Tropical Atlantic geostrophic currents and ship drifts

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Abstract
Historical ship drifts and geostrophic surface currents obtained from hydrographic data are compared in the tropical Atlantic Ocean. The major components of the current system (North Equatorial Current, North Equatorial Countercurrent, South Equatorial Current) are clearly depicted by the two data sets. The main difference between the two fields is the weakness of the geostrophic currents except in the near‐equatorial band, where on the contrary, they are much larger than the ship drifts. The amplitudes of the annual signals differ, but the phases are in rather good agreement. The uncertainty concerning the ship drift data, and the limitation of the geostrophic assumption, can explain a part of the differences encountered in this study. A much better comparison is obtained if we add to the geostrophic current ug an Ekman drift current ue computed from the climatological wind stress and a mean constant vertical viscosity value. Then the total velocity u = ug + ue corresponds very closely in phase and amplitude to the ship drift velocity, except at the equator.