Kuroshio Water Intrusion Into The South China Sea Inferred From The Lagrangian Coherent Structure

Previous studies have showed that Kuroshio patterns in the vicinity of Luzon Strait displayed significant transient features and were usually categorized into"Kuroshio loop"?"Kuroshio branch"?"anticyclonic eddy detached from Kuroshio loop"?"Kuroshio leap Luzon Strait" types in previous studies.In this study,the surface water transport in Luzon Strait was analyzed in detail by using satellite-tracked surface drifters from 2000-2017,surface velocity data from 1993-2017,warm eddy data from winter cruise in 2010 and cold eddy data from May 2017.Here we show a Lagrangian view of upper water exchanges across the Luzon Strait based on the finite time Lyapunov exponents(FTLE)fields computed from altimetric geostrophic current.The Lagrangian coherent structures(LCSs)extracted from FTLE fields well identify the typical flow patterns and eddy activities related to the Kuroshio intrusion.In addition,these LCSs also reveal the intricate transport trajectories and fluid domains around the Luzon Strait,which are validated by the tracks of satellite-tracked surface drifters and cannot be visually recognized in the velocity maps.The FTLE fields indicate that there are mainly four types of particle trajectories near the Luzon Strait,among which the Kuroshio northward-flowing"leaping" pattern and the clockwise rotating "looping" pattern occur more frequently than the direct Kuroshio branch into the SCS called "leaking" pattern or the SCS water outflowing to Pacific.The Kuroshio's eddy shedding events at the Luzon Strait are further analyzed and the importance of considering LCSs in estimating transport by eddies is highlighted.The anticyclonic eddy(ACE)shedding cases reveal that ACEs mainly originate from the looping paths of Kuroshio and thus could effectively trap the Kuroshio water before eddy detachments.During the 21 ACE shedding events from 1993 to 2017,there were 18 events with Kuroshio water captured by ACE.Th capture ratios ususally increased first and then decreased in the 30 days before shedding.LCSs provide useful information to predict the positions of the upstream waters which finally enter the ACEs.However,due to the different intensity of the "Kuroshio loop" and the strong nonlinear interaction during the development of the eddy,the capture ratios in these eddy shedding events are significantly different.In addition,under the background of warm eddy shedding,Pacific water does not necessarily get involved in the warm eddy and enter SCS with warm eddy.When there is a strong cyclone cold eddy in the northwest of Luzon island,the interaction between the Kuroshio loop current and the cold eddy or the eddy pair can significantly regulate the transport trajectory.At this time,Kuroshio water can enter SCS directly through the path at the boundary of the eddy pair.In contrast to the behaviors of ACEs,during the formations of single cyclonic eddies(CEs)west of the Luzon Strait,the LCS snapshots exhibit strong barriers between the CEs and the Kuroshio mainstream,indicating that the upstream Kuroshio waters could hardly enter the CEs through the mesoscale advections.So CEs have little contribution to the upper water exchanges from the Pacific into the SCS.The distinct eddy-trapping processes in ACEs and CEs are supported by the cruise observations.