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

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, this regression with 15-m currents yields an apparent mixing depth of 34?38 m. which is much less than the observed 60-m depth of the mixed layer. New three-parameter models for turbulent stress are presented based on these observed depth scales and regression coefficients. The model stresses rotate from downwind to crosswind at the base of the mixed layer. The model currents rotate from approximately 60° to the right of the wind vector at the surface to 180° to the right of the wind vector at the mixed layer base.