Interplay of Wind Forcing and Buoyant Discharge off Montauk Point: Seasonal Changes to Velocity Structure and a Coastal Front

Abstract

Seasonal-mean currents in fall, winter, and spring on the bathymetrically complex continental shelf 15?65 m deep off Montauk Point, outside Block Island Sound, are analyzed using moored profiling current-meter records from a 2.5-yr period. A sharp boundary, or coastal front, occurs where strong, shallow, generally southwestward flow that weakens nearly linearly with increasing depth meets deeper flow nearly opposite in direction (fall/winter) or markedly weaker (spring). Velocities veer clockwise (counterclockwise) with increasing depth inshore (offshore) of the front. Evidence is presented that thermal wind balance holds without major frictional modification: it accounts for the veering, and the seasonal-mean horizontal density gradients it implies are generally toward the southeast quadrant in agreement with limited hydrographic measurements. Substantial seasonal changes in flow and frontal attributes occur because of the interplay of annual cycles in wind forcing and buoyant discharge. In fall and winter, upwelling-favorable wind causes nearshore setdown of sea surface height and an inshore-directed barotropic pressure gradient: shallow down-coast (alongshore to the south and west) currents weaken, and deep currents are up-coast. In spring, buoyant discharge peaks and winds weaken: shallow down-coast flow strengthens and deep flow weakens. The front extends to the seafloor, with attachment depth shallowest in winter, deepest in spring, and intermediate in fall; its width is smaller in fall than in winter. Buoyant discharge theory based on bottom boundary layer density advection dynamics captures these seasonal shifts. Cross-shelf circulation includes offshore (onshore) shallow (deep) motion; deep onshore motion appears enhanced near a canyon and persistent through all three seasons, suggesting wind-driven upwelling is not solely responsible.