Eulerian mean surface velocity field derived by combining drifter and satellite altimeter data

Abstract

Combining surface drifter and satellite altimeter data,we estimate Eulerian mean sea‐surface velocity with a resolution of quarter degrees in both latitude and longitude. The Eulerian mean velocity is calculated by subtracting the altimeter‐derived velocity anomaly (i.e., deviation from the temporal mean) at the time when a drifter measured the surface velocity, from this drifter‐measured velocity. The method is applied to the surface flow of the North Pacific, using TOPEX/POSEIDON and ERS‐1/2 altimeter data, and WOCE‐TOGA surface drifter data obtained during October 1992 to January 2001. The estimated Eulerian mean velocities for the Kuroshio south of Japan and the Kuroshio Extension region tend to be smaller than simple averages of drifter‐derived velocities by 20 to 50 cm sec−1. The instantaneous velocities are obtained every ten days as sums of the Eulerian mean velocities and velocity anomalies. They agree well with in situ surface velocities measured by acoustic Doppler current profilers.