The predictability of eddy shedding from Kuroshio loop in the Luzon Strait: insights from ensemble forecast

Abstract

The Kuroshio often intrudes into the South China Sea (SCS) in the form of a loop through the Luzon Strait in winter, accompanied by eddy shedding, significantly impacting the circulation and thermodynamic state in the northern SCS. The eddy shedding of the Kuroshio loop into the SCS is influenced not only by local winds but also modulated by the western Pacific (WP) circulation. Given the uncertainties in simulating the WP circulation, accurately forecasting the process of the Kuroshio loop and eddy shedding into the SCS poses a considerable challenge. Ensemble forecast based on Ensemble Kalman Filter (EnKF) assimilation represents an advanced forecasting methodology. Compared to deterministic forecast from a single model, ensemble forecast effectively considers model error uncertainties throughout the forecasting process. Utilizing the high-resolution SCS-WP EnKF assimilation forecast system, our study focuses on the predictability of a strong Kuroshio loop and subsequent eddy shedding into the SCS during the winter of 2016-2017. The findings indicate that the successful forecast of this strong Kuroshio loop and eddy shedding primarily attributed to the assimilation of ocean observations in the Subtropical Countercurrent (STCC) region, partially supported by the assimilation of observations in the North Equatorial Current (NEC) region. Specifically, assimilating observations in the STCC region reproduced observed westward-propagating cyclonic eddies, which, upon collision with the Kuroshio in the Luzon Strait, intensified its intrusion into the SCS and led to eddy shedding. Meanwhile, assimilation in the NEC region represented the weak background current of the Kuroshio, further promoting the formation of a strong Kuroshio background loop and the occurrence of eddy shedding in the Luzon Strait according to the teapot effect.