Adjustment of a Coastal Buoyant Outflow Under Tidal and Wind Forcing

The discharge of brackish water from estuaries typically forms distinct coastal plumes, often visible through their color signature due to sediment load and particulate matter from rivers. The processes of mixing and dispersion of coastal plumes are subjected to natural variations in the magnitude and timing of freshwater inflows, tides and meteorological conditions. This study presents shipboard observation of the bulge region of a buoyant plume off Winyah Bay, SC. The observation comprises downward looking 600 and 1200 kHz ADCP (Acoustic Doppler Current Profiler) and CTD measurements. Along with standard CTD casts, water samples were collected to analyze and determine mass sediment concentration using standard filtering techniques. Auxiliary data such as wind, river discharge and coastal sea level measurements were collected from WeatherFlow, USGS streamflow and NOAA tide gauge stations, respectively. The study addresses the evolution of bulge region of the plume under the conditions of low freshwater discharge and light wind forcing. The study also examines the fate of the suspended sediments under high discharge condition. The spatial and temporal salinity and temperature structures from CTD measurements are analyzed to inspect the evolution of the bulge region. The impact of suspended sediments on modifying the density anomaly within the plume is also assessed. Gradient Richardson number is calculated to examine the influence of suspended sediments on the mixing processes within the plume. Analysis of the observations demonstrate that the buoyant water was not dispersed by the wind forcing but formed a well pronounced baroclinic jet with associated front. The buoyant outflow occurring at semidiurnal tidal frequency first propagated northward with the wind-driven currents, but then turned anticyclonically and continued southward, against the wind-driven current. However, this baroclinic jet never reached the coastline to form a coastal current. Due to wind forcing, the frontal zone width exceeded the baroclinic Rossby radius, and in some cases multiple frontal structures were observed. Overall, the results demonstrate that under certain forcing conditions all buoyant outflow is deflected into a growing bulge and ultimately spreads offshore contributing to the cross-shelf exchange processes