Observation and simulation of wind-driven lateral circulation in Chesapeake Bay

Three-month long mooring data are collected in Chesapeake Bay to investigate the lateral circulation driven by wind. The clockwise and counterclockwise lateral circulations (look into estuary) derived by the Ekman transport are observed during up-estuary and down-estuary winds, respectively. Different from the clockwise circulation, the counterclockwise circulation shows an asymmetry because the lateral velocity in the left side of the circulation is significantly weakened. In the entire observation period, the observed surface lateral (sub-tidal) velocity and along-channel wind speed have a linear relationship, but it occurs only in the right (eastern) side of the bay because of the asymmetry. During the set-down phase of up-estuary and down-estuary winds, enhanced bottom lateral currents can be observed. However, the enhanced bottom current during down-estuary wind only appears in the right side of the bay, while it can be found in the entire cross-channel section during up-estuary wind. In the deep channel (right side of the bay), there is a linear correlation between the bottom lateral current and along-channel sub-tidal velocity, suggesting that the enhanced bottom lateral current is generated by the bottom stress associated with the along-channel current. A realistic ROMS model reproduces temporal variation of the surface and bottom lateral circulations during up-estuary and down-estuary well. The diagnostic analyses of the momentum equation in ROMS model suggest that the lateral pressure gradient, which is induced by water accumulation in the left side of the bay and steepening of isopycnals caused by the counterclockwise circulation, inhibits surface Ekman transport under the down-estuary wind and generates a clockwise lateral circulation in the end of the down-estuary wind in the left side of the bay. Our observations and numerical simulations indicate that the sub-tidal lateral circulation in Chesapeake Bay is dominated by the Ekman transport caused by wind.