Simulating the Delaware Bay Buoyant Outflow: Comparison with Observations

Abstract

Coastal buoyant outflows from rivers and estuaries previously have been studied with field research, laboratory experiments, and numerical models. There is a dire need to evaluate model performance in light of coastal current observations. This research simulates the Delaware Bay outflow and compares results with observations of estuarine and shelf conditions. Observations include an estuarine salinity climatology, a record of freshwater delivery to the shelf, coastal current salinity mappings, and surface drifter data. Simulation efforts focus on spring 1993 and spring 1994, the primary field study period. The simulation is forced with river discharge, winds, and tides; only tidal-averaged results are discussed. Estuarine salinity results are consistent with the observed lateral salinity pattern, vertical structure, and response to river discharge. Salinities within the lower bay agree with observations, but the simulation overestimates the along-estuary salinity gradient. Observed and simulated freshwater delivery exhibit the same amplitude of response to river discharge and winds. The simulation produces a buoyant outflow that is generally consistent with the observed buoyancy signature, width, length, and vertical structure over a variety of river discharge and wind conditions. The simulated coastal current, however, tends to be somewhat shorter and fresher than observed. Simulated surface drifter paths exhibit the observed onshore advection during downwelling winds as well as offshore transport and current reversals during upwelling winds. A statistical evaluation based on shelf salinity mappings indicates that the model reproduces the observed variance and has only a small bias (less than 10% of plume buoyancy signature). The rms error of 1.2 psu is linked to the shorter and fresher nature of the simulated coastal current. Observational comparisons discussed in this paper indicate that the model can simulate many coastal current features and its response to river discharge and wind forcing.