Flood-Ebb and Spring-Neap Variations of Lateral Circulation in the James River Estuary

Lateral circulation in estuaries has received increasing attention in recent years because advection by the lateral flow appears to play a leading role in driving estuarine exchange flows. Previous numerical modeling and observational studies suggested that the lateral circulation has strong flood-ebb asymmetry and undergoes large spring-neap variations. However, recent observations in the James River estuary showed that the lateral circulations at the spring and neap tides were of similar magnitude despite large differences in the vertical stratification. A numerical model is developed to investigate the temporal evolution of the lateral circulation in the James River estuary. The spring-neap variations of the lateral circulation strength can be interpreted in terms of a three-way balance in the streamwise vorticity equation: vorticity generation due to the tilting of planetary vorticity by the vertical shear in the along-channel current, lateral baroclinic forcing, and turbulent diffusion. Strong spring mixing increases the turbulent diffusion of vorticity while reducing the vertical shear in the along-channel current and vorticity generation. The lateral barolinic forcing is tied to the vertical stratification and is larger during the neap tide than during the spring tide. The overall balance in the vorticity budget is similar between the spring and neap tides, resulting in small differences in the lateral circulation strength.