Impact of Mesoscale Eddies on Kuroshio Intrusion Variations at Northeast of Taiwan

In the North Pacific Subtropical Countercurrent region, abundant mesoscale eddies are generated and propagated to the western boundary, which has significant impact on the Kuroshio and its intrusion at northeast of Taiwan. To date, neither in situ continuous ocean current observations nor a conclusive dynamical explanation on eddy activities modulating the Kuroshio intrusion has been given. Examinations of nine-year data assimilative Hybrid Coordinate Ocean Model outputs were used to investigate the impact of eddies on the Kuroshio and its intrusion in detail, especially at subsurface. In the lower layer of the ocean in the shelf break region, cyclonic eddies decrease the current velocity and intensify the ocean stratification, thus weaken the constraint of topography on the current in the upper ocean and favor more Kuroshio intrusion. Further diagnostic analyses indicate that, the forcing from negative potential vorticity advection carried by cyclonic eddies could decrease the potential vorticity (PV) gradient across the shelf break, which enables the Kuroshio water to overcome the PV barrier induced by the steep slope and intrude onto the East China Sea shelf. The impact of anticyclonic eddies is opposite to cyclonic eddies’. Furthermore, one Kuroshio intrusion event induced by a cyclonic eddy was captured by mooring observations during late autumn in 2008 northeast of Taiwan. The in situ ocean current observations along with other data during the same period further confirmed the dynamical interpretations.