Modeling the Generation of Internal Waves by Lateral Circulation in an Idealized Estuary with Channel-Shoal Bathymetry

Renjian Li and Ming Li, University of Maryland Center for Environmental Science Horn Point Laboratory, Cambridge, MD, United States
Abstract:
Recent observations in Chesapeake Bay showed that internal lee waves were generated near the edge of the deep channel and propagated onto shallow shoals, leading to wave overturning and enhanced turbulent mixing. It was suggested that lateral circulation interacted with the channel-shoal bathymetry to generate the internal lee wave. However, it is not clear if this lateral wave generation mechanism is applicable to other coastal plain estuaries. Using an idealized straight estuarine model with archetypical channel-shoal bathymetry, we conducted numerical simulations to investigate the internal wave generation under different tidal and river flow forcing conditions. In a wide estuary where lateral Ekman forcing drives one circulation cell, internal lee waves are generated over the deep channel during ebb tides under certain river flow and tidal forcing condition. As the river flow increases, the strength of lateral circulation decreases due to stronger stratification and the lateral flow is not strong enough to generate internal lee waves. As tidal forcing increases, the stratification in the estuary decreases and experiences large fluctuation over a flood-ebb tidal cycle, and internal waves disappear as the lower layer becomes well mixed. In a narrow estuary where differential advection generates two counter-rotating cells, frontogenesis tends to develop at the shoal-channel interface due to the convergence of the lateral flows.