Gulf Stream structure, transport, and recirculation near 68°W

Abstract

An analysis of the structure and transport of the Gulf Stream is undertaken using direct current meter observations from a 13-mooring array deployed near 68øW from June 1988 to August 1990. The analysis is based on a "stream-coordinate" approach, in which velocities are rotated into a local, downstream coordinate frame and averaged according to their relative cross-stream location within the current. The picture so obtained represents the average synoptic structure of the Gulf Stream, rather than the Eulerian-averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations. Many familiar features of the Gulf Stream are reproduced in the analysis, including an asymmetric velocity profile with larger shear on the cyclonic (shoreward) side of the current, an offshore displacement of the velocity core with depth, and a subsurface velocity maximum on the offshore side of the current. Westward recirculations are also seen on both sides of the Gulf Stream. Maximum downstream speeds at the axis of the Gulf Stream reach approximately 2.0 m/s at the surface and 0.7 m/s at 1000 m, roughly twice the corresponding Eulerian-averaged values. The analysis also reveals a deep extension of the Gulf Stream at 3500 rn depth with a width of 130 km and average speeds of 3-4 cm/s. The transport of the Gulf Stream in the stream-coordinate frame is 113 ---8 Sv, approximately 30% larger than the Eulerian-averaged transport of 88 Sv. On the basis of these results and other recent studies the downstream transport increase of the Gulf Stream and the inflow structure to the Gulf Stream are reconsidered. It is concluded that approximately 30 Sv, or over half of the transport increase between Cape Hatteras and 68øW, is fed by inflow from the northern side of the Gulf Stream and that this inflow is concentrated near Cape Hatteras and 68øW, where the Gulf Stream flows steeply across isobaths converging from the north. [Worthington, 1976; Halkin and Rossby, 1985; Hogg, 1983]. Knauss [1969] showed that in the first 1000 km downstream of Cape Hatteras the Gulf Stream transport increases from 65 Sv (1 Sv = 10 6 m3/s) to approximately 150 Sv (at 65øW), an average rate of increase of 8 Sv per 100 km. This large transport, of order 150 Sv, is apparently maintained at least as far as 55øW [Hendry, 1982; Hogg, 1992]. Despite this large increase in transport, hydrographic sections show that the Gulf Stream has a very similar baroclinic structure over this domain, in terms of its horizontal and vertical scales and total density contrast across the current. The evolution of the Gulf Stream downstream of 55øW is not well understood at this time, but near the Grand Banks, there appears to be a distinct change in the character of the Gulf Stream, from a single meandering front to multiple, branching fronts feeding into the Azores and North Atlantic Currents [Krauss, 1986]. The region east of 55øW is also thought to be where most of the detrainment from the Gulf Stream to the bordering recirculati...

The Gulf Stream pathway and the impacts of the eddy-driven abyssal circulation and the Deep Western Boundary Current

“…Positive relative vorticity is generated while the abyssal current passes under the stream, and after passing under the stream, the abyssal current immediately retroflects eastward onto the abyssal plain, thus eliminating the component of flow normal to the stream and preventing further southward abyssal current advection of the Gulf Stream pathway. As a result, specific features of the bottom topography and feedback from the influence of the Gulf Stream on the abyssal current pathway determined the latitude of the stream at 68.5-69" W. The speed and direction of the associated abyssal currents are consistent with those observed by Johns et al (1995).…”

“…2 have mean maximum surface layer current speeds (see Table 2 in Section 3.4) in line with observations reported by Halkin and Rossby (1985), Joyce et al (1986). Johns et al (1995), and Schmitz (1996). Thus, differences in the separation pathway cannot be attributed to differences in the inertial character of the simulated currents.…”

“…Observationally, this behavior is illustrated at 3500 m depth in Fig. 2 of Johns et al (1995) for the abyssal current cross-under at 68-69°W. These results suggest that the termini of steeper escarpments associated with the continental slope help determine the latitude of the Gulf Stream pathway at 68.5-69°W and 71-73°W.…”

“…1). Additionally, the model mean gyre centered near 68°W, 37.8°N is supported by 26-month means from an array of 15 current meter moorings at 3500 m depth, again with similar mean speeds (Johns et al, 1995, their Fig. 2).…”

Near-Surface Transport Pathways in the North Atlantic Ocean: Looking for Throughput from the Subtropical to the Subpolar Gyre

“…Similarly, the general expectation that waters constituting the upper limb of the MOC are carried continuously into the subpolar gyre via the Gulf Stream/ North Atlantic Current has come under some recent suspicion. Prior studies have shown that the Gulf Stream transports approximately 65 Sv (Sv [ 10 6 m 3 s 21 ) at Cape Hatteras (e.g., Johns et al 1995) and that about 20 Sv of this flow makes its way northward into the subpolar gyre via the North Atlantic Current. However, recent studies based on surface drifters show very little intergyre exchange in the North Atlantic, leaving open the question of how, where, or when the waters of the MOC upper limb enter the subpolar gyre to eventually return to their deep-water formation sites.…”