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.

Cyclonic entrainment of preconditioned shelf waters into a frontal eddy

“…Frontal eddies are cyclonic eddies that regularly characterize the landward side of all western boundary currents, e.g., Gulf Stream [ Glenn and Ebbesmeyer , ] and the East Australian Current [ Everett et al ., ], and eastern boundary currents, e.g., California Current [ Kim et al ., ] and Benguela Current [ Shannon et al ., ]. These frontal eddies are formed as a result of velocity shear between the current and the continental shelf, or from meanders and instabilities in the boundary current.…”

Wind and Gulf Stream influences on along‐shelf transport and off‐shelf export at Cape Hatteras, North Carolina

“…The cross-shelf increase in current magnitude is also likely a Gulf Stream effect, either through entrainment or cross-shelf sea surface slope effects. Gulf Stream filaments often overwash the entire shelf just south of Cape Hatteras [Glenn and Ebbesmeyer, 1994b]. Gulf Stream offshore location, separation angle, and path curvature at the Cape affect filament size and production, coastal sea level, and cross-shelf sea level slope.…”

“…Hatteras,Glenn and Ebbesmeyer [1994b] noted enhanced shoreward progression of Gulf Stream filament waters when the main Gulf Stream front is located farther from shore. In fact, during the period of along-shelf divergence shown inFigure 18the Gulf Stream had moved offshore briefly, as indicated by the B4• pressure record (Cross spectra between nearshore transport convergence and forcing functions: (left) cross spectra between along-shelf transport convergence (B l-A1 transports) and CHLV2 along-shelf wind (solid line) and between along-shelf transport convergence and DSLN7 along-shelf wind (dashed line), (right) cross spectra between nearshore along-shelf transport convergence and PB4• (solid line), (top) magnitude squared coherence (drawn to the same scale), (middle) the phase lag in degrees, and (bottom) the phase lag in days, converted from degrees by 0aav -0aeg/(f360), where f is frequency.…”

An extraordinary breach of the Gulf Stream north wall by a cold water intrusion

“…The initial intrusion began as a fairly typical interaction between continental shelf waters, exiting from the continental shelf, near Cape Hatteras and a Gulf Stream frontal eddy (Panel 1). Mesoscale eddies form frequently in this region and often draw coastal waters to them as they migrate past the Carolina Capes region [ Glenn and Ebbesmeyer , 1994]. Typically as the eddy progresses further downstream from Cape Hatteras the intrusion of coastal water is stretched into an elongated filament that propagates or is dragged along with the Stream, attached to the North Wall [ Churchill and Berger , 1998].…”