TRANSPORT AND FATE OF LOW-DENSITY WATER IN A COASTAL FRONTAL ZONE

We have examined the transport and fate of low-density water in a coas tal frontal zone located on the continental shelf of the southeastern United States. About 800 m3 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 coasta l frontal zone is transported seaward to the middle shelf. There, the transport is augmented by alongshelf transport of low-salinity water a pparently 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 diffus ion coefficient of O(100) m2 s-1 represented the observed loss. Among the factors contributing to the loss were: (1) decreased alongshelf wi nd 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 co uld partially account for some of the observed cross-shelf transport o f low-density water. Estimates based on analyses of tidal currents ind icate 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 est imates of the offshore transport of particulate aluminum.During wind r elaxation or reversal, the pressure field collapses and briefly bifurc ates the alongshelf currents inside the 20 m isobath. Bifurcation is c onfirmed in observations and models. This mechanism would coincide wit h an offshore tilt of the coastal frontal zone and would efficiently r emove low-density water in surface layers, thus confirming earlier hyp otheses.