. Particularly interesting are equatorward undercurrents along the western boundary, one of which has already been confirmed in recent measurements off French Guyana. In winter it connects with the EUC in the model, in summer with the NECC. A northward undercurrent in the model exists off Brazil, between 5-degrees and 10-degrees-S, but that is already close to the southern boundary of the model domain. The annual mean throughflow from the southern hemisphere into the Caribbean along the western boundary is small in the model, and in particular, there is no enhanced throughflow in winter, when the cross-equatorial North Brazil Current transport is not taken up by the NECC.