Surface drifter observations are decomposed into mean, seasonal (annual and semiannual) and eddy components via Gauss-Markov estimation. This approach helps separate seasonal fluctuations and mean values in the observationally-sparse tropical and South Atlantic, where monthly mean values cannot be calculated at the spatial scale of the major currents. In some regions, large differences are found between these means and those obtained by simple binned averaging. The differences are attributed to inhomogeneous sampling of seasonal variability, and to the inherent bias of Lagrangian observations towards periods of low velocity. The analysis reveals strong seasonal variations of some surface currents, including a significant late spring reversal of the western North Equatorial Counter-Current.