Kuroshio intrusion (KI) is a critical linkage between the Pacific and the South China Sea (SCS), profoundly influencing the variability of marine dynamical and ecological processes of the SCS. Due to the complex mechanism and the lack of predictability study on KI, the accuracy of KI prediction remains limited. This study obtains the fastest growing initial errors (FGIEs) of KI using the Regional Ocean Modelling System (ROMS) and conditional nonlinear optimal perturbation (CNOP) method. Specifically, the CNOP, which is an effective method in calculating FGIEs in a nonlinear system, refers to the perturbation that can lead to the maximum of an objective function at a target time under certain constraints. The calculation results reveal two types of FGIEs with similar spatial patterns but opposite signs. When superimposed on the background field, both types of errors exhibit rapid growth and northwestward propagation. At prediction time, the CNOP+ (with positive sea surface height error) and CNOP- (with negative sea surface height error) errors respectively cause significant overestimation and underestimation of KI. Notably, CNOP- errors may even lead to complete failure in predicting the occurrence of KI. The rapid error growth primarily originates from barotropic instability induced by the zonal velocity shear of the reference state. Sensitive areas for targeted observations, identified through vertical integration of initial total energy error, extend northwestward from the southern Luzon Strait to the interior SCS, centered near 120.5°E, 20°N. Remarkably, removing initial errors within this sensitive area (covering merely 0.1 % of the total model domain) can improve KI prediction accuracy most effectively, by 25 %∼38 %. This research provides an effective guidance for the design of targeted observation strategies, having great significance in improving the prediction skill of KI.
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