Comparison of characteristics of hydrographic structures in Aleutian eddies

Thursday, 18 December 2014
Rui Saito1, Ichiro Yasuda1, Kosei Komatsu1, Hiromu Ishiyama2, Hiromichi Ueno3, Hiroji Onishi4, Takeshi Setou5 and Manabu Shimizu5, (1)The University of Tokyo, Atmosphere and Ocean Research Institute, Kashiwa, Japan, (2)Hokkaido University, Sapporo, Japan, (3)Hokkaido University, Faculty of Fisheries Sciences, Sapporo, Japan, (4)Hokkaido University, Faculty of Fisheries Sciences, Hakodate, Japan, (5)Natl Res Inst Fisheries Sci, Yokohama, Japan
Mesoscale anticylonic eddies formed in the Alaskan Stream region south of the Aleutian Islands between the 180° meridian and about 170°E are called Aleutian eddies. Many of these eddies propagate in the southwestward direction, and often reach western subarctic gyre. The objective of present study is to compare hydrographic structures of three Aleutian eddies with distinctive propagation paths and evaluate potential impacts of physical environment along the paths on those eddies. We observed three eddies during the summers of 2010 and 2012, in which two of the eddies (A and C) were observed west of 172°E and one (B) east of 172°E. In each eddy, a subsurface cold dichothermal water (3.0-4.0°C) was observed to be located above a subsurface warm mesothermal water (4.0-4.5°C), and the two waters were separated by 26.5-26.6σθ isopycnals. The minimum temperature in the dichothermal water at ~26.4σθ was lower in the eddies A and C (2.8-2.9°C) than in the eddy B (3.2°C). This difference could be ascribed to eddy history of wintertime cooling and influence of warm Alaskan Stream. The Aleutian eddies were originated from the Alaskan Stream, and propagated westward after isolation from the Alaskan Stream. Winter cooling could make the dichothermal water cooler for eddies which were isolated for longer time. This is the case for the eddy A. The particle tracking experiments using the 1/10° eddy resolving ocean model called FRA-ROMS (Fisheries Research Agency of Japan) demonstrated that the water within the eddy was mostly originated from cold water around the propagation path in the early spring, suggesting continuous cooling of dichothermal water after the winter-time cooling. The eddy B stayed near the Alaskan Stream which could provide warm water to the eddy, to make less cold dichothermal water. The particle tacking experiments using FRA-ROMS confirmed that the water in the eddy was mainly originated from warm Alaskan Stream in the early spring, suggesting relative warming of dichothermal water. These are also confirmed from the climatological winter-mean temperature (WOA13) at 26.4σθ which was lower (< 3.0°C) around the propagation paths of eddies A and C which moved to the west of 171°E during winter than that of eddy B.