Tropical Pacific near-surface currents estimated from altimeter, wind, anddrifter data

Tropical surface currents are estimated from satellite-derived surface topo graphy and wind stress using a physically based statistical model calibrate d by 15 m drogue drifters. The model, assumes a surface layer dominated by steady geostrophic and Ekman dynamics. Geostrophy varies smoothly from a be ta plane formulation at the equator to an f plane formulation in midlatitud e, with the transition occurring at similar to 2 degrees-3 degrees latitude . The transition is treated with a Gaussian weight function having a meridi onal decay scale that is found to be approximately the Rossby radius (simil ar to 2.2 degrees latitude). The two-parameter Ekman model represents drift er motion relative to wind stress, with downwind flow along the equator and turning with latitude. Velocities computed from satellite data are evaluat ed statistically against drifter velocities and equatorial current moorings . Examples of the geostrophic and Ekman flow fields in the western Pacific during a westerly wind burst in late December 1992 depict a strong eastward flow and equatorial convergence. A comparison between December 1996 and Ju ne 1997 illustrates the basin-wide reversal of equatorial surface flow duri ng the onset of the 1997 El Nino.