Local Winds Induce the Upper-Layer Kuroshio Intrusion Northeast of the Taiwan Island

Abstract

The shoreward intrusion of upper-layer Kuroshio water northeast of the Taiwan Island is a key process for ocean-shelf sea exchanges in the western boundary of subtropical Pacific Ocean. Yet, the underlying dynamic mechanisms remains unclear. Here in this study, causality analysis was first conducted based on satellite and reanalysis data, which suggested that local winds rather than the upstream Kuroshio transport or the surface cooling play a primary role. Numerical model experiments further confirmed this point. In winter, the downwelling-favorable northerly wind reduces the sea level in the middle shelf through Ekman pumping; whereas, the high sea level at the shelf break, due to the northward-flowing Kuroshio, slightly increases. The enhanced shoreward barotropic gradient force then pushes Kuroshio to the shelf once it loses the support of the island. Consequently, a horizontal difference in temperature, thus density, is formed by the intrusive warm water, which is further amplified by the differential air-sea heat flux. The resultant baroclinic effect regulates the path of intrusive water, forming an anticyclonic loop. In summer, the absence of these two effects results in a much weaker upper-layer Kuroshio intrusion. In autumn and spring, the differential cooling effects are negligible, and the barotropic effect of episodic northerly winds alone can induce a weak intrusion, but the anticyclonic loop is absent. In short, local winds directly trigger the upper-layer Kuroshio intrusion northeast of the Taiwan Island; other effects, such as differential cooling, are secondary.