Abstract

Since the fall of 1992, an acoustic Doppler current profiler mounted on a freighter, the CMV Oleander, has been measuring upper-ocean currents between New Jersey and Bermuda on a weekly basis. The extensive database that results from the frequent, systematic, and sustained sampling enables the exploration of a number of questions regarding currents in the northwest Atlantic. This paper reports on interannual variations in transport in the Gulf Stream and adjacent waters. The repeat sampling greatly increases the ability to discern even rather subtle variations in near-surface transport and explore their possible causes. The transect is divided into three subregions: the Gulf Stream is defined by a high velocity core with an instantaneous width set by where the downstream component of velocity changes sign; the Slope Sea exists between the Gulf Stream and the continental shelfbreak; the Sargasso Sea lies between the Gulf Stream and Bermuda. These three regions exhibit quite different signatures of variability. Over the eleven years of operation to date annual averages of Gulf Stream transport have a standard deviation of 6% but a 23% peak-to-peak range. No discernable trend in transport is evident in the eleven-year record. The westward transport in the Slope and Sargasso seas can both vary by a factor two in magnitude but they have quite different temporal characteristics: the Slope Sea transport changes take place gradually whereas the Sargasso Sea exhibits much larger variations on shorter time scales. It is conjectured that the Slope Sea time scales are set by high-latitude buoyancy-related forcing, whereas the Sargasso Sea and Gulf Stream variability reflects tropical and subtropical mechanical forcing. The lateral position of the Gulf Stream exhibits a correlated behavior with westward transport in the Slope Sea. When Slope Sea transport increases, the Gulf Stream shifts to the south with a concomitant hint of increased Gulf Stream transport. The southward shift of the Gulf Stream may be part of a dynamical response to this increased circulation in the Slope Sea since the Slope Sea flow is blocked in the west by the Gulf Stream at Cape Hatteras suggesting that the path of the Gulf Stream is governed more by thermohaline- than wind-driven forcing. The fast time scales of transport in the stream, on the other hand, point to wind-driven forcing from the tropics and subtropics. Thus Gulf Stream position and transport would appear to be driven by quite different physical processes.

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