Observations of the response of a buoyant estuarine plume to upwelling favorable winds

Abstract

The response of the buoyant plume from the Chesapeake Bay to upwelling favorable winds and during conditions of low freshwater discharge is examined using an offshore array of bottom-deployed acoustic Doppler current profilers (ADCPs) as well as ancillary data. During periods of northward (upwelling favorable) winds the observed response is strong offshore flow (U > 0.2 m/s) in a shallow layer of plume water and a rapid reversal of the near-surface flow from the usual southward flow associated with inertial turning of the plume. The offshore migration of the plume front is deduced from sequential arrivals at the various mooring locations. The orientation of the plume front is approximately alongshore (north-south), suggesting that the upwelling response is two-dimensional. A plume thickness calculated from the ADCP data is relatively constant at about 4 m as the plume moves across the moorings. The offshore propagation speed of the plume front of about 0.20 m/s matched the advection speed predicted by Ekman theory using the calculated plume layer thickness. The mooring velocity profiles provide a view of the across-plume structure in a reference frame fixed to the front. This reveals a pattern of circulatory motion with sinking of plume and ambient water at the front and an ascending motion occurring inshore, suggesting a recirculation between the seaward and inshore portions of the plume. These results are consistent with recent model simulations of the response of a river plume during upwelling favorable winds.