The seasonal variability of current velocities in the tropical Atlantic was studied by grouping ship drift velocity observations into 2¿¿5¿ boxes and calculating monthly mean velocity values. These values were used to calculate and map the annual mean velocity, the seasonal variation about the mean, the annual and semiannual harmonics, and the first two empirical orthogonal functions (EOFs).

The seasonal variation is primarily zonal in the equatorial band and in the North Equatorial Countercurrent (NECC) and primarily alongshore near the coast of South America. Maxima of seasonal variation of 23 cm/s are centered in the NECC near 6¿N, 42.5¿W and in the Gulf of Guinea near 2¿N, 7.5¿W. Most (~80%) of the variance in the NECC and along the western boundary of the area studied has an annual period; most of the variance along the equator in the mid-Atlantic has a semiannual period. Over the whole region, 49% of the seasonal variance is explained by the annual harmonic, and 69% is explained by a combination of the annual and semiannual harmonics. The second EOF contains 29% of the variance of the data set and shows a simultaneous speeding (slowing) of the major equatorial currents (the South Equatorial Current (SEC), the North Brazil Current, the NECC, and the Guinea Current) along their principal axes of variation with a concurrent slowing (speeding) of the Guyana Current and the Brazil Current. The pattern of variation resembles a large equatorial gyre centered along 4¿N that flows clockwise from May to September and counterclockwise most of the rest of the year. The first EOF accounts for 35% of the variance and shows a simultaneous speeding (slowing) of eastward velocity from 5¿S to 10¿N except in the western SEC, the western end of the North Brazil Current, and the Guinea Current. The out-of-phase fluctuations apparent in the western SEC near the equator are explained by the slowing and reversal of the South Equatorial Current near 2¿N, 40¿W between April and June. This slowing of the SEC is associated with the start up of the NECC, which accelerates eastward in May, June, and July and flows eastward across the Atlantic from July through December.

The SEC divides seasonally near the eastern tip of Brazil, where residual alongshore velocities are northward for half the year (peaking during May and June) and southward for the other half of the year. A second division can be seen north of the Amazon, where the North Brazil Current continues up the coast into the Guyana Current during the first half of the year but partially retroflects into the NECC during the last half of the year. A fast eastward flow in the NECC coincides with below average velocities in the Guyana Current and above average velocities in the western sections of the North Brazil Current and the SEC (North of the equator).