Asymmetric impact of Central Pacific ENSO on the reduction of tropical cyclone genesis frequency over the western North Pacific since the late 1990s

Abstract

Tropical cyclone (TC) genesis frequency over the western North Pacific (WNP) is significantly reduced since the late 1990s, coinciding with a Pacific decadal oscillation (PDO) phase transition from positive to negative. In this study, the underlying mechanism for this change is investigated through analysis of asymmetric central Pacific (CP) El Nino-Southern Oscillation (ENSO) properties induced by the negative PDO phase. Our results suggest that the significant reduction is caused by asymmetric CP ENSO properties, in which the CP La Nina is more frequent than the CP El Nino during negative PDO phases; furthermore, stronger CP La Nina occurs during a negative PDO phase than during a positive PDO phase. CP La Nina (El Nino) events generate an anticyclonic (cyclonic) Rossby wave response over the eastern WNP, leading to a significant decrease (increase) in eastern WNP TC genesis. Therefore, more frequent CP La Nina events and the less frequent CP El Nino events reduce the eastern WNP mean TC genesis frequency during a negative PDO phase. In addition, stronger CP La Nina events during a negative PDO phase reinforce the reduction in eastern WNP TC genesis. The dependency of CP ENSO properties on the PDO phase is confirmed using a long-term climate model simulation, which supports our observational results. Our results will also improve understanding of TC in other basins, since both PDO and CP ENSO variability influence global dynamics.