Intratidal Dynamics of Fronts and Lateral Circulation at the Shoal-Channel Interface in a Partially Stratified Estuary

In partially stratified shoal-channel estuaries, variations in bathymetry at the shoal-channel interface generate horizontal shears and density gradients, and foam lines characteristic of convergence fronts are frequently observed. This study is based on transect data collected every 30 min across the shoal-channel interface in south San Francisco Bay to analyze the dynamics of the transverse circulation and convergence fronts at this interface throughout a tidal cycle. During the ebb, a lateral density gradient develops as a result of the competition between differential advection of the longitudinal salinity gradient; lateral variations in vertical mixing; and nonlinear advection terms, which are strongest when convergence fronts develop early and late in the ebb. The lateral circulation over the slope is characterized by a large intratidal variability, reversing three times during the ebb. This variability is driven by a balance dominated by inertia and lateral baroclinic pressure gradient during the ebb but also involving Coriolis and advection terms at leading order. The convergence fronts developing at the edge of the shoal during the ebb are greatly affected by the direction of the lateral circulation on the slope and display similar intratidal variability as a result. Observations from moored instruments suggest that these processes are involved in all partially stratified spring ebbs in south San Francisco Bay and are more sensitive to variations in tidal amplitude than density stratification on the slope.