Response of a Large Stratified Estuary to Wind Events: Observations, Simulations, and Theory for Long Island Sound

The response to wind events in the Long Island Sound (LIS), a large macrotidal estuary influenced by rotation and stratification, is studied using long-term ferry-based current observations near the mouth, unstratified and stratified numerical simulations forced with along-estuary winds, and analytic solutions based on linear barotropic theory. The observed wind-event velocity anomalies for down-estuary winds have surface-intensified downwind flows flanking a deeper central upwind flow. Response to up-estuary wind events has a weaker magnitude and a broader and thicker downwind flow. The downwind and upwind flows are more laterally aligned than vertically layered, as determined by a newly defined dimensionless lateral alignment index. Simulation results and analytic solutions share the gross spatial patterns of the observed response, though statistical measures indicate weak agreement. Along-estuary variations in the simulation results and analytic solutions follow similar trends and are strongly influenced by variations of the bathymetric cross section. Wind-event anomalies in the section-averaged dynamics are dominated by the along-estuary pressure gradient opposing wind stress. In the stratified simulation, wind-driven density advection, isopycnal straining, and stirring modify stratification, eddy viscosities, and baroclinic pressure gradients. The wind-event response of the baroclinic pressure gradient is 15% of the barotropic gradient but is dynamically linked to response differences to up-estuary and down-estuary winds. The wind-event response asymmetries near the mouth are in qualitative agreement with observations and are opposite to asymmetries closer to the head.