Gulf Stream bimodality and variability downstream of the Charleston bump

Abstract

Gulf Stream mean flow and variability have been studied in the Gulf Stream Deflection and Meander Energetics Experiment (DAMEX). This study was conducted between September 1981 and April 1982 in the area of the Charleston bump, off the coast of the southeastern United States. Using data from an array of current meters and satellite imagery, it has been shown that the seaward deflection of the Gulf Stream by the bump has a bimodal character and that the Gulf Stream meander field downstream of the bump differs between the two states of deflection. The two deflection conditions have been termed the weakly deflected state and the strongly deflected state in this paper. A weakly deflected Gulf Stream flows past the Charleston bump and close enough to the edge of the continental shelf that its shoreward sea surface temperature (SST) front remains inshore of the 600‐m isobath between Charleston and Cape Hatteras. A strongly deflected Gulf Stream turns sharply eastward at the bump and flows seaward far enough to have its shoreward SST front offshore of the 600‐m isobath for several tens of kilometers along the continental slope downstream of the bump. The transitions between deflection states may occur very rapidly, and once in a strongly deflected state the Gulf Stream may remain so for up to several months. Two transitions were observed during DAMEX, one from a weakly to a strongly deflected state and one in the other direction. The transition from weakly to strongly deflected state took only a few days at a given site, while the signatures of the other transition were not as definite in the data records. The strongly deflected state occurred during the fall and winter in DAMEX (October through January), and satellite observations from earlier years suggest that it is this time of year in which the strongly deflected state preferentially occurs. It was not clear from the DAMEX data alone how the transition from a weakly deflected to a strongly deflected state may be induced, but path or transport changes in the Gulf Stream as it approaches the bump and interactions between the current and rings or eddies are candidate mechanisms. The bimodal character of the stream's path affects the low‐frequency variability of the current between the Charleston bump and Cape Hatteras in the following manner. Typical meander/frontal eddy patterns dominate the mesoscale fluctuation field during times of weak deflection, while larger‐amplitude meanders of the Gulf Stream jet, which can have very sharp curvature and strong anticyclonic circulations in their crests, exist during times of strong deflection. Furthermore, these larger‐amplitude meanders, which we call the DAMEX biweekly meanders, have generally slower phase propagation speeds than do the typical meander/frontal eddy events (about 20–25 km/day compared with 35–60 km/day) as well as longer periods (about 16 days compared to 1 week).