Lagrangian Observations of an Ebb Tidal Plume under Low Discharge Conditions, Main Pass, Mobile Bay, AL

Tidal inlets, separating barrier islands, are common throughout the Northern Gulf of Mexico and often exhibit discharge plumes that deliver pulses of buoyancy, nutrients, and sediments to the adjacent coastal zone. These tidal inlet exchanges have a major impact on the overall fate of the material transported to the shelf. In order to better understand the circulation patterns at these critical estuarine-shelf interfaces, a Lagrangian experiment was conducted using surface drifters during the Fall of 2015. It quantified the flow patterns associated with the ebb tidal plume exiting from Mobile Bay, a major source of river discharge to the Mississippi Bight. Each “Davis Drifter” included GPS, conductivity, and temperature measurements, and was deployed with consistent temporal and spatial intervals at the mouth of the bay on ebb tides under light wind conditions for 6-24 hours. The drifters experienced significant dispersion after passing through an ebb-flow delta system. At the mouth, maximum speeds of 1.3 m s-¹ were observed by the drifters along the thalweg of the shipping channel and the ebb tidal front extended approximately 15 km offshore. The flow trajectories were asymmetric, favoring the downshelf westward direction which was consistent with the intermediate Kelvin Number, O(0.5-1.0). This suggests that under low discharge conditions Coriolis forcing is important to the ebb flow dispersion at the mouth. Additional drifter releases are being conducted through the Fall of 2015 and other analyses of the flow structure of the near-field plume are under way. The study is expected to characterize the dispersion patterns and salt entrainment of the near-field under a wide range of environmental conditions. The findings from this study will significantly enhance our ability to understand the near shore circulation in the Gulf of Mexico.