Observations of Momentum Transfer in the Upper Ocean: Did Ekman Get It Right?

The first theory of wind-driven flow is that of Ekman, who assumed a uniform eddy viscosity and derived the velocity spiral that bears his name. Over the years, observations of wind stress and current have improved to the point that frictional momentum transfer in the upper ocean can be quantified. Observations of velocity spirals are reviewed to address such issues as the vertical structure and penetration of wind-driven momentum and the frequency dependence of momentum transfer. Observed response is more slab-like than the theoretical Ekman spiral. The penetration depth does not vary with frequency as Ekman theory predicts, suggesting regulation by the mixed layer. Wind-driven momentum penetrates beneath the mixed-layer base, into a stratified region dominated by inertial shear.