Factors affecting extreme rainfall events in the South Pacific

Abstract

Extreme rainfall events in the South Pacific are widespread and affected by various factors on different time scales. We use daily rainfall data from 20 stations over the South Pacific to investigate the characteristics of extreme rainfall events from 1979 to 2018. For regional analysis, we group the stations into three clusters characterizing the western, the central, and the far eastern regions of the South Pacific Convergence Zone (SPCZ). Extreme rainfall events contribute to roughly 20% of the seasonal mean rainfall in all three clusters. Among all four factors considered, tropical cyclones (TCs) cause the highest increase in the probability (ΔpwesternSPCZ~286%, ΔpcentralSPCZ~84%, ΔpfareasternSPCZ~189%) of extreme rainfall events. The Madden-Julian Oscillation (MJO) is the second most important factor affecting the probability of extreme rainfall events, increasing it by 30–60% when the MJO is active over the western SPCZ in phases 5–6, over the central SPCZ in phases 6–7, and over the far eastern SPCZ in phases 8–1. The probability is reduced by the same order of magnitudes during the opposite dry phases of the MJO, i.e., phases 1–3 for the western and central SPCZ, and 3–6 for far the eastern SPCZ region. The probability of extreme rainfall events increases during La-Niña (El-Niño) conditions to the southwest (southeast) of the mean SPCZ by 27% (31%); however, the impact of the El-Niño and Southern Oscillation (ENSO) along the SPCZ is not apparent. Dynamical analysis shows that the favorable conditions for generating extreme rainfall events are associated with northwesterly moisture transport and its convergence. The impact of TCs, MJO, and ENSO on rainfall extreme events can be partly understood considering this dynamical analysis. Extratropical Rossby waves can trigger tropical disturbances, but their impact on extreme rainfall events is generally less important than of the TCs, MJO, and ENSO.