Seasonal and interannual variability of the North Equatorial Current, the Mindanao Current, and the Kuroshio along the Pacific western boundary

Along the Philippine coast in the western Pacific, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current. Using both the linear, time-dependent Sverdrup theory and a high-resolution, nonlinear reduced-gravity model, this study investigated the changes in the NEC-Mindanao Current-Kuroshio (NMK) system induced by large-scale surface wind forcings. Using the Florida State University monthly wind stress data from 1961 through 1992, we show that the seasonal bifurcation of the NEC occurs at the northernmost position in October and the southernmost position in February. While the meridional migration of the basin-wide trade wind has a relatively small effect in shifting the bifurcation latitude (by about 100 km), the monsoonal wind along the low-latitude western Pacific is effective in inducing a large northward excursion of the NEC's bifurcation in the fall season. On the interannual timeseale, the positive wind stress curl of the trade wind tends to intensify and shifts the zero wind stress curl line northward prior to El Niño-Southern Oscillation (ENSO) events. With a lag of about 1 year this shift induces the bifurcation of the NEC to occur at a higher latitude. During the La Niña years the NEC generally bifurcates at a lower latitude. No significant seasonal fluctuations are found in the transport of the NEC near the Philippine coast. Seasonal changes in the Mindanao Current and the Kuroshio are, however, significant, and their transports tend to fluctuate 180° out of phase, due to the different speeds of the baroclinic Rossby waves at their respective latitudes. The Kuroshio (the Mindanao Current) has a seasonal minimum (maximum) transport in fall when the NEC bifurcates at the seasonally northernmost latitude. The interannual changes in the inflow NEC are largely controlled by the basin-wide, wind stress curl anomalies. While the quasi-biennial changes are confined only to the southern limb of the NMK system, signals with ENSO timescales are found to influence the midlatitude, subtropical circulation via the Kuroshio.