Regeneration of a warm anticyclonic ring by cold water masses within the western subarctic gyre of the North Pacific
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Regeneration of a warm anticyclonic ring as a result of interaction with cold water masses was observed within the western subarctic gyre of the North Pacific. Satellite, profiling float, and shipboard observations revealed that a warm-core ring originated from the Kuroshio Extension, propagating northeastwards, entrained cold and fresh water masses from the coastal area of Hokkaido, which are typically recognized within the ring as water that is colder than 2.5 °C. The potential temperature and planetary contribution of potential vorticity of the cold water in the coastal area of Hokkaido were <2 °C and 15 × 10−11 m−1s−1, respectively, suggesting that it originated from the Sea of Okhotsk. After the intrusion, the warm core of the ring cooled, freshened, and contracted, while the outer and lower parts became occupied by the cold and fresh water; however, even after the cooling, the positive surface elevation and downward depression of the main pycnocline, typical of an anticyclonic ring, were still evident. The ring continued to propagate northeastwards, with the main part of its structure occupied by the cold water, but changed its direction of travel from northwest to west-southwest 8 months after the cold-water event, and was finally absorbed into another warm-core ring. It is suggested that these anticyclonic rings, which transported and mixed warm and cold water masses, play important roles in the cross-gyre exchange of subtropical and subarctic waters in the North Pacific.
KeywordsAnticyclonic ring Regeneration Warm core Cold core Kuroshio Sea of Okhotsk Cross-gyre exchange Profiling float
The authors thank members of the Fisheries Environmental Oceanography group of the Atmosphere and Ocean Research Institute, The University of Tokyo, for their help in acquiring the profiling float data. The hydrographic data obtained by the R/V Wakataka-Maru were provided by the Tohoku National Fisheries Research Institute. This research was supported by the Ministry of Education, Science, Sports and Culture of Japan via a Grant-in-Aid for Scientific Research (KAKENHI) (S) 20221002, (B) 23310002, and Innovative Areas 24121002, and the Global Warming Project supported by the Ministry of Agriculture, Forestry and Fisheries Japan.
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