A prescriptive bulk model of periodic estuarine stratification driven by density currents and tidal straining

A prescriptive bulk model of periodic estuarine stratification is developed using potential energy anomaly arguments and a bulk vertical eddy viscosity model derived a priori. In the model, vertical mixing is dominated by tidal stirring except around slack water when other weaker 'residual' mixing mechanisms, such as due to the shear induced by the gravitational circulation and wind stirring, dominate. The model under certain conditions exhibits bi-stable solutions corresponding to the well-mixed and stratified states found on either side of a tidal mixing front. In the transition from spring and neap tidal cycles, a stratification state change can occur depending on the strength of residual mixing. The model can account for the phase lag observed between changes in tidal stirring and the onset and persistence of stratification, and leads to new theoretical criteria for the position of tidal mixing fronts.