Subsurface hydrographic structures and the temporal variations of Aleutian eddies

Abstract

Aleutian eddies are mesoscale anticyclonic eddies formed within the Alaskan Stream region between 180° meridian and 170° E south of the Aleutian Islands. They propagate southwestward after the isolation from the Alaskan Stream and pass through the Western Subarctic Gyre. We compared hydrographic structures of three Aleutian eddies observed during summer, west of 170° E (Eddy A) and east of 170° E (Eddies B and C). In each eddy, a subsurface dichothermal water (3.0–4.0 °C) was observed above a subsurface mesothermal water (4.0–4.5 °C). The minimum temperature in the dichothermal water at around a depth of 100 m was colder in Eddy A (2.8 °C) than in Eddies B and C (3.0–3.2 °C). This difference could be ascribed to wintertime cooling and influence of surrounding waters during spring warming period. The wintertime cooling makes the dichothermal water colder for eddies isolated from the Alaskan Stream region for a longer time. Particle-tracking experiments using re-analysis products from a data-assimilative eddy resolving ocean model suggested that the dichothermal water within Eddy A was cooled by the entrainment of surrounding colder water even during the spring warming period. The mesothermal waters at depth around 250 m demonstrated similarity among the observed eddies, and the maximum temperature in the mesothermal water within Eddy A (4.3 °C) was close to that of Eddies B and C (4.2 °C) in the in situ observations. These results indicated that the dichothermal water of Aleutian eddies modifies over time, whereas the mesothermal water maintains the original feature as they propagate southwestward from the Alaskan Stream region to the Western Subarctic Gyre.