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

Flowing along the U.S. eastern seaboard from Nantucket Shoals to Cape Hatte ras is a low-salinity (< 34 practical salinity units) water mass known as M iddle Atlantic Eight (MAB) shelf water. It has long been recognized that a large quantity of this water is entrained into the Gulf Stream near Cape Ha tteras. We examine this process using moored instrument, hydrographic, and drifter track data acquired in the Cape Hatteras region over 1992-1994. The se measurements reveal that the cross section and volume transport of MAB s helf water tend to shrink considerably going southward toward and past Cape Hatteras. In particular, our hydrographic data show shelf water cross sect ion to be reduced by about a factor of 8 over a region stretching 80 km nor th of Diamond Shoals (between 35 degrees 20'N and 36 degrees 10'N). There a ppear to be two fairly distinct zones of export in this region. Our data su ggest that shelf water loss in the northern zone,over 35 degrees 40'N-36 de grees 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 effectin g 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 me asured by moored current meters. Contributing to this export is a seaward f low 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 Eigh t. The offshore transport of shelf water at the edge of this front is princ ipally observed when the front is migrating shoreward across the shelf. Thi s intermittency in the seaward transport of shelf water at the front may be partly responsible for the variability of the presence and transport of sh elf water seen at the edge of the Gulf Stream.