A note on the validity of the Sverdrup balance in the Atlantic North Equatorial Countercurrent

An ocean general circulation model of the tropical Atlantic Ocean is used to study the vertically integrated vorticity equation for the annual mean flow in the Atlantic North Equatorial Countercurrent (NECC). It is found that the nonlinear terms play an important role in the vorticity budget, in the western part of the basin. Sverdrup balance does not hold in the region of the North Brazil Current retroflection, where advection of relative vorticity by the mean flow and by the eddies is important. In the eastern part of the basin, these nonlinearities are negligible and the flow appears to be in Sverdrup balance. In the model, the cut-off location occurs at 32 W: The results suggest that in the western part of the basin, observations relying on hydrographic data neglect two important contributions to the vorticity balance: advection of planetary vorticity by the deep meridional flow and advection of relative vorticity. r 2004 Elsevier Ltd. All rights reserved.