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.