Observations and Modelling of Periodic Stratification in the Upper York River Estuary, Virginia

Two current meter moorings were deployed simultaneously for 61 days during the winter of 1989-90 in the Upper York River Estuary, Virginia, at a separation along the estuary axis of one tidal excursion. Both moorings were equipped with four current meters measuring conductivity, temperature, and current velocity, and a surface conductivity-temperature recorder, giving a detailed picture of the vertical structure of the water column. The time series of the vertical density structure measured by both moorings show closely similar periods of mixed and stratified conditions along the estuary. Comparison with the tidal currents over the same period demonstrates lower stratification or complete mixing to be associated with the strong spring tidal currents, while significant 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 with peaks in the surface wind stress. A numerical model, used successfully 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 closure scheme to relate the efficiency of vertical turbulent mixing to the local water-column stability. Running the model with a depth-dependent horizontal density gradient results in no significant stratification predicted for the entire deployment. By incorporating depth variability of the horizontal density gradient, estimated from the observations from the two moorings, the model reproduces the qualitative features of the observed evolution of stratification.