Seasonal Variability in the Surface Currents of the Equatorial Pacific

Buoy drifts and current meter records between January 1987 and April 1992 are used to investigate the seasonal variability of the equatorial Pacific Ocean currents at a depth of 15 m. The buoy drifts and current meter data are well correlated, and their differences are small, although slightly larger currents may be given by the buoy drifts. The seasonal cycle in the currents is analyzed between 20-degrees-N and 20-degrees-S on a 1-degree-x5-degrees grid using a function-fitting algorithm which somewhat smoothes the zonal structure but retains the meridional structure. The analysis captures a large, zonally coherent seasonal variability of the currents within 15-degrees of the equator, which significantly exceeds the estimated errors that originate from the limited sampling of the interannual, intraseasonal, and higher-frequency fluctuations of the currents. Many features of the new climatology are shared with other analyses of the surface currents in the equatorial Pacific, particularly the timing of the seasonal cycle of the main currents. There are, however, differences in the current velocities that are illustrated by a comparison with the ship drift data, which are analyzed here with the same spatial resolution. The analysis of the ship drifts presents larger meridional scales which are probably the result of the spatial smoothing involved in estimating a ship drift. The ship drifts are noticeably downwind of the 15-m currents. At the equator, they are also more westward than in the analysis of the 15-m currents between November and March near the date line and January and July in the eastern Pacific which at least partially results from differences in the climatic conditions sampled in the two data sets.