Effects of Wind Straining on Estuarine Stratification: A Combined Observational and Modeling Study

A combined observational and numerical modeling study was conducted to clarify the effects of wind straining on estuarine stratification. Long‐term mooring observations in the middle of Chesapeake Bay showed an asymmetric stratification response to along‐channel winds. The stratification decreased under up‐estuary winds. Under down‐estuary winds, however, the stratification increased at moderate wind speeds but decreased at high wind speeds. In concert with numerical modeling, the mooring and ship‐based survey data were analyzed to test the wind‐straining mechanisms. In the middle and upper parts of the estuary, the straining of the density field by the vertical shear of the along‐channel current dominates the stratification response, enhancing stratification under the down‐estuary winds but reducing it under the up‐estuary winds. A regime diagram was constructed to place the wind‐induced stratification change in terms of the dimensionless Wedderburn number W : it is a linearly function of W for W  > 0 (up‐estuary winds) but a parabolic function of W for W  < 0 (down‐estuary winds). In the lower part of the estuary, however, the stratification response to the wind direction was opposite to that in the middle and upper Bay. The wind‐driven lateral circulation was much stronger there, and the cross‐channel straining of the density field by the vertical shear in the lateral currents drove the stratification response in the lower Bay. Therefore, both the along‐channel and cross‐channel straining regulate the estuarine stratification, and the net stratification change over a wind event is determined by the relative strength of along‐channel and cross‐channel straining.