Observations of the creation and evolution of small-scale oceanic frontal cusps and slicks

Airborne microwave radar imagery and coincident in situ data collected off Cape Hatteras, NC (USA) are used to examine the small-scale horizontal structure of a frontal region, which formed through intrusion of relatively dense Gulf Stream water onto the continental shelf. The frontal outcrop is shown to have a kilometer-wavelength scalloped structure consisting of sharp angular features (cusps) alternating with broad, gently curved regions (troughs). There is also an associated pattern of slicks lying on the buoyant side of the front and asymmetrically offset from the cusps. These slicks appear to originate from biophysical processes associated with the front itself and to trace out cyclonic trajectories of surface fluid particles. It is conjectured that the distinctive horizontal pattern of frontal cusps and slicks arises from shear-flow instability modified by the requirement for convergence of buoyant water along the front.