Hysteresis-like Behavior of the Monthly Mean Density-Driven Circulation in Tampa Bay

A nonlinear relation between the salinity field and the subtidal exchange circulation in the Tampa Bay estuary is demonstrated using observational data from 1999-2011. The data are averaged to form mean monthly climatological values of total freshwater discharge (Q), axial and vertical salinity gradients, and subtidal vertical shear. Over the mean annual cycle the hydrodynamics are found to form a hysteresis-like loop in parameter space that passes through three dynamical regimes: Regime I is relatively dry with weak salinity gradients and exchange circulation. Regime II is the wet season (June-September) in which all quantities rapidly increase. In regime III the exchange flow persists even though Q and the axial salinity gradient are again low. Gradient Richardson numbers and Simpson numbers also form a loop in parameter space with Ri remaining subcritical (turbulent) until the wet season when Ri rises above criticality (weak vertical mixing) where it remains through the end of regime III. The Simpson number is in a narrow range around 0.2, indicating that the horizontal salinity gradient is always a driver of the exchange circulation. The vertical eddy viscosity (Az), estimated from a parameterization of the Richardson number, decreases by almost an order of magnitude from regime I to II. The Az remains low during III, indicating the persistent stratification is insulating the exchange flow from destruction by tidal mixing during this time period. Early results from other large estuaries indicate similar behavior.