Interaction of a Slope Eddy with the Shelfbreak Front in the Middle Atlantic Bight

Abstract

Spring conditions at the shelf break in the Middle Atlantic Bight mark the transition period between the generally well-mixed shelf water in winter and the highly stratified shelf conditions during summer. A high-resolution hydrographic survey made during early May 1996 is used to describe the thermohaline and velocity structure of the shelfbreak front. The front was strongly affected by the presence of a slope eddy immediately offshore of the front. The eddy, which had a diameter of 25 km, was anticyclonic with onshore/offshore flows of 0.2 m s?1 on opposing sides. On the western side of the eddy, where the flow seaward of the front was predominantly onshore, the front was very steep and the frontal jet was particularly strong, with maximum near-surface velocities of 0.5 m s?1. On the eastern side of the eddy, the front was drawn offshore and was much less steep, with near-surface velocities of only 0.2 m s?1. A surprising feature was the presence of a second jet over the foot of the front, shoreward of the main, surface-trapped jet. These observations suggest that eddies over the slope have a large impact on shelfbreak frontal structure, and result in strong alongfront gradients within the frontal jet.