Transport and fate of low-density water in a coastal frontal zone

Abstract

We have examined the transport and fate of low-density water in a coastal frontal zone located on the continental shelf of the southeastern United States. About 800 m 3 s −1 of freshwater in the frontal zone was lost seaward over an alongshelf distance of 250 km. During moderate to strong southward wind stress events, low-salinity water in the coastal frontal zone is transported seaward to the middle shelf. There, the transport is augmented by alongshelf transport of low-salinity water apparently lost from the frontal zone further upstream. Most of the low-salinity water is lost to the outer shelf south of the center of the bight-shaped coastline. A simple Fickian diffusion model with a diffusion coefficient of O(100) m 2 s −1 represented the observed loss. Among the factors contributing to the loss were: (1) decreased alongshelf wind stress in the southern part of the area due primarily to the bight-shaped nature of the coast; and (2) cross-shelf mixing due to the time-dependent flow. Dispersion due to vertical shear in tidal currents could partially account for some of the observed cross-shelf transport of low-density water. Estimates based on analyses of tidal currents indicate that a cloud of freshwater at the coast would spread seaward at a rate of about 2 km per day. This rate is consistent with earlier estimates of the offshore transport of particulate aluminum. During wind relaxation or reversal, the pressure field collapses and briefly bifurcates the alongshelf currents inside the 20 m isobath. Bifurcation is confirmed in observations and models. This mechanism would coincide with an offshore tilt of the coastal frontal zone and would efficiently remove low-density water in surface layers, thus confirming earlier hypotheses.