Observations of Gulf Stream frontal eddies in the vicinity of Cape Hatteras

Abstract

The Frontal Eddy Dynamics (FRED) experiment was conducted offshore North Carolina between Cape Fear and Cape Hatteras from May through November 1987. Frontal eddies propagating northward along the North Carolina shelf break were observed during a 3‐week intensive shipboard and aircraft survey phase, followed by a six‐month (long‐term) mooring and remote sensing monitoring phase. Using observations of a frontal eddy named Abbott from the intensive phase, it has been established that a frontal eddy cold dome can propagate out of the South Atlantic Bight past Cape Hatteras. The focus here therefore is on frontal eddy observations in the long‐term data set in the immediate vicinity of Cape Hatteras. The long‐term moorings indicate that 39 northward propagating frontal eddies were observed immediately south of Cape Hatteras, giving a recurrence interval of once every 4–5 days. Two types of frontal eddies are identified, dependent on the bimodal character of the amplitude of the parent Gulf Stream meanders. During the small‐meander mode, the mean Gulf Stream front is approximately aligned with the 100‐m isobath, and the small warm filaments rarely penetrate beyond mid‐shelf. During the large‐meander mode, the mean Gulf Stream front is displaced offshore, but the larger warm filaments often penetrate onto the inner shelf. The duration of each meander mode at Cape Hatteras is 2–3 months, with rapid (several day) transitions between the modes. The transition to the large‐meander mode appears to be related to local interactions with cold core rings in the Sargasso Sea rather than the offshore deflections of the Gulf Stream at the Charleston Bump. The intense observations of Eddy Abbott occur during the small‐meander mode, demonstrating that even when the Gulf Stream is closer to the confining topography of the continental shelf, frontal eddy cold domes can propagate beyond Cape Hatteras and form new warm filaments in the Middle Atlantic Bight.