The Kuroshio intrusion has a profound influence on the marine dynamic and ecological environment in the East China Sea (ECS) shelf. In most previous studies, the intrusion has been evaluated by the volume transport across the 200 m isobath.
A 3-D high-resolution ROMS model covering the ECS, Huang Hai and Bohai Sea is established in this study. Based on this model, a whole-region passive-tracing method is proposed, and the target water body of the study area is completely traced by releasing the passive tracer at the initial position of the target water body in the study area. The concentration of the tracer in a certain area represents the volume ratio of the target water body in that area. When calculating the volume transport of the target water body, the volume ratio of the water body is further introduced into the calculation.
However, this approximate substitution leads to an underestimation of the Kuroshio intruding flux as it fails to separate the onshore and offshore transport of Kuroshio and shelf water. In this paper, a whole-region passive-tracing modeling method is proposed to quantitatively explore the bidirectional exchange of Kuroshio and shelf water, by introducing Kuroshio water concentration as a passive tracer into the flux calculations. In terms of the climatological annual mean, the net Kuroshio transport (NKT) onto the ECS shelf is as high as 1.85 Sv (1 Sv = 106 m3 s−1), with net shelf water transport (NST) of 0.42 Sv. The NKT is about 30% higher than the net volume transport (NVT) across the 200 m isobath (1.43 Sv), which indicates that the influence of the Kuroshio intrusion on the ECS may be far greater than believed before. The volume exchange on the inner shelf of the ECS is also discussed.
Analyses of the momentum balances indicate that the exchanges on both the shelf break and the inner shelf are generally dominated by the geostrophic balance, but the exchange on the shelf break is more susceptible to nonlinear effects. Results of a set of sensitivity experiments show that variation of the Kuroshio intrusion is mainlyregulated by the combined effect of Kuroshio transport, the Taiwan Strait current and local wind stress. In contrast, the local wind stress has a predominant effect on the outflow of shelf water.
For the water exchanges across the 200 m isobath，the Kuroshio and TWS current mainly affect the Kuroshio intrusion northeast Taiwan. By changing the density field, the surface heat flux has an important impact on the Kuroshio intrusion in northeast Taiwan and southwest Kyushu. While the influence of local wind on water exchanges are widely distributed throughout the upper layer of the entire shelf break. The influence of summer monsoon on shelf water exchange is multiple. On one hand, it provides more shelf water for the ECS by increasing the TWS current. On the other hand, it strengthens the shelf water drainage by inducing the offshore Ekman transport. For the water exchanges across the 100 m isobath, the TWS current plays a predominant role, and the influence of local wind cannot be ignored. The weak TWS current greatly promotes the Kuroshio onshore invasion. For shelf water drainage across the 100 m isobath, both the summer monsoon and strong TWS current have an important role.