Hysteresis of the Annual Exchange Circulation in the Tampa Bay Estuary

Abstract:

A nonlinear relation between the salinity field and the subtidal exchange circulation in the Tampa Bay estuary is demonstrated using 1999-2011 measurements of river discharge, salinity and velocity. These data are binned and averaged to form mean monthly values of freshwater input Q, axial (vertical) salinity gradient ΔS (Δ_zS), and subtidal vertical shear σ, respectively. These interrelated quantities are found to cycle through three dynamical regimes. The first regime (January to May/June) is a vertically well-mixed state (Δ_zS≈0) when Q is low and a complex relation between ΔS and σ occurs. This time period has high Δ_zS with an estimated gradient Richardson number R_i ~0.25. A value of R_i > 0.25 is usually stably stratified. Regimes II and III are both partially mixed states and both show a near-linear relation σ=cΔS+c₀. Regime II (July-Sept) is characterized by rapidly increasing Q, high ΔS, c=-0.053 km psu^-1s^-1 and R_i~0.6. The third regime (Oct-Dec) has low Q, and includes high to minimal ΔS, relatively high ΔS, c=-0.011 km psu^-1s^-1 and R_i ~0.4. This asymmetric annual cycle generates hysteresis in the exchange circulation relative to the axial salinity gradient where σ is multi-valued with regard to ΔS, forming a loop in state space. The loop forms because the σ developed during II persists through III even when the original source of its formation (ΔS) has declined to minimal levels. We hypothesize this large Richardson number during III suppresses vertical mixing of momentum and allows the exchange circulation to maintain higher values.