Shelfbreak frontal eddies over the continental slope north of Cape Hatteras

摘要

Shelfbreak and slope eddies have been implicated as important agents in the exchange of water between the shelf and slope domains of the Middle Atlantic Bight (MAB). Here we present temperature, salinity, and velocity data from a series of shipboard transects that intercepted a rich eddy field over the slope of the southern MAB. Attention is focused on a well-sampled cyclonic eddy, of roughly 60-km diameter and 300-m depth, that translated southward at 0.1 m s?1. The eddy was composed of a mix of water masses including MAB shelf and slope water, Gulf Stream water, and water from the MAB shelfbreak front. Gradient Richardson numbers suggest that these water masses were subject to vigorous turbulent vertical mixing. The transport of shelfbreak frontal water contained within the eddy was substantial. In the upper 100 m, shelfbreak frontal water comprised ～75% of the eddy's volume. This frontal water fraction moved southward with a transport of ～0.4 Sv, comparable with the volume transport within the shelfbreak frontal jet. A number of factors indicate that this highly energetic eddy, with maximum azimuthal velocity of 0.7 m s?1, was generated through instability of the shelfbreak frontal jet. The eddy had apparently developed rapidly (in <3 days), consistent with models of eddy generation through baroclinic instability of the shelfbreak frontal jet. The eddy's potential temperature/salinity (/S) properties and energy density closely matched the /S properties and energy density found in the frontal jet to the north of the eddy.