Interacting Influences of Diurnal Tides, Winds, and River Discharge on a Large Coastal Plume

The dispersal of large river plumes in the coastal ocean depends on multiple factors, and in some cases, can be categorized into distinct dynamical regimes: a tidally dominated near-field, a rotational mid-field, and a coastal current far-field. In this study, observations and modeling are used to evaluate the factors controlling the variability in the buoyant plume from Mobile Bay. Rather than distinct dynamical regimes, the Mobile Bay plume depends on forcings that act at overlapping temporal and spatial scales: diurnal tides, river discharge events, and winds. Satellite synthetic aperture radar images along with shipboard in-situ sampling and marine radar are used to observe plume fronts in spring 2021. Hydrodynamic model simulations are compared with observations and used to characterize a large coastal plume at consistent tidal phase across a range of forcing conditions. The along-shore position of the plume depends primarily on advection by wind-driven surface currents. The cross-shore extent and plume area depend primarily on the tidal amplitude and river discharge, and secondarily on northerly (seaward) winds. Along-shore winds influence the buoyancy anomaly by altering salinity in the estuary and offshore. Upwelling winds increase the buoyancy anomaly and advect previous plumes away from the mouth. Downwelling winds reduce the buoyancy anomaly by trapping previous plumes near the coast and directing freshwater discharge toward a secondary outlet. Thus, the combined, overlapping influences of the tide, wind, and discharge dominate the variability in freshwater delivery to the shelf at time scales of days and distances of tens of km. Buoyant river plumes discharge into the coastal ocean and alter the distribution of freshwater and material like sediment and nutrients. This study examines the factors affecting the plume from Mobile Bay in the Gulf of Mexico. The research integrates shipboard measurements, satellite imagery, and numerical modeling to capture the variability in the plume in spring 2021. Plumes are often characterized with distinct regions where particular physical processes dominate-the tide or river discharge close to the mouth, and the earth's rotation and wind as the plume spreads and transitions into a coastal current. However, for Mobile Bay the winds, tides, and river discharge all contribute significantly to the size and position of the plume. Along-shore winds drive coastal currents that transport the plume, upwelling or downwelling winds move the plume offshore or trap it near the coast, and shallow stratification increases the direct influence of the wind. Diurnal tides drive outflows that are much larger than the scale of influence of the earth's rotation, and river discharge varies substantially with meteorological events. The influences of the wind, tides and discharge cannot be cleanly separated in determining the fate of outflow from Mobile Bay and similar systems. Diurnal tides, river discharge, and winds all contribute substantially to determining the size and location of the Mobile Bay plume Along-shore winds advect the plume and influence the buoyancy anomaly through upwelling/downwelling and the freshwater outflow distribution Response of the plume to forcing at tidal-to-meteorological time scales inhibits formation of a bulge and geostrophic coastal current