The effect of wind direction on cross-shelf transport on an initially stratified inner shelf

Cross-shelf and along-shelf wind stresses both independently have been shown to drive transport across the inner shelf. We investigate how the circulation and density fields respond to simultaneous cross- and along-shelf wind forcing using a set of model experiments on an initially stratified inner shelf. For all wind directions with a downwelling along-shelf component, the inner shelf is unstratified and the cross-shelf wind dominates transport. For wind directions with an upwelling along-shelf component, the inner shelf can be stratified and the transition from inner to midshelf occurs over a wide swath of shelf. In this transition region, the effects of the cross- and along-shelf wind stresses are not separable, because the response to each component is tied to the other by the density field on which they both act. A consequence of this asymmetry is that offshore transport occurs for more compass directions, and more strongly than onshore transport, making offshore transport more likely under variable wind conditions in the field, even if the average wind stress were near zero. These model results demonstrate a strongly asymmetric directional dependence of wind-driven, cross-shelf transport on the inner shelf. Field observations from the inner shelf of Martha's Vineyard, Massachusetts, substantiate both the directional dependence and magnitude of the transport predicted by the model.