A simple model of estuarine subtidal fluctuations forced by local and remote wind stress

Abstract

Observations of estuarine low subtidal sea level and current fluctuations have often shown domination by the remote effects of the wind, acting on the adjacent coastal ocean, over the local surface stress, acting on the estuary itself. The remote effects are transmitted to the estuary by impressment on its mouth of sea level change induced by the onshore component of coastal Ekman transport and become increasingly dominant as the frequency decreases. A simple, barotropic model is developed to investigate the joint action of these two wind forcing mechanisms. The relative shortness of most estuaries relative to low subtidal estuarine wavelengths explains the dominance of the remote effect for both sea level and barotropic current fluctuations in the estuary. For the same reason, however, surface slope is dominated by local wind setup. For an estuary with axis nearly parallel to the coast the two effects will operate either in concert or in opposition, depending on hemisphere and orientatipn of the estuary axis relative to the coast. For the geometries of both Chesapeake Bay and the Delaware Estuary the model predicts opposition with the remote effect dominant at lower frequencies, consistent with recent observations.