OBSERVATIONS OF BAROTROPIC AND BAROCLINIC EXCHANGES IN THE LOWER CHESAPEAKE BAY

Current meter observations from four instruments deployed in two moori ngs in the area of the Chesapeake Bay mouth have been analyzed to rela te the flow structure to meteorological and tidal forcing. The current velocities and water temperatures recorded by these instruments have been compared to meterologial and sea level data from a tide gauge loc ated in the same area. The data were collected in the period July-Sept ember 1993, during the passage of hurricane Emily over the coast of No rth Carolina and off the coast of Virginia. Intratidal velocity fluctu ations coincide with near-bottom temperature variations. Near-bottom t emperature oscillations are of greater magnitude than those near the s urface and are predominantly semidiurnal. During flood periods, bottom temperature typically drops 6 degrees C with respect to its ebb value . Temperature behavior suggests self-adjustment of a longitudinal temp erature gradient during neap tides and tidal advection of such gradien t in spring tides. Low-pass filtered fluctuations of temperature and f low, and hence water exchange at the bay mouth, appear to be caused by the superposition of wind events and gravitational circulation modula ted 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 a nd near-bottom inflow, occurs during neap tides.