A Wind-Induced Mesoscale Eddy Over the Vancouver Island Continental Slope

We use Nimbus 7 coastal zone color scanner imagery and direct oceanic measurements to investigate an extensive mesoscale circulation pattern that developed off the coast of Vancouver Island, British Columbia, in July 1979. Particular emphasis is on a well-defined cyclonic eddy that formed in 1000 m of water over the central portion of the continental slope. Characteristic features of the eddy included a core diameter of 25 km, a vertical scale of 500 m, azimuthal current speeds of 0.20±0.05 m s−1, and an alongshore propagation speed of 1.7±0.6(×10−2) m s−1. According to our interpretation, the eddy, and the attendant mesoscale circulation pattern, evolved through mixed barotropic-baroclinic instability of a poleward flowing coastal “jet” that had been generated during a 1-week period of strong 0(10 m s−1) southeast winds. Formation of the eddy appears to have taken place 5–7 days after onset of the jet and may have been triggered through perturbation of the mean flow by a 0(10-km) “bump” in the continental slope. Results imply that eddylike circulation observed off the coast of Vancouver Island is frequently initiated by “local” alongshore winds rather than through instability of the prevailing California Current regime, as previously suggested in the literature. The results also indicate that thermal imagery often fails to delineate major oceanic surface features off the coast.