Channel-Shoal Hydrodynamic Interaction in Partially Stratified Estuaries: A Barotropic Versus Baroclinic Perspective

Drowned-river estuaries are usually composed of a deep channel laterally bounded by shallow shoals. The channel-shoal interface in partially stratified estuaries is a dynamically important region where complicated lateral circulations can develop. Despite their relatively smaller magnitude with respect to the along-channel currents, lateral flows play an important role in both longitudinal and lateral dynamics of estuaries. In a previous study by Zhou et al. (Journal of Geophysical Research: Oceans, vol. 125, e2020JC016135) that focuses on channel-shoal interaction with only baroclinic forcing, lateral barotropic forcing arising from the change of water surface elevation was neglected by making a rigid-lid assumption. This limitation may be important especially on the shallow shoal, where strong lateral barotropic transport could be observed when the shoal is ventilated at low water. In this study, three-dimensional idealized numerical simulations were performed to investigate the coaction of barotropic and baroclinic forcings as a driver of channel-shoal hydrodynamic interaction in partially stratified estuaries. It was found that the shoalward advection of highly-salinity water mass energizes shoal water away from the channel-shoal interface. On the shallow shoals, barotropic and baroclinic lateral circulations show strong spatiotemporal correlativity, with their relative importance determined by the strengths of tidal and river forcing. Relevance of the findings from the idealized simulations to the hydrodynamics of a realistic channel-shoal estuary was also provided.