Water discharged from the Gulf Stream north of Cape Hatteras

Abstract

Satellite radiometer‐derived sea surface temperature images together with moored instrument and hydrographic survey data indicate that water ejected from the Gulf Stream often occupies the upper 200 m of the water column over the continental slope between Cape Hatteras and Hudson Canyon. At times this water resembles the energetic Gulf Stream frontal eddies commonly seen to the south of Cape Hatteras. However, many of the observed parcels of this water differ markedly from frontal eddies and Gulf Stream warm‐core rings and so appear to form a class of discharged Gulf Stream water distinct from any previously reported. These parcels generally cover a broad area (compared with frontal eddies), contain relatively weak currents (generally < 40 cm s−1 at 100 m), and are remarkably long‐lived (sometimes distinguishable for more than 2 months). Salinity anomaly distributions indicate that despite their persistence, these water masses contain intrusions of and mix with surrounding lower‐salinity water, particularly in the upper 50 m. Continuity of various tracers along σθ surfaces reveals that the discharged Gulf Stream water observed near the sea surface originated within the nutrient‐bearing stratum of the Gulf Stream, having upwelled hundreds of meters along density surfaces. As a result, it tends to enhance nutrient concentrations over the continental slope, to a significant extent at 100 m depth but only marginally at 50 m depth. By contrast, this water does not carry unusually large kinetic energy densities into the slope region, but apparently gives up a good deal of kinetic energy, on the order of 104 cm2 s−2 per unit mass, while leaving the Gulf Stream.