Interannual Variability in the Mid- and Low-Latitude Western North Pacific

Abstract

Twenty-two years (1967?88) of hydrographic data collected by the Japan Meteorological Agency along the 137°E meridian and surface wind data compiled by Florida State University (FSU) were analyzed to study the interannual variability in the western North Pacific. In the midlatitude region north of 22°N, the dominant signal in the dynamic height field was the interannual path variations of the Kuroshio. Whereas the eastward transport of the Kuroshio itself had no significant changes between the straight-path and meander-path years, the net transport of the Kuroshio system including recirculations showed a 30% increase during the meander-path years. In the straight-path years when the net transport was small, the Kuroshio tended to take a straight path with a strong recirculation developed to the south. The interannual path variations of the Kuroshio strongly influenced the water-mass movement in the midlatitudes. During the Kuroshio meander years, we found that a significant portion of the North Pacific Intermediate Water east of the Kuroshio meander was blocked from subducting farther westward. In the middepth layer of 1500?2500 m, analysis of the ??S relation revealed a water-mass movement negatively correlated to the upper-layer Kuroshio path changes, implying a possible compensating flow in the middepth layer for the cold-core eddy emerging north of the Kuroshio. In the low-latitude region along 137°E, fluctuations in the surface height anomaly field had a meridionally coherent structure, and large surface height drops coincided with the ENSO events in the tropical Pacific. Accompanying the surface height drops in the ENSO years was an increase in the transport of both the North Equatorial Current (NEC) and the North Equatorial Countercurrent (NECC) and a southward shift in the boundary of the NEC and NECC. Based on the FSU surface wind data, we found that these interannual fluctuations of the NEC and the NECC were highly correlated to the Sverdrup transport fluctuations estimated from the basinwide wind-stress curl field. Using a reduced-gravity model and simplified patterns of wind forcing, we showed that this high correlation came about because the center of the interannual signal of the wind-stress curl field is close to the western Pacific (near the date line) and because the thermocline tilt in the NEC region attenuates the strong latitude dependence of the phase speed of the long baroclinic Rossby wave.