The Kuroshio Current is a northward flowing Western Boundary Current (WBC) in the Pacific Ocean. It is a bifurcation arm of the North Equatorial Current and consists of northwestern Pacific Ocean water. The other arm is the southward flowing Mindanao Current. The Kuroshio Current flows along the eastern Philippine coast, up to 13.7 Sv... of it leaking into the Luzon Strait - the gap between the Philippines and Taiwan - before continuing along the Japanese coast. Some of the leaked water manages to intrude into the South China Sea (SCS). This affects the heat and salt budgets and circulation and eddy generation mechanisms in the SCS. There are various theories about possible intrusion paths and what mechanisms initiate them.
Intrusion paths
From satellite data, Nan, et al. (2011) concluded there are three intrusion paths for the Kuroshio Current into the SCS. A northward flowing WBC (like the Kuroshio Current) can deform at a gap in a western boundary and form an anticyclonic current loop if the gap is wide enough. This results in a looping path, where water from the Kuroshio flows through the middle of the Luzon Strait into the SCS and out in the north of the strait. The current loop in the SCS forms due to Ekman transport resulting from northeasterly winds that push Kuroshio surface water westward. Anticyclonic eddies can shed from the current loop and penetrate farther into the SCS, as has been observed by Li (1997) During the winter monsoon season, Kuroshio intrusion strengthens. Winds blow in the northwestward direction, thereby pushing Kuroshio surface water into the Luzon Strait. This can result in an anticyclonic bending of the Kuroshio flow into the Luzon Strait, from which a branch detaches into the SCS. A cyclonic gyre forms northwestward of the Luzon Strait as a result of this leaking path. This theory is based on observations by D.Z. Qiu, et al. (1984) from floaters but more recently no such branch has been observed The Kuroshio can also take a leaping path across the Luzon Strait and into the SCS. This is seen as a strengthening of the Luzon Cyclonic Gyre to the west of the strait while the Kuroshio continues northwards along the eastern Taiwanese coast. The anticyclonic gyre normally present in the SCS is significantly weakened as a result.
Intrusion mechanisms
Wind forcing The Luzon Strait and SCS experience seasonally reversing monsoon winds; these are southwestward and stronger in the boreal winter and northeastward and weaker in the boreal summer. This results in negative wind-driven Ekman transport in the winter, strengthening Kuroshio intrusion, and positive transport in the boreal summer, weakening intrusion. Wind-driven Ekman transport could therefore contribute to westward flow through the Luzon Strait and hence to Kuroshio leakage into the SCS. However, research has shown that less than 10% of Luzon Strait transport is due to purely wind-driven Ekman flow. Nevertheless, wind-driven Ekman drift still influences the inflow angle and speed of Kuroshio intrusion
Inter-basin pressure gradient A build up of water has been observed on the Pacific side of the Luzon Strait by Y. T. Song (2006), which results in a pressure gradient across the strait. This could initiate the bending of the Kuroshio Current into the Luzon Strait, thereby resulting in eventual leakage. Satellite data show a decreasing trend in Kuroshio intrusion strength over time, which correlates with a decrease in the cross-Luzon Strait pressure gradient, thereby supporting this theory. However, the exact mechanism for a pressure gradient-induced intrusion is not yet fully understood. The proposed equation describing the transport Q {\displaystyle Q} between the SCS and the Pacific Ocean basins is based on a two-layer ocean model.
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