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

Abstract

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.

Large Projected Decline in Dissolved Oxygen in a Eutrophic Estuary Due to Climate Change

“…The combined effects of sea level rise and larger winter‐spring runoff lead to stronger stratification in the future climate, resulting in larger reductions in the vertical diffusive and advective O 2 fluxes to the bottom water. While turbulent mixing is known to be suppressed in stratified water, previous studies (Lerczak & Geyer, ; Scully, ; Li et al, ; Xie & Li, ) also showed that vertical advection due to lateral circulation decreases under stronger stratification. Therefore, one certain aspect about the impact of climate change on estuarine hypoxia is the increasing stratification and decreasing vertical supply of O 2 to the bottom water.…”

Controls on Carbonate System Dynamics in a Coastal Plain Estuary: A Modeling Study

“…The hydrodynamic model used in this study was based upon the Regional Ocean Modeling System (ROMS). ROMS has been validated against a wide range of observational data and has exhibited considerable capacity in reproducing estuarine dynamics at tidal, synoptic, and seasonal time scales in Chesapeake Bay (M. Li et al, , ; Xie & Li, ; Zhong & Li, ). The horizontal mesh grid is 80 × 120 cells with ~1‐km cell width (Figure ).…”

Axial Wind Effects on Stratification and Longitudinal Sediment Transport in a Convergent Estuary During Wet Season

“…This pattern is also applicable to the narrow upper bay (low Ke) of a convergent estuary but not applicable to the wide lower bay. The lateral current and lateral straining are stronger than the longitudinal ones in the lower bay, as proposed by Xie and Li (2018).…”

“…Compared to the N 2 changes during the neap tide, changes in stratification were less significant during spring tides. The opposite response of the upper and Journal of Geophysical Research: Oceans lower bay regions to axial winds is not consistent with the effects of longitudinal wind straining (Scully et al, 2005), and lateral wind straining and wind-driven mixing (Chen & Sanford, 2009a), but this pattern may be induced by the combined contributions of along-channel wind straining and cross-channel wind straining, as proposed by Xie and Li (2018). The associated mechanism is discussed in section 5.1.…”