Optimal precursor of the transition from Kuroshio large meander to straight path

We used the conditional nonlinear optimal perturbation (CNOP) method to explore the optimal precursor of the transition from Kuroshio large meander (LM) to straight path within a barotropic inflowout-flow model, and found that large amplitudes of the optimal precursor are mainly located in the east of Kyushu, which implies that perturbations in the region are important for the transition from LM to straight path. Furthermore, we investigated the transition processes caused by the optimal precursor, and found that these processes could be divided into three stages. In the first stage, a cyclonic eddy is advected to the formation region of the Kuroshio large meander, which enhances the LM path and causes a cyclonic eddy to shed from the Kuroshio mainstream. This process causes the LM path to change into a small meander path. Subsequently, the small meander is maintained for a period because the vorticity advection is balanced by the beta effect in the second stage. In the third stage, the small meander weakens and the straight path ultimately forms. The positive vorticity advecting downstream is responsible for this process. The exploration of the optimal precursor will conduce to improve the prediction of the transition processes from LM path to straight path, and its spatial structure can be used to guide Kuroshio targeted observation studies.