TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 60 (No. 2), pp. 453-462, 2004

Formation Process of North Pacific Intermediate Water Revealed by Profiling Floats Set to Drift on 26.7sq Isopycnal Surface

Yugo Shimizu1*, Takanori Iwao2, Ichiro Yasuda3, Shin-ichi Ito1, Tomowo Watanabe4, Kazuyuki Uehara5, Nobuyuki Shikama6 and Toshiya Nakano7

1Tohoku National Fisheries Research Institute, Shinhama-cho, Shiogama, Miyagi 985-0001, Japan
2Hakodate Marine Observatory, Mihara, Hakodate, Hokkaido 041-0806, Japan
3Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Hongo Bunkyo-ku, Tokyo 113-0033, Japan
4National Research Institute of Fisheries Science, Fukuura, Kanazawa-ku, Yokohama, Kanagawa 236-8648, Japan
5National Research Institute of Far Seas Fisheries, Shimizu-Orito, Shizuoka, Shizuoka 424-8633, Japan
6Frontier Observational Research System for Global Change, Natsushima, Yokosuka, Kanagawa 237-0061, Japan 7Meteorological Research Institute, Nagamine, Tsukuba, Ibaraki 305-0052, Japan

(Received 21 February 2003; in revised form 23 October 2003; accepted 23 October 2003)

Abstract: Six newly developed floats, which were set to drift on the 26.7sq isopycnal surface and to profile temperature, salinity and pressure above 1000 dbar once a week, were deployed in the Oyashio and Kuroshio Extension (KE) in order to examine the circulation, formation site and time scale of newly formed North Pacific Intermediate Water (NPIW). The floats were deployed in February or May 2001, and the data from their deployments to December 2002 are analyzed here. Four of the six floats were deployed near the KE axis at around the first meander crest, and they moved eastward to 157°E-176°W at latitudes of 30°N-45°N. The other two floats deployed in the Oyashio water with low-potential vorticity near the south coast of Hokkaido moved southward to reach the KE front and then moved eastward to the same region as the first four floats. The temperature and salinity at 26.7sq measured by the profiling floats indicate that the source waters of NPIW, Oyashio and Kuroshio waters are drastically mixed and modified in the mixed water region west of 160°E. The floats were separated into the three paths east of 160°E between the Kuroshio Extension front and the north of Water-Mass front (nearly subarctic front). New NPIW is judged to be formed along these three paths since the vertical profiles of temperature and salinity are quite smooth, having a salinity minimum at about 26.7sq along each path. Kuroshio-Oyashio isopycnal mixing ratios of the new NPIW are 7:3, 6:4 and 5:5 at 26.7sq along the southern, middle and northern paths, respectively. Potential vorticity converges to about 14-15 10-11 m-1s-1 along these paths. The time scale of new NPIW formation is estimated to be 1-1.5 years from the merger of Oyashio and Kuroshio waters to the formation of the new NPIW.

*Corresponding author E-mail: yugo@affrc.go.jp

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