Observations of barotropic and baroclinic exchanges in the lower Chesapeake Bay

Abstract

Current meter observations from four instruments deployed in two moorings in the area of the Chesapeake Bay mouth have been analyzed to relate the flow structure to meteorological and tidal forcing. The current velocities and water temperatures recorded by these instruments have been compared to meteorologial and sea level data from a tide gauge located in the same area. The data were collected in the period July–September 1993, during the passage of hurricane Emily over the coast of North Carolina and off the coast of Virginia. Intratidal velocity fluctuations coincide with near-bottom temperature variations. Near-bottom temperature oscillations are of greater magnitude than those near the surface and are predominantly semidiurnal. During flood periods, bottom temperature typically drops 6°C with respect to its ebb value. Temperature behavior suggests self-adjustment of a longitudinal temperature gradient during neap tides and tidal advection of such gradient in spring tides. Low-pass filtered fluctuations of temperature and flow, and hence water exchange at the bay mouth, appear to be caused by the superposition of wind events and gravitational circulation modulated by the spring-neap tidal cycle. Wind stress produces a barotropic response on the residual flow. Southwestward winds drive coastal ocean water into the bay and northeastward winds drive water out of the bay. The development of gravitational circulation, near-surface outflow and near-bottom inflow, occurs during neap tides.