Wind-driven currents in the tropical Pacific

An analysis is made of the ensemble mean ageostrophic near-surface circulation measured by 1503 Lagrangian drifters drogued to 15-m depth in the tropical Pacific between 1987 and 1994. It was found that the physical model of the wind-driven currents in a weakly stratified upper layer, in which the Richardson number remains near unity, accounts for 40% of the vector variance of the observations. In such a model, the amplitude of the current is proportional to u(*)(N/f)(1/2) and the depth scale of the wind-driven layer is proportional to u(*)(N/f)(-1/2). When the ageostrophic currents at 15-m depth are binned by the scaled Ekman depth, a net rotation of 0.87 radians was observed through the layer depth. These measurements suggest that the vertical austausch coefficient of the upper ocean is proportional to u(*)(2)/N. Ekman proposed such a model based on two reports of wind-driven currents in 1905 at widely separated latitudes.