Short-range prediction experiments of the Kuroshio path variabilities south of Japan

• Published in: Ocean dynamics Vol. 56; no. 5-6; pp. 607 - 623
• Main Authors: Usui, Norihisa, Tsujino, Hiroyuki, Fujii, Yosuke, Kamachi, Masafumi
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.12.2006
• Subjects: Ocean currents; Oceanography; Predictions; Japan
• ISSN: 1616-7341; 1616-7228
• DOI: 10.1007/s10236-006-0084-z

Assimilation and prediction experiments of the Kuroshio path variability south of Japan were conducted to investigate the predictability of the Kuroshio path. The assimilation and prediction system is composed of an eddy-resolving model and a three-dimensional variational analysis scheme with vertical coupled temperature-salinity empirical orthogonal function modes. The sea surface height (SSH) variability and the variations of the Kuroshio path of the assimilation fields are in good agreement with those observed. The results of the assimilation are then used as the initial conditions for 138 cases of 90-day prediction experiments conducted from 1993 to 2004. The predictive limit of our system is assessed by the SSH anomaly in the assimilation field and is found to be around 40-60 days, which is much longer than that of the persistence. The prediction results show good performance in the transition stage from a straight to a meandering path. For example, a large meandering event that occurred in August 2004 is successfully predicted in a 2-month forecast. Two types of failure cases are investigated. One is a case where the eastward propagation speed of the meander is faster than a real state. The dynamical response of the model to the assimilation revealed that an initial shock, caused by the dynamically unbalanced initial condition, induces the fast eastward propagation of the meander. The other case exhibits an unrealistic meander. In this case, a cold anomaly at an intermediate layer in the initial condition grows rapidly and results in the unrealistic meander. This implies that the Kuroshio path south of Japan has a chaotic nature. These facts revealed by the failure cases give us some insight for improving the predictive skill of the Kuroshio path variability. [PUBLICATION ABSTRACT]