Temporal Variability of the Large-Scale Geostrophic Surface Velocity in the Northeast Pacific*

Data from Argos-tracked mixed layer drifters in fall and winter 1987 (49 drifters) and 1989 (16 drifters) are used to investigate the differences in the large-scale surface velocity and eddy activity in the northeast 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 variance, indicates that the currents are at 60° to the right of the wind. The magnitude of the currents is 30% of the magnitude of the wind stress. In 1987–88, the geostrophic motion in the region from 46.5° to 48.5°N, 142° to 133°W was characterized by an eastward flow of 0.9 (±0.4) cm s−1 and a northward flow of 0.7 (±0.4) cm s−1. In 1989–90, for the same region, the geostrophic 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 presence of several persistent, warm core mesoscale eddies. In 1989–90 there is no evidence of any significant eddy activity. The mean speed of the 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 same, drifters in 1987–88 take a longer time to travel eastward because of the significant north–south excursions due to the mesoscale eddies. Data from two drifter experiments have shown that the variability of mesoscale eddies can result in large interannual differences in estimates of mean velocity.