The seasonal variations of surface currents in the tropical Atlantic Ocean: A comparison of ship drift data with results from a general circulation model

Historical ship drifts from the tropical Atlantic Ocean are compared with surface currents from a general circulation model forced with monthly mean climatological winds. The model accurately reproduces the spatial structure of the currents and its time dependence, which varies considerably from the eastern side of the basin, where a semiannual harmonic is prominent, to the western side, where an annual harmonic is dominant. However, the amplitude of the simulated surface currents in the western side of the basin is too large. Mixing processes in the model appear to be too weak, especially when the winds are weak in regions where the thermocline is deep. High-frequency fluctuations of the winds need to be taken into account, and parameterization of the mixing needs to be improved, especially when the Richardson number is small.