WIND-DRIVEN MOTIONS IN THE NORTHEAST PACIFIC AS MEASURED BY LAGRANGIAN DRIFTERS

Analysis is presented of the time-dependent motion of 47 surface drift ers in the northeast Pacific during fall 1987 and 16 drifters in fall and winter 1989/90. The drifters were drogued at 15-m depth and were d esigned to have wind-produced slips less than 2 cm s(-1) for wind spee ds up to 20 m s(-1). The coherence of velocity and local wind is prese nted for motions with periods between 1 day and 40 days. For periods b etween 5 and 20 days, drogue motion at 15-m depth is found to be highl y coherent with local wind with an average phase of 70 degrees 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 desig n is used to provide a possible explanation of the differences. A line ar regression, which accounts for 20%-40% of the current variance, giv es water motion at 0.5% of wind speed and 68 degrees to the right of t he wind vector. Assuming an Ekman-type balance, this regression with 1 5-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-param eter models for turbulent stress are presented based on these observed depth scales and regression coefficients. The model stresses rotate f rom downwind to crosswind at the base of the mixed layer. The model cu rrents rotate from approximately 60 degrees to the right of the wind v ector at the surface to 180 degrees to the right of the wind vector at the mixed layer base.