Equatorial Undercurrent and North Equatorial Countercurrent at 38°W:: A new perspective from direct velocity data

The western Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC) are investigated at 38 degrees W using contemporaneous subsurface ( ADCP) and high-resolution near-surface ( drifters + satellite altimeter) velocity measurements, together with hydrographic (CTDO2) data that were collected from 1998 to 2006. The observations reveal an EUC with a strong semiannual pattern of intensification. Direct measurements also confirm the existence of a northern branch of the NECC (nNECC), observed here for the first time in the western tropical Atlantic. The NECC displays an annual cycle of northward migration on the basin, driven by the Sverdrup transport generated by the wind field. In this cycle the nNECC is a semipersistent feature fed by waters from the Northern Hemisphere and the residual nNECC flow from the previous year.