On the inclusion of Stokes and Ekman surface currents into hydrodynamic models

Abstract

Recently published results (Mao and Heron, J. Phys. Oceanog., 2008) comparing sea surface currents, measured by a 30 MHz ocean radar with wind speed and direction under a range of conditions in Bass Strait, Victoria, Australia indicate that the coupling between wind and surface currents depends on the maturity of the sea wave spectrum. For a mature sea the main contribution to surface current is usually Stokes drift, while in developing sea conditions the Stokes drift is reduced and Ekman-type stress is the main contributor to surface currents. At short fetches where stress driven currents dominate the net surface current has the Ekman angular offset of up to 45 degrees. At long fetches this angle falls to a value around 10-15 degrees and depends on wind speed. The inclusion of these parameters into hydrodynamic models requires knowledge of the age of the wave spectrum and wind speed and direction at any given site. The Stokes component of the surface current is in the direction of the dominant waves, and the Ekman component of the surface current veers in a direction consistent with the Coriolis parameter.