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

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 68°W 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 m 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 68°W, is fed by inflow from the northern side of the Gulf Stream and that this inflow is concentrated near Cape Hatteras and 68°W, where the Gulf Stream flows steeply across isobaths converging from the north.