Abstract

Long-term current measurements made with surface drifters, SOFAR floats at 700 m and 2000 m, and current meters at 4000 m have been combined to produce for the first time a vertical section of directly measured average zonal currents in and adjacent to the Gulf Stream. The results from the different data sets are remarkably consistent in showing three vertically coherent zonal jets—the Gulf Stream and two flanking countercurrents. The consistency in location, velocity and transport (per unit width) of these currents supports the conclusion that these are real features of the long-term average velocity field. The current jets coincide with a region of high eddy kinetic energy and its gradient, implying a dynamical connection.The Gulf Stream is defined to be the eastward current bounded by the countercurrents. The surface Stream is ∼900 km wide and has a maximum eastward velocity of 28 cm s−1 at 39.5N, averaged over a one degree latitude band. The Stream extends to the sea floor about 200 km south of its surface axis. The deep Stream is only ∼200 km wide and has a maximum mean velocity of 7 cm s−1. The total mean volume transport of the Stream is estimated to be 93 × 106 m3 s−1 by fitting a smooth transport profile to the four directly measured values. About a third of this is depth-independent. Including estimates of local wind-driven surface velocity diminishes the total transport by 8 × 106 m3 s−1, to 85 × 106 m3 s−1. The mean transport is significantly less than the estimated synoptic transport of the Stream in this region, approximately 150 × 106 m3 s−1.The subsurface Stream is flanked by relatively narrow, westward flowing countercurrents. The combined westward transport of the countercurrents is more than enough to locally recirculate the increase of Gulf Stream transport over the 30 × 106 m3 s−1 wind-driven component of the subtropical gyre, returned in a gyre scale flow. The northern countercurrent, located between the Stream and continental shelf and rise, transports 41 × 106 m3 s−1. Part of this westward transport is the mean southwestward flow of the Western Boundary Undercurrent, which transports 10 to 15 × 106 m3 s−1. The southern countercurrent is located between 35–37N and transports about 29 × 106 m3 s−1. Part of this transport (∼12 × 106 m3 s−1) is probably recirculated as eastward flow south of 35N. The net transport across 55W north of 35N is eastward and equal to 11–23 × 106 m3 s−1 (the low value includes the wind correction).

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