Frictional Currents in the Mixed Layer at the Sea Surface

Abstract

The wind-driven flow in the mixed layer between the free surface and the top stable sheet of a thermocline is usually turbulent. The frictional component of such a flow may be separated from a depth-independent general current. The turbulent frictional current is governed by a small number of external parameters and should be subject to quite simple laws. Explicit approximate solutions may be obtained with the assumption of constant eddy viscosity and a lower boundary condition of zero stress at the bottom of the mixed layer: these show a variety of different “Ekman spirals,” depending only on the parameter fh/u^* where f is the Coriolis parameter, h the mixed layer depth, and u^* the shear velocity. Experimental data collected within the mixed layer on Lake Huron are reported and compared with the simple theory. Encouraging agreement between theory and observation is found and some quantitative relationships are tentatively established.