Observation of a Baroclinic Eddy: An Example of Mesoscale Variability in the Bering Sea

Abstract

Drift buoys with shallow (17 m) drogues, released during May 1977 and tracked by satellite, delineated an eddy in the southeastern Bering Sea. Located above complex topography having a depth range of 200 to 3000 m, the eddy had a diameter of about 150 km. Mean rotational speeds ?50 km from the eddy's center were 20 cm s?1, but speeds up to 50 cm s?1 were measured. A CTD survey during July defined the eddy from 200 to 1500 m depth in temperature and salinity distributions, but no hydrographic evidence for the eddy existed at the surface. A geostrophic calculation relative to 1500 m agreed qualitatively with drifter data, but was ?5 cm s?1 less than mean drifter speeds. Examination of the T-S correlation showed that water masses at the eddy's core were the same as those at its periphery, in contrast with a cyclonic ring observed nearby in July 1974. The last drifter left the eddy in October, and a second CTD survey in February 1978 showed that the eddy had either dissipated or moved. An earlier STD survey of the region in summer 1971 had shown neither an eddy like that seen in 1977 nor a ring like that seen in 1974. In spite of the ubiquitous inclusion of permanent eddies and steady currents in Bering Sea circulation schemes, recent evidence from synoptic data suggests that the hydrographic and velocity fields are highly variable on scales of 50 to 200 km and a few weeks to a few years. While we do not understand the generating mechanism for this eddy, current instability, wind forcing and topographic interaction all remain plausible candidates. Because of the eddy's size and location, we believe that it formed within the southeastern Bering Sea.