Wind Influence on Gravitational Circulation in a Partially Mixed Estuary

In a study aimed at determining the role of wind and wind mixing in estuarine circulation we examined both immediate and seasonal responses. Employing field observations in Chesapeake Bay and numerical experiments, we sought to separate wind vs. tide in driving the classical flow, but also to improve our description of the direct wind response. We found that the phased, two-layer response to an applied wind stress is superposed on the quarter-wave seiche response, with upward phase propagation depending on longitudinal position in the estuary. For the gravitational flow, a traditional measure has been the mean or low-frequency component. Long-term records show that the wind can dominate the bulk shear, not only over event scales, but over seasons, with river flow a secondary influence. The strength of the gravitational flow is modulated by local hydraulics. Remote hydraulics influence mid-estuary gravitational flows through high-salinity intrusions at a lower-estuary control point. Wind appears to be the primary regulator of these intrusions. The emerging picture of estuarine circulation in partially mixed estuaries such as Chesapeake Bay is one of nearly continuous gravitational adjustment to wind perturbations rather than a steady flow driven by tidal mixing. The complexity of wind’s dominant contribution, through direct driving, mixing, and wind-triggered intrusions hampers a proper separation and description of the slower-varying gravitational flow in this pulse-response system.