Observation of an Anticyclonic Eddy near the Continental Shelf Break South of New England

Abstract

A small scale but energetic and highly nonlinear anticyclonic eddy was observed near the New England Shelf Break in July 1983. Satellite images show a tongue of shelf water 15 km wide flowing offshore then turning anticyclonically westward. Subsequent images show the circulation closing on itself as the eddy expands and slowly drifts southwestward along the shelf break. Acoustic Doppler current data show anticyclonic circulation extending to at least 200 m depth. The center of the eddy appears to undergo solid body rotation with a peak azimuthal velocity of 50 cm s⁻¹ at a radius of 7 km to give an angular velocity ω = −7 × 10⁻⁵ s⁻¹ and Rossby number Ro = |ω|/f = 0.74. The resulting nonlinearity from the cyclostropic term produces a shoaling of the near surface pycnocline and a vertical current shear with sign opposite to that of the thermal wind shear. The eddy is not a warm core ring. Its size, lifetime and water type composition suggest a local origin. It is possible that the anticyclonic vorticity is generated by the thinning of a lense of shelf water as it moves offshore. Pressure work then leads to anticyclogenisis deeper in the water column. Comparison of the observed velocity field with calculations from a two-layer eddy model of Olson and co-authors agree within a factor of 3. The energy source for this feature is not readily identified. Wind stress is not a likely source. The proximity of warm core ring 83-E suggests a potential role in the eddy formation, perhaps through the generation of baroclinic instability of the shelf/slope water front.