A Basin-scale Oceanic Instability Event in the Gulf of Alaska

A composite NOAA 14 satellite AVHRR (Advanced Very High Resolution Radiometer) image of the Gulf of Alaska for early March 1995 reveals a sequence of six anticyclonic oceanic eddies spanning more than 1500 km of the continental margin of North America from central British Columbia to central Alaska. Although isolated mesoscale eddies have previously been documented for this eastern boundary region, the composite image provides the first evidence that trains of such eddies can develop "simultaneously" along the entire coast. The 80 km radius and 250 km spacing of these warm-core eddies are consistent with a baroclinic instability event in the poleward coastal current that prevails along the northwest continental margin of North America in “winter” (roughly October through March). Wind data from offshore meteorological buoys, combined with partial cloud-free thermal images for January and February, suggest that the event was triggered in mid January by an abrupt, coast-wide reversal in the prevailing poleward wind. The instability event disrupted the alongshore transport of relatively warm, high salinity water and presumably resulted in a greatly enhanced crossshore flux of brackish, nutrient-rich coastal water into the open ocean. Based on its basin-scale extent and more than two month duration, it is feasible that the event had a major impact on the early spring recruitment and survival of zooplankton and pelagic fish within the highly productive fishery zones of coastal British Columbia and Alaska. Less extensive eddy sequences found in several earlier satellite images for the region suggest that these instability events are fairly common. If so, this indicates that the poleward current that forms over the eastern Pacific continental slope in late fall (herein, the “Northeast Pacific Coastal Current”) is marginally stable and likely to deform into a series of mesoscale eddies following abrupt, basin-scale reversals in the seasonally dominant wind. Provided these wind events occur early enough in the season, the coastal current could reform with the onset of poleward winter winds.