Eddies in the eastern Gulf of Alaska from TOPEX POSEIDON altimetry

Eddy activity in the Gulf of Alaska (GOA) is evidenced to fluctuate with the El Nino-Southern Oscillation (ENSO) cycle. Altimetry from TOPEX/POSEIDON is used to study eddy formation along a single descending track in the far eastern GOA from late 1992 through late 1998. The track is sampled between 45 degrees N and 60 degrees N, where it is roughly parallel to the eastern boundary. The geoid and mean currents are removed from the data through extraction of the temporal mean at each point. The resulting anomalies are passed through a matched filter designed to detect Gaussian signals embedded in noise. The filter is optimized to detect eddies with amplitude greater than or similar to 10 cm and diameters roughly 50-350 km. Both anticyclonic and cyclonic structures are examined. The distribution of eddies in space is similar for both anticyclonic and cyclonic eddies, with most occurring between 50 degrees N and the northern limits of the GOA. Few eddies of either sign were found south of 50 degrees N. Anticyclonic eddies are stronger and have a broader distribution of amplitudes (typically 5-30 cm) compared to cyclonic features, which are more symmetrically distributed over 5-15 cm. The detection of weak (similar or equal to 15 cm) eddies is sensitive to the filter threshold, but detection of larger-amplitude eddies is robust. The average amplitude of anticyclonic eddies increases following the 1993, 1994/1995, and 1998 El Nino events. Eddy amplitude appears to vary with ENSO strength: the strongest eddies detected form in early 1998. These results support previous numerical studies of eddy development in the eastern GOA.