Fresh water delivery to the continental shelf and subsequent mixing: An observational study

We address two questions related to buoyant coastal discharge: (1) What agents control the delivery of estuarine fresh water to the shelf? (2) How is this fresh water mixed with shelf water? Our subject was the Delaware Coastal Current, the buoyancy driven current originating in the Delaware estuary. The delivery of freshwater to the shelf at subtidal frequencies is controlled by two forcing agents: upland freshwater discharge into the estuary and the component of the wind parallel to the estuarine axis. The mean current we observed in the source region showed a strong decline in speed and large counterclockwise veering with increased depth. These variations in the mean current are best explained by thermal wind shear. From the moored instrument records we found evidence that in the source region mixing events of several hours duration are common at tidal frequencies when the local Richardson number Ri drops to a minimum level below 1/4. Furthermore, there is a pronounced modulation of this minimum level for Ri controlled by the action of coastal Ekman circulation. Data from the satellite-tracked drifters show the striking difference between the coastal current configuration during downwelling and upwelling events. During downwelling the flow is downshelf and weakly onshore with particle trajectories orthogonal to the mean horizontal salinity gradient. In contrast, during upwelling the flow is strongly offshore and somewhat upshelf with particle trajectories down the mean salinity gradient, implying rapid mixing of plume water with shelf water. Corresponding values for horizontal dispersion of plume water showed modest values for both the along-shelf and across-shelf directions under downwelling but a very large value for the across-shelf dispersion under upwelling. We conclude that wind stress, acting through the mediation of the strain field produced by coastal upwelling circulation, is the primary means for completing the mixing of fresh water within the plume with shelf water.