A numerical study of residual circulation induced by asymmetric tidal mixing in tidally dominated estuaries

Residual currents induced by asymmetric tidal mixing (ATM) were examined using a series of idealized numerical experiments for weakly stratified, partially mixed, and highly stratified narrow estuaries that neglected lateral variations of bathymetry and the effects of Earth's rotation. The Eulerian residual currents were decomposed into four components, i.e., river-induced, density-driven, nonlinearities-induced, and ATM-induced flows such that the longitudinal distribution and strength of each component can be depicted and evaluated. In weakly stratified estuaries, ATM-induced flow has a two-layer structure similar to that of density-driven flow. It reinforces the estuarine exchange flow. In partially mixed and highly stratified estuaries, the ATM-induced flow tends to have a three-layer structure with landward flows near the surface and the bottom and seaward flow in the middle of the water column. It appears to act against the estuarine exchange flow in parts of the water column. The relative importance of ATM-induced flow to estuarine residual currents varies in different types of estuaries. Compared to the density-driven flow, the relative importance of ATM-induced flow decreases as stratification increases. In the central regime of the estuary, the strength of the ATM-induced flow is greater than that of density-driven flow under weak stratification and tends to be smaller under strong stratification.