Response of a Coastal Plume Formed by Tidally Modulated Estuarine Outflow to Light Upwelling-Favorable Wind

This study presents observations of a buoyant plume off Winyah Bay, South Carolina, which was formed under conditions of high freshwater discharge and upwelling-favorable wind forcing. Analysis of observations demonstrates that the response of the anticyclonic bulge formed by tidally modulated estuarine outflow to the light upwelling-favorable wind is more complex than the previously studied far-field response. The latter can be described by a slab-like model with mixing concentrating at the offshore edge of a buoyant layer. The observed plume depth increased from ~3 m near the mouth to 6 m at the offshore edge, with plume depth changing in a steplike fashion rather than continuously. CTD profiles near these steps revealed overturning indicative of vigorous mixing. Estimates of the gradient Richardson number confirmed the likelihood of mixing/entrainment not only at the offshore edge of the plume but also in the proximity of the observed steps. We hypothesize that these steps represent tidal fronts that undergo geostrophic adjustment and are advected offshore by the superimposed Ekman drift. Scaling analysis suggests that mixing and entrainment at the observed interior fronts can be enhanced by superposition of geostrophic and wind-induced shear.