The Evolution of a California Undercurrent Submesoscale Eddy (Cuddy)

Long-lived subthermocline eddies generated from the California Undercurrent, termed "Cuddies", have long been recognized as elements of the California Current System that carry warm, saline, low oxygen Undercurrent waters offshore. Multiple autonomous underwater vehicles (Seagliders) were used to identify and continuously occupy a Cuddy offshore of Washington state and Vancouver Island, British Columbia, Canada, for four months during the winter of 2013-2014. Repeated sections across the eddy enabled characterization of its water properties and current structure. The Cuddy surveyed consisted of an isolated mass of high spice water centered near 200 m depth with a radius of roughly 12km. It had little discernible surface expression, while beneath its core, isopycnal deflection extended to at least 500 m depth. Rotating anti-cyclonically, a core region in solid body rotation was observed out to ~¾ of the radius of maximum velocity where speeds reach ~0.2 m/s. On either side of the radius of maximum velocity, a transition region existed where exterior waters slowly eroded the Cuddy. In this region layered lateral intrusions were observed, presumed sites for enhanced lateral and vertical mixing. At radii greater than twice the radius of maximum velocity, currents became indistinguishable from background flows. During its survey the Cuddy traveled northwest in the along-slope direction over 400km at speeds ranging up to 0.08 m/s. After an initial period of near stationarity, it translated northward rapidly until reaching a second period of slow movement by which its rotary strength was markedly weakened. A second, much larger Cuddy was likely observed at the very end of the glider missions.