The Summer Kuroshio Intrusion Into the East China Sea Revealed by a New Mixed-Layer Water Mass Analysis

Though the Kuroshio intrusion (KI) into the East China Sea (ECS) is relatively weaker during summer, it is of great importance to the ECS shelf ecosystem in this biologically-active season. The interannual variability of the summer intrusion is less explored as the long-term observations of the oceanic current are insufficient to draw a complete and unbiased conclusion. Using 3-year in situ measurements, we develop and validate a new tracer-based mixed-layer optimal multi-parameter (MLOMP) water mass analysis, which can well capture the intrusion pattern and year-to-year variation. The result exhibits a two-layer intrusion with decoupled interannual variations. The surface intrusion is controlled by the interaction between the Kuroshio and steep topography. During the year with abnormally weak upstream transport, the Kuroshio surface water may notably enter the ECS shelf and can reach farther north, as opposed to the very weak summer intrusion revealed by previous seasonal studies. The intrusion of Kuroshio subsurface water is characterized by a northward nearshore branch current, which is regulated by the offshore Ekman transport and shoreward pressure gradient at the interannual scale. The new MLOMP is also applied to the satellite temperature and salinity to examine the surface intrusion. The results still depict a reasonable spatial distribution and year-to-year variability of the intrusion, implying potentially a more practical use for longer-term intrusion analysis. The western Pacific boundary current, that is, Kuroshio, may enter the East China Sea off the northeast Taiwan, which transports rich nutrients into the shelf region and stimulates the primary productivity of the local ecosystem, particularly in summer. However, the interannual variability of the Kuroshio intrusion (KI) is rarely investigated mainly because the long-term current variability is conventionally observed through surface drifters or derived from satellite altimetry, which suffers from the low resolution as well as the sampling biases. Here we extract the year-to-year variability of the Kuroshio intrusion by a new water mass analysis method based on the in situ measurements of temperature, salinity, and chemical tracers. The results reveal a two-layer intrusion structure with different interannual variations. The surface intrusion is related to the upstream Kuroshio intensity while the subsurface is mainly controlled by the offshore Ekman transport. Particularly, the summer intrusion is not always weak, as opposed to the traditional conclusion from seasonal studies. We also examine this tracer-based water mass analysis with satellite temperature and salinity. It turns out that the intrusion patterns and interannual variations are still reasonably captured, suggesting that the new method can be an effective tool for longer-term current intrusion study. The new mixed-layer optimal multi-parameter water mass analysis reveals that the summer Kuroshio intrusion has a two-layer structure with distinct year-to-year variation The interannual summer Kuroshio surface water intrusion is controlled by the upstream Kuroshio intensity and the Kuroshio subsurface water by the offshore Ekman transport The mixed-layer water mass analysis can be an effective tool for studying the current intrusion even with satellite data