A salinity front and current rip near Cape Hatteras, North Carolina

Shipboard hydrographic and acoustic Doppler current profiler data collected in September 1991 show a shallow front separating brackish coastal 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 surface convergence) and an adjoining band of short, steep waves (indicating wave-current interaction). Sinking flow of 15 cm/s and a 10-dB change in acoustic backscatter strength occur over a cross-front distance of 10–20 m. While northward surface currents in the Gulf Stream water 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 cm/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 by 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.