Fluctuations of Gulf Stream frontal position between Cape Hatteras and the Straits of Florida

Abstract

Low‐frequency variations in the onshore/offshore position of the Gulf Stream thermal front have been examined at 10 cross‐isobath transects equally spaced along the South Atlantic Bight between the Straits of Florida and Cape Hatteras. The data were obtained from the U.S. Navy's frontal analysis charts for the 12‐year period, 1976–1988. A time domain empirical orthogonal function (EOF) analysis revealed two modes of fluctuation about the mean Gulf Stream position, which together explain the well‐known “weakly and strongly deflected” frontal position states which occur on timescales of a few months in the vicinity of the Charleston Bump. Analyses of fluctuations at the annual period revealed a 3‐month phase lag between frontal position, as shown by the most significant time domain EOF mode, and volume transport through the Straits of Florida. In addition, frequency domain EOF and cross‐spectral analyses of frontal position revealed annual waves which appear to propagate to the south (north) in the region south (north) of the bump. For transect pairs having significant coherence and nonzero phase estimates, the phase speed and wavelength were about 10 cm/s southward (7 cm/s northward) and 3000 km (2000 km) in the southern (northern) region. This suggests an annual generation of a perturbation in the vicinity of the bump which radiates away as a long annual wave and which may be related to, or part of, the mechanism which produces the annual modulation of Gulf Stream transport.