Observations of the wind-induced exchange at the entrance to Chesapeake Bay

Abstract

Water density and velocity data from two ;75-day deployments across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the transverse structure of wind-induced subtidal exchange. Acoustic Doppler current proe lers, electromagnetic current meters, and conductivity-temperature-depth recorders were deployed at the entrance to the bay from mid-April to early July of 1999 and from early September to mid-November of 1999. Three main scenarios of wind-induced exchange were identie ed: (1) Northeasterly (NE) winds consistently drove water from the coast toward the lower Chesapeake Bay as well as water from the upper bay to the lower bay, which was indicated by the surface elevation slopes across the lower bay and along the bay. This resulted in water piling up against the southwestern corner of the bay. The subtidal e ow over the southern portion of the bay entrance was directed to the left of the wind direction, likely the result of the ine uence of Coriolis and centripetal accelerations on the adjustment of the sea level gradients. Over the northern shallow half of the entrance, the subtidal e ows were nearly depth-independent and in the same direction as the wind. (2) Southwesterly (SW) winds caused opposite sea level gradients (relative to NE winds), which translated into near-surface oute ows throughout the entrance and near-bottom ine ows restricted to the channels. This windinduced circulation enhanced the two-way exchange between the estuary and the adjacent ocean. (3) Northwesterly winds produced the same exchange pattern as NE winds. Water piled up against the southwestern corner of the bay causing net oute ow in the deep, southern area and downwind e ow over the shallow areas. Northwesterly winds greater than 12 m/s caused the most efe cient e ushing of the bay, driving water out over the entire mouth of the estuary.