IMPACTS OF WINDS AND RIVER FLOW ON ESTUARINE DYNAMICS AND HYPOXIA IN CHESAPEAKE BAY

Abstract

In the stratified rotating estuary of Chesapeake Bay, the driving mechanisms of wind-induced lateral circulation are examined using a three-dimensional hydrodynamic model (ROMS). A new approach based on the streamwise vorticity dynamics is developed, and the analysis reveals a balance among three terms: the conversion of the planetary vorticity by along-channel current shear, baroclinicity due to cross-channel density gradient, and turbulent diffusion. It is found that the lateral flow in the Bay is mainly driven by the Ekman forcing, but the lateral baroclinicity creates asymmetry in the streamwise vorticity between down- and up-estuary winds.

The traditional view of wind-driven circulation in estuaries ignores the lateral circulation, but wind-induced lateral flows can affect subtidal estuarine circulation and stratification. Coriolis acceleration associated with the lateral flows is of first-order importance in the along-channel momentum balance, with the sign opposite to the stress divergence in the surface layer and the pressure gradient in the bottom layer, thereby reducing the shear in the along-channel current. Moreover, the lateral straining of the density field by lateral circulation offsets the along-channel straining to control the overall stratification. Regime diagrams are constructed using the dimensionless Wedderburn (<italic>W</italic>) and Kelvin (<italic>Ke</italic>) numbers to clarify the net wind effects.

A coupled hydrodynamic-biogeochemical model is developed to simulate the seasonal cycle of dissolved oxygen in Chesapeake Bay and investigate key processes which regulate summer hypoxia in the estuary. Diagnostic analysis of the oxygen budget for the bottom water reveals a balance between physical transport and biological consumption. In addition to the vertical diffusive flux, the along-channel and cross-channel advective fluxes are found to be important contributors in supplying oxygen to the bottom water. While the vertical diffusive oxygen flux varies over the spring-neap tidal cycle and is enhanced during wind events, the advective oxygen fluxes show long-term averages due to the gravitational estuarine circulation but display strong oscillations due to wind-driven circulations. It is found that water column respiration comprises about 74% of the total consumption and sediment oxygen demand contributes 26%. Sensitivity-analysis model runs are conducted to further quantify the effects of river flow, winds, water column respiration and sediment oxygen demand on the hypoxic volume in the estuary.