Key Dynamical Factors Driving the Kuroshio Subsurface Water to Reach the Zhejiang Coastal Area

Observations indicate a seasonal variation of the Kuroshio subsurface water (KSSW) intrusion into the coastal region off the Zhejiang Province of China. The observed temperature and salinity distributions suggest a stronger intrusion in June and weaker intrusion in December. A hydrodynamic model well reproduces the seasonal variation of temperature and salinity characteristics and the current structure over the continental shelf. A passive tracer in the subsurface layer is used to investigate the movement of the KSSW. Tracer experiments confirm the seasonal variation of the KSSW intrusion, showing that the intrusion is stronger in summer and weaker in autumn and winter. Moreover, tracer concentrations suggest that in general it takes at least 1.5 months for the KSSW from east of Taiwan to reach the Zhejiang coastal area. Analyses of the momentum balances indicate that the KSSW's northward movement is regulated by the geostrophic current, while the shoreward intrusion results from the bottom Ekman effect. The seasonal changes in pressure gradients and vertical eddy viscosities contribute to the KSSW intrusion into the Zhejiang coastal area in June rather than in December. Sensitivity experiments show that the seasonal variation of the KSSW intrusion is mainly induced by the different wind stresses in summer and winter. The stronger southerly monsoon, the weaker and warmer Taiwan Warm Current, and the stronger and colder Kuroshio Current are favorable to the KSSW intrusion. Plain Language Summary In this work, we focus on the key dynamical mechanisms to drive the Kuroshio subsurface water (KSSW) to reach the Zhejiang coastal area. The temperature and salinity observational data and numerical model show that there is a seasonal variation of the Kuroshio subsurface water intrusion, which is stronger in summer and weaker in autumn and winter. The KSSW intrusion is mainly induced by the different winds between summer and winter. The stronger southerly monsoon, the weaker and warmer Taiwan Warm Current, and the stronger and colder Kuroshio Current are favorable to the KSSW intrusion. This study indicates that during summer, the KSSW intrusion provides a pathway for the delivery of nutrients into the Zhejiang Province for phytoplankton growth. Thus, it will improve further understanding of the coastal marine ecosystem's response to these nutrient sources. Key Points Observations show a seasonal variation of the Kuroshio subsurface water intrusion, which is stronge