Low-Frequency Variations in Currents near the Shelf Break: Northeast Gulf of Alaska

Abstract

We present current observations from depths of 20, 50, 100 and 175 m obtained over a 3-year period near the northeast Gulf of Alaska shelf break, and emphasize a 2-year continuous segment from 50 m depth. These records indicate that a moderate (?16 cm s?1 at 50 m) mean flow was directed, at all depths, along-shelf toward the northwest. Variance was high and had nearly normal distributions in along- and across-shelf components. The mean alongshelf speed varied from ?12 cm s?1 in summer to ?20 cm s?1 in winter and a more distinct seasonal cycle was evident in monthly kinetic energies of wind and current. Very low-frequency (VLF) (<0.1 cpd) current fluctuations were prominent flow features and were vertically coherent between 50 and 100 m depth (coherency squared >0.8 at 95% level). Fluctuating kinetic energy of the current contained in the VLF band was a significant fraction (34%) of the total kinetic energy, and displayed an increase over the two years with no seasonal trend. Trends in the wind kinetic energy in the VLF band were much different and most (?66%) of the VLF current kinetic energy was not correlated with that of the winds. These current fluctuations were probably related to oceanic-scale features such as eddies or meanders in the Alaska Current.