Transport of Middle Atlantic Bight shelf water to the Gulf Stream near Cape Hatteras

Abstract

Flowing along the U.S. eastern seaboard from Nantucket Shoals to Cape Hatteras is a low‐salinity (<34 practical salinity units) water mass known as Middle Atlantic Bight (MAB) shelf water. It has long been recognized that a large quantity of this water is entrained into the Gulf Stream near Cape Hatteras. We examine this process using moored instrument, hydrographic, and drifter track data acquired in the Cape Hatteras region over 1992–1994. These measurements reveal that the cross section and volume transport of MAB shelf water tend to shrink considerably going southward toward and past Cape Hatteras. In particular, our hydrographic data show shelf water cross section to be reduced by about a factor of 8 over a region stretching 80 km north of Diamond Shoals (between 35° 20′N and 36°10′N). There appear to be two fairly distinct zones of export in this region. Our data suggest that shelf water loss in the northern zone, over 35°40′N–36°10′N, occurs principally seaward of the shelf edge. Seaward movement of shelf water into Gulf Stream meanders and into the circulation of water discharged from the Gulf Stream are identified as important agents effecting this loss. Export of shelf water in the southern zone occurs over middle and outer shelf, as indicated by 5–10 cm s−1 mean off shelf velocities measured by moored current meters. Contributing to this export is a seaward flow of shelf water in a strong current at the edge of the front separating MAB shelf water from the more saline shelf water of the South Atlantic Bight. The offshore transport of shelf water at the edge of this front is principally observed when the front is migrating shoreward across the shelf. This intermittency in the seaward transport of shelf water at the front may be partly responsible for the variability of the presence and transport of shelf water seen at the edge of the Gulf Stream.