Observations of hysteresis in the annual exchange circulation of a large microtidal estuary

Abstract

A nonlinear relation between the salinity field and the subtidal exchange circulation in the Tampa Bay estuary is demonstrated using observational data from 1999 to 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. Well-known steady state solutions indicate that the exchange circulation is linearly proportional to the horizontal salinity gradient, assuming a constant vertical eddy viscosity (A eff ). The exchange flow is found to be multivalued with respect to the horizontal salinity gradient, forming a hysteresis 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 A eff , estimated from a parameterization of the Richardson number, decreases by almost an order of magnitude from regimes I to II. It remains low during III, indicating that the persistent stratification is insulating the exchange flow from destruction by tidal mixing during this time period.

The effect of waves in hydrodynamics, stratification, and salt wedge intrusion in a microtidal estuary

“…The potential energy anomaly is defined as In addition to the stratification parameters mentioned, the Simpson dimensionless number (Simpson et al, 1990) was used. The Simpson number (Si) evaluates the balance between the contribution of the longitudinal density gradient as a driver of circulation and stratification versus the input of the bottom friction velocity as a mixing force in estuaries (Meyers et al, 2015). To evaluate the effect of waves on stratification, the bottom friction velocity scale was modified (U* 2 = t bed r ), where t bed is the bed shear stress considering the tide, and tidal effects with the waves.…”

Seasonal Variability of Tidal Currents in Tampa Bay, Florida

“…Water temperature is seasonal, however tends to be more horizontally and vertically uniform than salinity (Zervas 1993). Relating the spatial gradients of temperature and salinity reveals that changes in density in the Bay are nearly entirely controlled by salinity (Meyers et al 2015).…”

“…Non-tidal circulation in the Bay is influenced by short-term factors like winds and freshwater input and was found to be significantly altered by extreme weather events like Hurricanes (Wilson et al 2006). On seasonal scales, variability in freshwater input and vertical stratification has been shown to significantly alter Tampa Bay's subtidal circulation (Meyers et al 2015).…”