Generation of a trigger meander for the 2004 Kuroshio large meander

Abstract

A generation process of a small meander southeast of Kyushu, which triggered the subsequent Kuroshio large meander in 2004, has been investigated using an ocean data assimilation and prediction system. The assimilation result reveals that the small meander is the most developed one since 1993. A cyclonic eddy propagating westward from the Kuroshio recirculation (KR) and a disturbance from the East China Sea (ESC) that accompanies a high potential vorticity (high‐PV) anomaly play important roles for the generation of the small meander. The cyclonic eddy triggers an initial cyclonic anomaly southeast of Kyushu. When the high‐PV anomaly passes through the Tokara Strait, a large positive vorticity is generated because the stretching of the water column is enhanced. The positive vorticity is brought in the cyclonic anomaly and accumulates there. As a result, the small meander is generated. From two sensitivity experiments an occurrence condition is determined by the cyclonic eddy from the KR by triggering the initial small‐scale meander, and the disturbance from the ECS contributes to the horizontal scale of the small meander. Tracing back the disturbance further to the upstream region of the Kuroshio, it is revealed that the disturbance is a frontal wave with a large horizontal scale generated by collision between a strong anticyclonic eddy and the Kuroshio east of Taiwan. The frontal wave propagates along the Kuroshio accompanying high‐PV generated around the continental shelf edge by viscosity. Thus the large frontal wave could contribute to the generation of the developed trigger meander by transporting a larger amount of high‐PV anomaly to the Tokara Strait.