An overview of the first 4.5 years of operation of a program to monitor the structure and variability of the Gulf Stream (GS) is presented. A container vessel that operates on a weekly schedule between Port Elizabeth, New Jersey, and Hamilton, Bermuda, is equipped with a 150-kHz narrowband acoustic Doppler current profiler to measure currents from the surface to ~300 m depth. A major objective of the multiyear program is to study the annual cycle and interannual variations in velocity structure and transport by the GS. In this survey the focus is on the transport and lateral structure of the current at 52-m depth.
The velocity maximum is constant at 2.07 ± 0.24 m s−1 (4 kt) with a seasonal range of ~0.1 m s−1 . Seasonal and interannual variations in total transport are observed but appear to be limited to the edges of the current, apparently reflecting low-frequency variations in the intensity of the recirculating waters adjacent to the stream. The transport by the central core of the current, defined as those waters moving at 1 m s−1 or faster, equals 0.9 × 105 m2 s−1 , has no seasonal signal, and is constant to within a few percent when averaged in half-year intervals. If the central core of the current is viewed as “insolated” from the effects of meandering, this result implies substantial stability to the large-scale wind-driven and thermohaline circulations during the observation program. Variations in poleward heat transport probably originate less in the GS and more from changing heat loss patterns at higher latitudes.
Other issues concerning the potential vorticity field and energy conversion rates are also discussed. This ongoing program illustrates the role commercially operated vessels can play in making repeat observations of the velocity structure (and other parameters) of the ocean on a regular basis.