Anticyclonic eddies in the Alaskan Stream

Abstract

Anticyclonic eddies propagating southwestward in the Alaskan Stream (AS) were investigated through analysis of altimetry data from satellite observations during 1992-2006 and hydrographic data from profiling float observations during 2001-06. Fifteen long-lived eddies were identified and categorized based on their area of first appearance. Three eddies were present at the beginning of the satellite observations; another three formed in the eastern Gulf of Alaska off Sitka, Alaska; and four were first detected at the head of the Gulf of Alaska near Yakutat, Alaska. The other five eddies formed along the AS between 157° and 169°W, and were named AS eddies. While the eddies that formed in the Gulf of Alaska mainly decayed before exiting the Gulf of Alaska, the AS eddies mostly crossed the 180° meridian and reached the western subarctic gyre. Four of five AS eddies formed under negative or weakly positive wind stress curls, which possibly caused AS separation from the coast. Comparison of eddy propagation speeds in the AS with the bottom slope showed that eddies propagated faster over steeper slopes, although eddy speeds were slower than those predicted by the topographic planetary wave dispersion relation. An AS eddy was observed by profiling floats in the western subarctic gyre after it detached from the AS. Intermediate-layer water near the eddy center had low potential vorticity compared with the surrounding water, suggesting that AS eddies provided the western subarctic gyre with water just south of the Aleutian Islands. © 2009 American Meteorological Society.