Effects of Locally Generated Wind Waves on the Momentum Budget and Subtidal Exchange in a Coastal Plain Estuary

A numerical model with a vortex force formalism is used to study the role of wind waves in the momentum budget and subtidal exchange of a shallow coastal plain estuary, Delaware Bay. Wave height and age in the bay have a spatial distribution that is controlled by bathymetry and fetch, with implications for the surface drag coefficient in young, underdeveloped seas. Inclusion of waves in the model leads to increases in the surface drag coefficient by up to 30% with respect to parameterizations in which surface drag is only a function of wind speed, in agreement with recent observations of air-sea fluxes in estuaries. The model was modified to prevent whitecapping wave dissipation from generating breaking forces since that contribution is integrally equivalent to the wind stress. The proposed adjustment is consistent with previous studies of wave-induced nearshore currents and with additional parameterizations for breaking forces in the model. The mean momentum balance during a simulated wind event was mainly between the pressure gradient force and surface stress, with negligible contributions by vortex, wave breaking (i.e., depth-induced), and Stokes-Coriolis forces. Modeled scenarios with realistic Delaware bathymetry suggest that the subtidal bay-ocean exchange at storm time scales is sensitive to wave-induced surface drag coefficient, wind direction, and mass transport due to the Stokes drift. Results herein are applicable to shallow coastal systems where the typical wave field is young (i.e., wind seas) and modulated by bathymetry. Plain Language Summary Water circulation and pollutant flushing in estuaries, bays, and similar coastal environments depend mainly on tidal currents, winds, and density differences between fresh and saltwater. However, winds normally coexist with waves, and the impact of waves on circulation patterns is often overlooked in most numerical models. In this study we used a numerical model to explore how Delaware Bay responds to a typical storm event. The model was configured to consider or ignore the effects of waves on circulation, and we used both options to contrast and compare the simulated results. The model that ignored waves could not reproduce the mechanisms that have been shown to control the energy transfer from the atmosphere to the water column. In agreement with recent observations in estuaries, the model indicates that wind waves in shallow water have an important regulating effect on the roughness of the sea surface, which is key for surface drag and circulation. We also report that even when the wind speed is the same above the water surface, the spatial distribution of waves determines the effective amount of energy that gets transferred from the air to the water column.