Annual Cycle of the Atlantic North Equatorial Countercurrent

An analysis of numerous meridional XBT sections near 28-degrees-W reveals that the geostrophic North Equatorial Countercurrent (NECC) continues to flow eastward throughout the year, fastest in fall and slowest in spring. Drifting buoys and historical ship drifts show that the near-surface Countercurrent reverses each spring even when systematic errors due to windage are taken into account. The seasonally fluctuating winds drive an Ekman surface current that is eastward in fall, adding to the geostrophic current, and westward in spring, countering and overwhelming the geostrophic current. The reversal of the Countercurrent in spring occurs in the near-surface layer and is driven by the Northeast Trades. Thus the near-surface velocity in the Countercurrent is determined by a competition between local wind stress and the larger field of wind stress curl, both of which have large seasonal variations in the tropical Atlantic.