The effect of the variable Kuroshio intrusion pathway on deep flow in the northern South China Sea (NSCS) is investigated using observational and theoretical analyses. With significant seasonality in path variation, the leaking path is found to be most frequent, whereas the looping (leaping) path is transient and prefers to occur in winter (summer). Such multi-path variability contributes greatly to the highest surface eddy kinetic energy in the NSCS, with the energy peaking for the looping path in winter, followed by the leaking and leaping paths. The information flow-based causality analysis suggests that the energetic surface perturbations in the Kuroshio intrusion region are causal to the intraseasonal variability in deep flow east of the Dongsha Islands, wherein the linkage is strongest within the Kuroshio Loop Current eddy shedding event related to the looping path. Therefore, the underlying dynamics for the vertical coupling is illustrated by this event, in which the eddy is highly compressed to the surface because of sloping topography and its energy is partially transferred downward through vertical pressure work to force the abyssal topographic Rossby waves. Moreover, the energetics and instability analyses reveal that barotropic instability dominates the energy transfer from background flow to eddies in the Kuroshio intrusion region, which energizes the surface perturbations most intensively for the looping path, followed by the leaking and leaping paths. The characteristic scales of the most unstable mode estimated by the reduced gravity model are analogous to the observations and account for the rapid growth of surface perturbations in the Kuroshio intrusion region.

Kuroshio intrusion,South China Sea,EKE,Deep flow