Decadal variability of tropical cyclogenesis and decay in the southwest Pacific

Tropical cyclones (TCs) represent a significant hazard to the southwest Pacific (SWP) region, impacting on properties, lives and infrastructure, accounting for similar to 76% of reported disasters within the region. A particular challenge that island nations face is the inherent degree of variability in TC risk from season to season, which hampers adaptation planning due to the difficulty in predicting how many TCs will form and the subsequent areas of impact. Therefore, this study aims to assess the historical variability of TC genesis and decay (i.e., the start and end of a TC life cycle) for the SWP over the last 70 years. This is achieved through a spatio-temporal analysis of TC Best Track Data from South Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) dataset from 1948 to 2017 for TCs that originate within 0 degrees-35 degrees S and 135 degrees E-120 degrees W. Results highlight substantial decadal variability in dominant regions of genesis and decay over the last seven decades. In particular, we observe a statistically significant displacement of TC genesis (decay) by 898 km (909 km) in an east-northeast (east-southeast) direction from 1948 to 2017. We also demonstrate that TCs undergoing extratropical transition have significantly decreased in terms of both frequency and duration during the last four decades, however, the length of the extratropical cyclone tracks has increased. We attribute these observations to conducive environmental conditions, including warmer sea surface temperatures in the eastern region of TC genesis, sufficient moisture content (relative humidity) and favourable vertical wind shear. Further, the observed ocean warming and decreased wind shear during recent decades across the south of the SWP region favour the east-southeasterly extension of extratropical cyclones. The findings of this study increase our understanding of decadal to multidecadal TC risk for SWP island nations and may assist in improving seasonal TC outlooks.