Role of the western tropical Pacific in the North Pacific regime shift in the winter of 1998/1999

Abstract

In this study, we examine the role of the western tropical Pacific in the North Pacific regime shift during the winter (December-January-February) of 1998/1999. The North Pacific regime shift in the winter of 1998/1999 is characterized by a dipole-like structure along 40 degrees N where significant warming is prominent in the western and central North Pacific. This shift might have been associated with an abrupt warming in the western tropical Pacific after 1998/1999, in the North Equatorial Current (NEC) bifurcation region (8 degrees N-18 degrees N, 125 degrees E-160 degrees E) in particular. The NEC bifurcation region experiences a regime shift from cooling to warming during the 1998/1999. Consequently, the Kuroshio current, which originates in the NEC bifurcation region, could transport the anomalous warm water into the North Pacific. In addition, a sudden shift in the NEC bifurcation latitude to the south also contributes to the 1998/1999 regime shift in the North Pacific. We suggest a possible mechanism that could have caused the regime shift in the NEC bifurcation region during the 1998/1999. An abrupt deepening of the thermocline depth in the NEC bifurcation region from 1997/1998 to 1998/1999, which is associated with the biggest El Nino in 1997/1998 and a subsequent La Nina event in 1998/1999, plays a role in initiating the shift in the NEC bifurcation region. Subsequently, the frequent occurrence of the La Nina events after 1998/1999 plays a role in maintaining the regime shift toward warming by sustaining a deepening of the thermocline depth.