- 2004 by the American Geophysical Union. An ocean data assimilation product from the Geophysical Fluid Dynamics Laboratory (GFDL) was used to investigate the seasonal variability of the ocean circulation in the northern tropical Atlantic. Analyses indicate that the seasonal variation of the circulation plays an important role in mediating meridional exchange between the subtropics and equatorial zone. In particular, seasonal changes in the Northern Equatorial Countercurrent (NECC) and in the cyclonic recirculation of the Guinea dome directly affect the trajectory of a subducted water parcel. Furthermore, seasonal variation of the zonal slope of the thermocline in the interior of the basin controls, to a large extent, the amount of water participating in the equatorial circulation. These changes can be explained in terms of a simple dynamical model where local Ekman pumping dominates thermocline variation in the western part of the basin, and Rossby wave adjustment comes into playing in the eastern basin.