Wind-Driven Motions in the Northeast Pacific as Measured by Lagrangian Drifters

Abstract

Abstract Analysis is presented of the time-dependent motion of 47 surface drifters in the northeast Pacific during fall 1987 and 16 drifters in fall and winter 1989/90. The drifters were designed at 15-m depth and were designed to have wind-produced slips less than 2 cm s−1 for wind speeds up to 20 m s−1. The coherence of velocity and local wind is presented for motions with periods between 1 day and 40 days. For periods between 5 and 20 days, drogue motion at 15-m depth is found to be highly coherent with local wind with an average phase of 70° to the right of the rotating wind vector. These results differ from analyses of FGGE-type drifters as reported by McNally et al. and Niiler in the same area. A model of wind-produced slip as a function of drifter design is used to provide a possible explanation of the differences. A linear regression, which accounts for 20%–40% of the current variance, gives water motion al 0.5% of wind speed and 68° to the right of the wind vector. Assuming an Ekman-type balance,...