OBSERVATIONS AND MODELING OF PERIODIC STRATIFICATION IN THE UPPER YORK RIVER ESTUARY, VIRGINIA

Two current meter moorings were deployed simultaneously for 61 days du ring the winter of 1989-90 in the Upper York River Estuary, Virginia, at a separation along the estuary axis of one tidal excursion. Both mo orings were equipped with four current meters measuring conductivity, temperature, and current velocity, and a surface conductivity-temperat ure recorder, giving a detailed picture of the vertical structure of t he water column. The time series of the vertical density structure mea sured by both moorings show closely similar periods of mixed and strat ified conditions along the estuary. Comparison with the tidal currents over the same period demonstrates lower stratification or complete mi xing to be associated with the strong spring tidal currents, while sig nificant stability develops during weaker currents. Such a spring-neap signal is caused by the modulation of tidal mixing energy which is in competition with the stratifying estuarine circulation. Superimposed on this tidal signal are more irregular remixing events associated wit h peaks in the surface wind stress. A numerical model, used successful ly in earlier work in a shelf sea region, is applied to the Upper York River regime. The model employs a Mellor-Yamada level 2 turbulence cl osure scheme to relate the efficiency of vertical turbulent mixing to the local water-column stability. Running the model with a depth-depen dent horizontal density gradient results in no significant stratificat ion predicted for the entire deployment. By incorporating depth variab ility of the horizontal density gradient, estimated from the observati ons from the two moorings, the model reproduces the qualitative featur es of the observed evolution of stratification.