Seasonal variation of a coastal jet in the Long Island Sound outflow region based on HF radar and Doppler current observations

[1] Surface current (HF radar) and velocity profile observations, obtained as part of the Front-Resolving Observational Network with Telemetry (FRONT) project over an approximately 2-year period, are used to describe the seasonal variability of a coastal jet in the Long Island Sound outflow region. The jet is observed in an area of the continental shelf where surface thermal fronts are frequently detected during both summer and winter. The current jet is coincident with a band of high summer frontal probability, and apparently arises from the interaction between Long Island Sound outflow and larger-scale alongshore currents on the shelf. The jet reaches peak strength in summer (transport of similar to0.07 Sv) and is weak or non-existent in winter. Flow is strongest near the surface and weakens with depth, with only moderate seasonal variations in the vertical shear. The relatively long data set of currents combined with historical hydrographic measurements and buoy wind observations is analyzed to examine the seasonal variability of the terms in the depth-averaged momentum balance. The depth-averaged pressure gradient is partitioned into a steric component, evaluated from the hydrography, and a non-steric component that is estimated as the residual of the computed terms in the momentum equation. The depth-averaged momentum balance is found to be approximately geostrophic in the across-shore direction. The seasonal variability in the jet arises due to the shifting balance between buoyancy-driven flow that is always downshelf but intensifies somewhat in summer and wind-driven flow which dominates in winter when wind stress becomes strongly upwelling favorable.