Subtidal Exchange in Eastern Long Island Sound

Abstract

ong Island Sound (LIS) is a large and wide macrotidal estuary with distributed river inputs, including the Connecticut River (the largest freshwater source) that flows into the eastern LIS near the mouth. In 2010, shipboard surveys of salinity, temperature, and currents were collected along an across-estuary transect in eastern LIS. Numerical model results are compared to these observations and used to study the spatial and temporal variability of salinity, velocity, and freshwater and salt fluxes over a 4-yr period. For all low wind conditions, observations and model results indicate an outward-flowing, low-salinity wedge on the south side with an inward-flowing, higher-salinity area underneath and to the north. Observations and model results during the low wind surveys indicate that stratification substantially decreases with increased tidal amplitude and decreased river discharge; the velocity field is less variable among surveys. Model analysis indicates strong sensitivities to both tides and river discharge; with discharge response strongest for salinity and freshwater flux and tidal response larger for velocities, volume flux, and salt flux. The long-term average net freshwater and salt fluxes are outward and inward, respectively. For both flux types, subtidal shear dispersion is twice tidal oscillatory diffusion, and both contributions are in the same direction as the net flux. The uniform flow contribution is small for freshwater flux, yet it is the largest single term for salt flux and partially counters the inward contributions.