A stochastic model for wind‐driven currents over the continental shelf

Abstract

A stochastic model for current, pressure, and density fluctuations over the continental shelf and slope is solved for coherence, phase, and gain between the oceanographic variables arid wind stress. Comparison of wind stress spectra and transfer functions show that free coastal‐trapped wave physics tends to exist in a frequency band bounded at the low end because of frictional predominance and at the high end because of the absence of wind energy at appropriate alongshore length scales. The transfer functions for density and especially cross‐shelf velocity show that these variables are sensitive to short length scales in the forcing and are thus difficult to predict in general. Model results are compared to observations of bottom pressure and alongshore velocity from the 1982 Coastal Ocean Dynamics Experiment off northern California. The results agree with observations, at least qualitatively, with regard to spatial and frequency patterns in coherence and gain. The poorest point of agreement is in the amplitude of wind stress‐current gains, which the model systematically underpredicts. The comparisons do demonstrate the importance of including accurate representations of bottom friction and of both cross‐shelf and alongshore gradients of wind stress amplitude.