A New Mechanism That Affects The Path Of The Kuroshio Set

In this study we show the Kuroshio intrusion into the South China Sea at the Luzon Strait can be remotely influenced by the downstream intrusion of the Kuroshio into the East China Sea northeast of Taiwan,at a lead time of about 220 days.This remarkable finding is revealed by a quantitive causality analysis which has just been rigorously established from first principles.It is originally motivated by the satellite observation of a cyclonic recirculation southeast of Taiwan formed as a wake of the loop current.It is found that the recirculation plays as a mediator bringing the influence exerted by the downstream intrusion in the East China Sea to the upstream loop current in the South China Sea.Further analysis indicates that a certain mode of the coastal trapped waves propagating southward along the eastern coast of Taiwan connects the two important intrusions of the Kuroshio.The onshore and offshore cross-shelf motions in association with the East China Sea intrusion excites coastal trapped waves on the continental shelf northeast of Taiwan.A low-frequency mode of the coastal trapped waves supported by both idealized model and numerical output,has a period close to that of a Rossby normal mode west of the Luzon Strait.Consequently,as the negative(positive)phase of the coastal trapped wave approaches,the southwestern and southeastern sides of Taiwan are occupied by an anticyclonic(a cyclonic)eddy and a cyclonic(an anticyclonic)eddy,respectively,with the former being a center of the normal mode.The southward(northward)meridional velocity of the negative(positive)phase intensifies the eddy dipoles,which work to strengthen(weaken)the recirculation and thus the loop current in the South China Sea.The particular mode of the coastal trapped waves has a phase speed of near 1.22 km per day and hence spends 220 days propagating from the northern to the southern tips of Taiwan,corresponding to the lead time as reported by the causal inference.