Journal of Oceanography, Vol. 60 (No. 2), pp. 269-282, 2004

Short-Range Prediction Experiments with Operational Data Assimilation System for the Kuroshio South of Japan

Masafumi Kamachi¹*, Tsurane Kuragano², Satoshi Sugimoto², Kumi Yoshita², Toshiyuki Sakurai², Toshiya Nakano¹, Norihisa Usui¹ and Francesco Uboldi³

¹Meteorological Research Institute, Nagamine, Tsukuba 305-0052, Japan
²Office of Marine Prediction, Japan Meteorological Agency, Ohtemachi, Tokyo 100-8122, Japan
³LEGOS/BRESM, 14, Av. Edouard Belin 31400 Toulouse, France

(Received 12 September 2003; in revised form 19 January 2004; accepted 19 January 2004)

Abstract: The short-range (one month) variability of the Kuroshio path was predicted in 84 experiments (90-day predictions) using a model in an operational data assimilation system based on data from 1993 to 1999. The predictions started from an initial condition or members of a set of initial conditions, obtained in a reanalysis experiment. The predictions represent the transition from straight to meander of the Kuroshio path, and the results have been analyzed according to previously proposed mechanisms of the transition with eddy propagation and interaction acting as a trigger of the meander and self-sustained oscillation. The reanalysis shows that the meander evolves due to eddy activity. Simulation (no assimilation) shows no meander state, even with the same atmospheric forcing as the prediction. It is suggested therefore that the initial condition contains information on the meander and the system can represent the evolution. Mean (standard deviation) values of the axis error for all 84 cases are 13, 17, and 20 (10, 10, and 12) km, in 138.5°E, in the 30-, 60-, and 90-day predictions respectively. The observed mean deviation from seasonal variation is 30 km. The predictive limit of the system is thus about 80 days. The time scale of the limit depends on which stage in the transition is adopted as the initial condition. The gradual decrease of the amplitude in a stage from meander to straight paths is also predicted. The predictive limit is about 20 days, which is shorter than the prediction of the opposite transition.