Observed tidal currents outside Block Island Sound: Offshore decay and effects of estuarine outflow

Abstract

An array of moored profiling current meters on the inner continental shelf 15–65 m deep is used to investigate tidal currents outside the Block Island Sound and Long Island Sound estuarine system. The M2 constituent dominates due to near‐resonant semi‐diurnal estuarine response. Vertical‐mean M2 currents rotate clockwise in time in ellipses elongated toward the estuary mouth with little sensitivity to complex local bathymetry; semi‐major axes decay sharply offshore (55 to 20 cm/s over 10 km) in agreement with the nearly inverse‐square radius dependence of a kinematic theory. Estimated tidal volume exchange out of Block Island Sound to the south is 2.9 × 105 m3/s. Observed vertical structure of M2 ellipses in the deepest 10–20 m (amplitude decay, ellipse flattening, major axis turning clockwise, phase advance) is generally captured well by optimally fit frictional solutions despite the fact that they omit bathymetry and include stratification only indirectly through its influence on eddy viscosity. In the upper water column, vertical structure varies seasonally: Mid‐depth ellipses enlarge in spring; near‐surface ellipses are larger (smaller) than optimal‐fit solutions in fall/winter (spring). Observed seasonal‐average estuarine outflow includes surface‐intensified Coriolis‐deflected mean flow that strengthens and spreads farther offshore in spring. Richardson numbers based on moored hydrographic profiler records suggest spring stratification suppresses deep eddy viscosities, which in frictional solutions can explain mid‐depth ellipse enlargement. An inviscid theory for mean‐flow modifications due to ambient vorticity of estuarine outflow currents is shown to be the most plausible explanation for observed seasonal changes in near‐surface tidal ellipses.