On the Annual Cycle of Equatorial Upwelling in the Central Atlantic Ocean

Abstract

The annual cycle of the upper ocean's vertical velocity component (w) on the equator at 28°W is examined by integrating the continuity equation using current meter data from the Seasonal Response of the Equatorial Atlantic Experiment. The annual cycle consists in part of an intense, but brief (?1 month), upwelling season beginning with the onset of strong easterly wind stress in boreal spring. This upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress. The record-length averaged w profile shows that maximum upwelling (0.6 ? 10?3 cm s?1) is located slightly above the core of the Equatorial Undercurrent and downwelling is located below the base of the thermocline. The standard deviations are about tenfold the magnitude of the means. Independent evidence supporting these results are that 1) sea surface temperature (SST) is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacements in agreement, 2) temperature and w are coherent and in quadrature within the thermocline over a broad range of frequencies exclusive of the instability wave band, 3) during the instability wave season, upwelling is associated with increasing SST and the vertical Reynolds' heat flux is maximum and divergent in the thermocline, and 4) after the instability waves abate, w and easterly wind stress are coherent and out-of-phase. The observed evolution of w differs from that implied by climatology, and these differences are attributed to the ocean's response to rapidly varying winds that are observed in-situ versus slowly varying winds characteristic of climatology.