Weakening of the Kuroshio Intrusion into the South China Sea under the Global Warming Hiatus

We analyze a 22-year (1993-2014) record of the Kuroshio intrusion (KI) index defined from maps of mean absolute dynamic topography west of Luzon Island in the Philippines to investigate the long-term trend of the KI into the South China Sea (SCS). The monotonically increasing trend of the KI index extracted by empirical mode decomposition implies a slight decreasing trend of the meridional pressure gradient across the Luzon Strait, suggesting a weakening of the KI into the SCS based on the geostrophy. This weakening is largely due to an intensification of the upstream Kuroshio east of Luzon, which is closely correlated with the latitude of the North Equatorial Current (NEC) bifurcation off the Philippines. We also find that the latitude of the NEC bifurcation has tended to migrate southward in recent years, coincident with the strengthening of the Kuroshio east of Luzon. The forcing of the trends of low-latitude circulation is attributable to changing surface wind fields in the tropical Pacific. The trade winds have intensified in response to the recent global warming hiatus, and an anomalous cyclonic wind field has appeared over the Philippine Sea. Both of these phenomena have contributed to the southward migration of the NEC bifurcation latitude in recent years. This has led to enhance the Kuroshio transport east of Luzon, and the Kuroshio has tended to bypass the Luzon Strait without significant westward encroachment.