TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 60 (No. 2), pp. 303-312, 2004

Operational Data Assimilation System for the Kuroshio South of Japan: Reanalysis and Validation

Masafumi Kamachi1*, Tsurane Kuragano2, Hiroshi Ichikawa3, Hirohiko Nakamura3, Ayako Nishina3, Atsuhiko Isobe4, Daisuke Ambe4, Masazumi Arai5, Noriaki Gohda5, Satoshi Sugimoto2, Kumi Yoshita2, Toshiyuki Sakurai2 and Francesco Uboldi6

1Meteorological Research Institute, Nagamine, Tsukuba 305-0052, Japan
2Office of Marine Prediction, Japan Meteorological Agency, Ohtemachi, Tokyo 100-8122, Japan
3Faculty of Fisheries, Kagoshima University, Shimoarata, Kagoshima 890-0056, Japan
4Department of Earth System Science and Technology, Interdisciplinary Graduate School of Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
5Faculty of Engineering, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8527, Japan
6LEGOS/BRESM, 14, Av. Edouard Belin 31400 Toulouse, France

(Received 11 September 2003; in revised form 2 February 2004; accepted 2 February 2004)

Abstract: We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis (historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5-10 Sv) to the east of Amami-Ohshima Island (20 Sv). Comparing the reanalysis with independent observations on the south-east slope of the Amami-Ohshima Island indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999 (November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong (weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the axis (64 km).

*Corresponding author E-mail: mkamachi@mri-jma.go.jp

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