On the influence of downwelling winds on the Chesapeake Bay outflow

With the purpose of studying the hydrography and flow structure off the mouth of the Chesapeake Bay, a series of transects were sampled continuously with a 600 kHz acoustic Doppler current profiler (ADCP) and a thermosalinograph during late September 1995. Hydrographic (CTD) stations were combined with underway measurements and occupied every 4 km along the transects to look at the vertical structure of the density field. This is the first time an ADCP has been towed in the waters off the Chesapeake Bay mouth. The study was carried out at the end of a two-day period of northeasterly winds. The surface salinity distribution showed that winds kept a well-defined Chesapeake Bay plume within a few kilometers from the coast. Near-bottom salinity fields displayed weaker horizontal gradients than the near-surface field. The ADCP observations yielded excellent resolution of the flow field from which a subtidal distribution was inferred. The subtidal near-surface flow showed a spatially coherent southward component in the area of observation in response to the downwelling winds. The southward coastal ambient flow advected the turning region of the plume to the south of the mouth of the estuary, which was consistent with numerical model results. The region influenced by the plume could be defined from the subtidal flows by an area where the difference between the near-surface and near-bottom flows was large. This area, off the Chesapeake Bay mouth, overlapped with that of subtidal flow divergence as calculated with the near-surface subtidal flow. Within the region of plume influence, the balance was probably semigeostrophic with modifications by friction in the along-flow direction. Outside that region, the momentum balance was apparently dominated by friction.