Evolution of Frontal Structure in a River Plume Subject to Strong Alongshore Tidal Currents

Fronts are a ubiquitous feature associated with tidally pulsed river plumes in the coastal ocean, and represent an energetic downwelling region responsible for significant mixing of est­uarine and ambient waters. Energetic fronts exiting from supercritical river mouths can contribute substantially to plume mixing through convergence and downwelling, but generally weaken as the plume evolves, due to active mixing, spreading, and the loss of energy to internal waves. We hypothesize that weakening of the front, and its contribution to plume mixing, is controlled by a ratio of the frontal length scale to an overall plume length scale, which decreases as the plume evolves due to diminishing connectivity between the plume front and the source through the ebb tide. This study focuses on the evolution of the frontal structure of the Connecticut River plume, which discharges into the strongly tidal waters of Long Island Sound, and is subject to strong alongshore tidal forcing. Due to the strong alongshore tidal currents, the Connecticut River plume exhibits less radial spreading that most of its open coast counterparts, and is an ideal venue for exploring frontal evolution due primarily to non-spreading related processes. Hydrographic data, including salinity, temperature and velocity, was collected over the first several hours outside the river mouth with an Autonomous Underwater Vehicle (AUV) making repeated passes through the front. The collected data will help to identify important frontal length scales and overall frontal structure in order to better understand the evolving dynamics of the front. Ultimately, this and similar data sets will enable better incorporation of frontal processes into coarse grid models.