Shelfbreak frontal structure on the continental shelf north of Cape Hatteras

Abstract

The continental shelf circulation north of Cape Hatteras is strongly affected by the proximity of the Gulf Stream. Previous observations have shown that this is an area in which Middle Atlantic Bight sub-pycnocline (“cold pool”) shelf water flows offshore, directly across isobaths, eventually becoming entrained in the north wall of the Gulf Stream. Hydrographic observations from the summers of 1990 and 1991 are used to show two different scenarios for the interaction of Gulf Stream water with the shelf flow. A high resolution cross-shelf transect was also sampled off New Jersey in each year to provide contrasting frontal structure well north of Cape Hatteras. In 1991 off New Jersey, shipboard velocity measurements indicated a baroclinic jet at the shelfbreak front, with a maximum along-shelf velocity of 0.5 m s −1. Large cross-frontal density gradients were located near both the bottom outcrop of the front as well as in the seasonal pycnocline, resulting in two distinct velocity maxima, one above and one below the seasonal pycnocline. The latter was associated with a pycnocline salinity maximum extending from the continental slope shoreward across the shelfbreak. The frontal structure near Cape Hatteras was very different, due to the presence of Gulf Stream water over the continental shelf and slope. In June 1990, the shelf water was diverted offshore 100 km north of Cape Hatteras, with Gulf Stream water present over the shelf farther south. Estimated geostrophic eastward velocities at the 1000-m isobath were 0.2 m s −1 (relative to 100 m). In 1991, Gulf Stream water was present over the upper slope, abutting the shelf water, but was not present over the shelf during the brief (3-day) survey. However, the near-surface Gulf Stream water over the upper slope was less dense than the shelf water and drove a northward flow of 0.2 m s −1 in the upper 30 m of the water column. In both cases, large along-isobath density gradients were present at the 1000-m isobath, consistent with geostrophic offshore flows of up to 0.2 m s −1. Thus, the presence of Gulf Stream water may affect the shelf circulation north of Cape Hatteras in at least two different ways: by either flooding the shelf and diverting the southwestward flow of water from the Middle Atlantic Bight directly offshore well north of Cape Hatteras, or by reversing the density gradients normally associated with the shelfbreak front and potentially allowing the shelf water to move offshore along downward-sloping isopycnals.