A SALINITY FRONT AND CURRENT RIP NEAR CAPE-HATTERAS, NORTH-CAROLINA

Shipboard hydrographic and acoustic Doppler current profiler data coll ected in September 1991 show a shallow front separating brackish coast al water from denser Gulf Stream water advancing onto the continental shelf near Cape Hatteras. The surface expression of the front has the hallmarks of a current rip: a corrugated line of flotsam (indicating s urface convergence) and an adjoining band of short, steep waves (indic ating wave-current interaction). Sinking flow of 15 cm/s and a 10-dB c hange in acoustic backscatter strength occur over a cross-front distan ce of 10-20 m. While northward surface currents in the Gulf Stream wat er move at about 60 cm/s, the front advects northward at only 35 cm/s; this difference implies frontal propagation to the south relative to the deeper fluid at a speed consistent with theoretical expectations. The surface convergence across the front is estimated to be about 50 c m/s, and the horizontal surface strain rate is estimated to be 0.025-0 .05 s-1. This supports the large strain rate needed by Jansen et al. ( 1993) to simulate the O(10 dB) radar modulations measured at the rip b y Askari (1992). Similar current rips may account for curvilinear high -backscatter features appearing in historical synthetic aperture radar images of the shelf area north of Cape Hatteras.