TEMPORAL VARIABILITY OF THE LARGE-SCALE GEOSTROPHIC SURFACE VELOCITY IN THE NORTHEAST PACIFIC

from Argos-tracked mixed layer drifters in fall and winter 1987 (49 dr ifters) and 1989 (16 drifters) are used to investigate the differences in the large-scale surface velocity and eddy activity in the northeas t Pacific. The velocities were corrected for wind-induced slippage and corrected for wind-driven (Ekman) flow by matching an Ekman model to the observed currents. The model, which explains 15%-30% of the varian ce, indicates that the currents are at 60 degrees ta the right of the wind. The magnitude of the currents is 30% of the magnitude of the win d stress. In 1987-88, the geostrophic motion in the region from 46.5 d egrees to 48.5 degrees M, 142 degrees to 133 degrees W was characteriz ed by an eastward flow of 0.9 (+/-0.4) cm s(-1) and a northward flow o f 0.7 (+/-0.4) cm s(-1). In 1989-90, for the same region, the geostrop hic eastward component was 3.8 (+/-0.5) cm s(-1). more than four times as large as in 1987-88, and the northward component was 0.3 (+/-0.5) cm s(-1). In this region ageostrophic contributions to the velocities are small. In 1987-88 the drifter tracks reveal evidence of the presen ce of several persistent. warm core mesoscale eddies. In 1989-90 there is no evidence of any significant eddy activity. The mean speed of th e drifters in 1987-88 was 7.0 (+/-0.3) cm s(-1) and in 1989-90 was 6.5 (+/-0.4) cm s(-1). So, although the average speed is the samel drifte rs in 1987-88 take a longer time to travel eastward because of the sig nificant north-south excursions due to the mesoscale eddies. Data from two drifter experiments have shown that the variability of mesoscale eddies ran result in large interannual differences in estimates of mea n velocity.