Residual circulation in western Long Island Sound

Current, salinity and temperature measurements from repeated ship transects, complemented by observations from long duration current profilers, are used to characterize the variability and structure of subtidal flow in western Long Island Sound, a region prone to seasonal hypoxia. Subtidal flow plays a leading role in the transport of oxygen and organic matter and must, therefore, be simulated as accurately as possible. We show that during periods of light wind in March and July, the subtidal along-sound flow is vertically and horizontally sheared with lower salinity water in the top 7 m moving eastward toward the ocean at approximately 10 cm s(-1) with a counterflow of similar magnitude elsewhere. Velocity contours were found to slope downward to the south, and maximum eastward velocities were found near the surface on the southern half of the section. We find that there is a net transport in the direction of the East River (westward.) The velocity distribution is broadly consistent with theoretical predictions for the steady, frictional, baroclinic pressure gradient driven flow modified by Coriolis acceleration, despite its neglect of advective effects which were found to be important in the present observational analysis. Our estimates of the pressure gradient, the Coriolis acceleration and stress divergence have similar magnitudes. Observation-based estimates of terms in the momentum balance suggest that advection is more important to along-estuary momentum than across-estuary momentum. Along-estuary advection is overestimated in recent hydrodynamic simulations when compared to observed values at sampling locations.