ON THE INFLUENCE OF WIND-STRESS CURL ON LOW-FREQUENCY SHELF WATER-FLOW

An approximate solution for wind-driven, near-barotropic now over the continental shelf is obtained. The solution shows that a wind stress with amplitude that increases offshore and then is relatively constant, as observed during the Coastal Ocean Dynamics Experiments (CODE), results in two types of shear. The horizontal shear is associated with the offshore increase of the wind stress, which strengthens the now. The vertical shear is associated with the alongshore wind-stress amplitude and its first two offshore derivatives. For a wind-stress structure similar to the one observed during CODE, the dominant vertical shear is associated with partial derivative(2) tau(y)/partial derivative X(2), where tau(y) is the alongshore wind stress and X is the distance seaward from the coast. Physically, partial derivative(2) tau(y)/partial derivative X(2) nonzero means that the surface Ekman pumping will vary with X, tilt isopycnals, and produce a vertical shear in the alongshore velocity in accordance with the thermal wind relation. The near-barotropic solution for the alongshore velocity field obtained with an exponential bathymetry and idealized wind forcing is very similar to the numerical solution obtained with the same parameters and is qualitatively similar to the observed structure and to numerical solutions obtained with a more realistic bathymetry. These results confirm previous work about the importance of wind-stress curl in determining the structure of the observed alongshore velocity flow during CODE and provide a simple physical explanation for the observed structure.