The long-term satellite altimeter and reanalysis data show that large seasonal variations of the Kuroshio intrusion into the South China Sea are associated with geostrophic transport through the Luzon Strait, but not with the current intensity, width, and axis position east of Luzon island. To address this issue, we examine the seasonal variability of surface intrusion patterns by a new streamline-based method. The along-streamline analysis reveals that the seasonality of geostrophic intrusion is only attributed to the cyclonic shear part of the flow, while the anticyclonic shear part always leaps across the Luzon Strait. A possible physical mechanism is proposed to accommodate these seasonal characteristics based globally on the vorticity (torque work) balance between the basinwide negative wind stress curl and the positive vorticity fluxes induced by the lateral wall, as well as locally on loss of balance between the torques of frictional stresses and normal stresses owing to the boundary gap. Through modifying the nearshore sea surface level, the northeasterly (southeasterly) monsoon increases (decreases) the positive vorticity fluxes in response to global vorticity balance, and simultaneously amplifies (alleviates) the local imbalance by enhancing (reducing) the positive frictional stress torque within the cyclonic shear layer. Therefore, in winter when the positive torque is large enough, the Kuroshio splits and the intrusion occurs, while in summer the stress torque is so weak that the entire current keeps flowing north. Significance Statement The Kuroshio, when flowing past the Luzon Strait, has a tendency of meandering into the Strait. The meandering current bifurcates with a branch into the South China Sea (SCS), which provides the primary exchange of momentum, heat, and biogeochemical tracers between the SCS and the Pacific. The intrusion occurs more frequently in winter than in summer; however, the reason for this seasonality remains unclear, though various theories have been suggested in a large volume of studies. Here we used a new streamline-based method to investigate this problem with the long-term satellite altimetry and reanalysis data. We found that the Kuroshio width, intensity, and position east of Luzon island barely change within a seasonal cycle, and the seasonal variability of intrusion depends mainly on the cyclonic shear part of the Kuroshio. These characteristics makes us look for a new interpretation for the seasonal Kuroshio intrusion. The global wind stress is responsible for the Kuroshio leaping across the Strait, and the unbalanced torque of the lateral friction when the current loses contact with the wall tends to push the Kuroshio into the SCS. The seasonally reversing monsoon modulates the frictional torque strength in response to the basin-scale wind curl input, which finally explains the seasonal variability of the Kuroshio intrusion.
