On meteorologically induced flushing in three U.S. east coast estuaries

Abstract

Meteorologically induced exchange with the coastal ocean is examined for three U.S. east coast estuaries using time series of subtidal volume flux over a common five-month period. A volume of water equal to that of the estuary was discharged every 86·4, 135·3 and 108·8 days for Narragansett, Delaware and Chesapeake Bays, respectively. However, a large fraction of the water discharged to the shelf is recirculated ocean water. For Chesapeake Bay, outflowing estuarine water can be considered as a mixture of 2·4–3·0% water of mean estuarine salinity and the remainder water of oceanic salinity. The degree of meteorological flushing is strongly influenced by mean depth, as a drop in sea level of a given amplitude will force a greater fraction of water from a shallow estuary than from a deep system. The relative importance of the meteorologically forced circulation and the gravitational circulation is frequency dependent, as illustrated by salinity variance at the entrance to Chesapeake Bay. Seasonal salinity variance must ultimately be controlled by seasonal changes in runoff, through the mechanism of gravitational circulation. However, the high coherence of wind-induced changes in bay volume and bay entrance salinity at periods of 2–20 days suggests that meteorologically forced advection is primarily responsible for salinity variance at these time scales.