Interior fronts in a coastal buoyant plume: scaling theory and observations

We propose a mechanism for the formation of interior fronts within the bulge area of a coastal buoyant plume. The fronts are produced by a combination of tidally-modulated estuarine outflow and a wind-induced offshore Ekman drift. The parameter space favorable for the interior front formation is defined in terms of the freshwater discharge, salinity anomaly of the estuarine outflow (indicative of the estuarine mixing), wind stress, and the Coriolis parameter. The existence of interior fronts with associated geostrophic jets depends on the competition between wind stress and buoyancy forcing: when the wind stress is excessive, the buoyant layer is homogenized through wind-induced advection and mixing, while for the weak wind stress conditions, consecutive tidal fronts are not spatially separated. Scaling theory is applied for the interpretation of two shipboard surveys conducted in October and November of 2015 off Winyah Bay, SC. During the first cruise, no interior fronts were observed, while the second cruise revealed the existence of one or two interior fronts within the anticyclonic bulge separated by ~6 km, consistent with scaling arguments. Interior fronts were characterized by step-like increase of the plume depth, lateral salinity gradients, and enhanced vertical mixing. The latter can be caused by superposition of geostrophic and wind-induced shear. Further examples of interior fronts in the study area are frequently seen in satellite images.