The Response of Surface Currents to Wind Investigated by HF Ocean Surface Radar

The momentum transfer from air to sea generates surface currents through both the wind shear stress and the Stokes drift induced by waves, corresponding to the two components in the surface current response to wind. Stokes drift depends on the sea state which is influenced by the fetch and the duration for wave development. This study focuses on the response of sea surface currents to wind under different fetch conditions using surface current data over a 30-day duration measured by HF (30MHz) ocean surface radar near the entrance to Port Phillip Bay in Victoria, Australia. Results suggest that the ratio of current speed to wind speed is larger under the long fetch condition and the angle between surface current vector and wind vector is smaller, suggesting the increasing significance of Stokes drift with fetch as the sea develops. Surface current data under different fetch conditions is analysed with an integration of Ekman theory and Stokes mass transport theory, which decomposes the currents into two components, i.e. Stokes drift and currents generated by wind stress. The analysed magnitude of Stokes drift agrees well with results derived from the empirical wave growth function. Results suggest that Stokes drift dominates the total currents in the mature sea state under the long fetch condition, whereas currents generated by wind shear stress is as important as Stokes drift under the short fetch condition when the sea is not well developed. As both the magnitude and the direction dependency of Stokes drift on wind are different from those of wind-generated currents, fetch plays a significant role in the total response of surface current to wind as it determines the sea state and therefore the contribution of Stokes drift in the total current. These results provide observational evidence that currents measured by HF ocean surface radar include Stokes drift, and therefore demonstrate the potential of HF radar measurement in providing insights into the mechanisms of the momentum transfer from air to sea.