PDO modulation of ENSO effect on tropical cyclone rapid intensification in the western North Pacific

Abstract

This study investigates how the Pacific Decadal Oscillation (PDO) modulates the effect of El Nino/Southern Oscillation (ENSO) on tropical cyclone rapid intensification (RI) in the western North Pacific. The analysis shows that the interannual relationship between ENSO and annual RI number in warm PDO phases is strong and statistically significant. In cold PDO phases, however, there is no significant correlation between ENSO and RI on the interannual timescale. The enhancement of the interannual ENSO–RI relationship in warm PDO phases is mainly attributable to the change of the environmental vertical wind shear. The PDO in warm (cold) phases can strengthen (weaken) an El Nino event to increase (reduce) the effects of the warm pool of water over the equatorial Pacific in typhoon season by local diabatic heating. El Nino events are accompanied by the stronger Walker circulation in the equatorial Pacific in the warm PDO phase than in the cold PDO phase. In contrast, the Walker circulation pattern and amplitude associated with La Nina events is less affected by the alternate PDO phase. This tends to make the atmospheric response to ENSO stronger (weaker) in warm (cold) PDO phase, and so is the atmospheric teleconnection of ENSO. Our results indicate that the stratification of ENSO-based statistical RI forecast by the PDO can greatly improve the accuracy of statistical RI predictions.