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

Abstract

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-wid e 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 timescale, the positive wind stress curl of the trade wind tends to intensify and shifts the zero wind stress curl line northward prior to E1 Nifio-Southern Oscillation (ENSO) events. With a lag of about i year this shift induces the bifurcation of the NEC to occur at a higher latitude. During the La Nifia years the NEC generally bifurcates at a lower latitude. No significant seasonal fiuctuation•s 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 ø 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. Introduction After encountering the western boundary along the Philippine coast, the North Equatorial CUrrent (NEC) in the Pacific Ocean bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current (Figure 1) [ Nitani, 1972 ]. If the surface wind forcing is steady, it is easy to show from the Sverdrup dynamics that the NEC's bifurcatiOn should occur where the zonally averaged transport stream function is zero. In reality, the surface wind field changes both in time and in space and these changes can exert a particularly large impact upon the NEC-Mindanao Current-

Roles of the Western North Pacific Wind Variation in Thermocline Adjustment and ENSO Phase Transition

“…The anticyclonic wind stress curl may also change the strength of the subtropical and equatorial gyre circulation. The latter, especially the Mindanao Current and the North Equatorial Countercurrent may affect heat storage in the WNP (Lukas, 1988;Qiu and Lukas, 1996). The results showed in Figs.…”

Distribution of ultraphytoplankton in the western part of the North Pacific subtropical gyre during a strong La Niña condition: relationship with the hydrological conditions

“…In the western part of the Pacific subtropical gyre the NEC bifurcates into Kuroshio Current (northward flowing) and the Mindanao Current (southward flowing) along the Philippine coast. The location of the bifurcation was suggested to vary depending on several factors including local monsoons and remote Rossby waves (Qiu and Lukas, 1996). However, bifurcation is also expected to vary seasonally and interannually in agreement with the ENSO value (Kim et al, 2004).…”

Influence of the North Equatorial Current on the population genetic structure of Tridacna crocea (Mollusca: Tridacnidae) along the eastern Philippine seaboard

“…Nonetheless, concordance of genetic patterns observed for T. crocea with those of other marine taxa (Magsino 2004), as well as with biogeographic patterns in reef fish community structure (Aliño & Gomez 1993), suggests that these patterns were in part shaped by biogeographic barriers to gene flow. However, the location of the biogeographic break may span a wide latitudinal range for various marine taxa, owing to the temporal and spatial variability in flow strength and bifurcation latitude of the NEC (Qiu & Lukas 1996). The historical origin of this biogeographic pattern, i.e.…”

“…1). The bifurcation latitude is estimated to lie between 11 and 14.5°N (Nitani 1972), and exhibits seasonal meridional shifting, from 14.8 up to 17°N in summer and winter, respectively (Qiu & Lukas 1996). The divergent current pattern generated by the NEC bifurcation has been suggested as one of the primary determinants in the north-south geographical differentiation of reef fish species composition along the eastern Philippines (Aliño & Gomez 1993).…”