Pacific authors, technical partners and region-wide universities are telling their stories on the science and impacts of climate change with the release of two new peer-review papers on “Tropical cyclones contributing to extreme rainfall over the southwest Pacific nations” and “Historical and future drought impacts in the Pacific islands and atolls”. The two papers are a product of a year long process of the Next Generation of Climate Projection for Western Pacific project implemented by the Commonwealth Scientific and Industrial Research (CSIRO) and Secretariat of the Pacific Regional Environment Programme (COSPPac, SPREP) funded by the Government of Australia. The overall goal is to increase the number of peer-reviewed papers on the Pacific and climate change. This initiative was made possible by the contributions of Pacific National Meteorological Services, Pacific and Australia Universities and all technical partners from around the region.
The first paper noted the Southwest Pacific (SWP) nations are among some of the worst impacted and most vulnerable globally in terms of tropical cyclone (TC)-induced flooding and accompanying risks. The study on TC and extreme rainfall, thus, first provides an improved understanding of the fractional contributions of seasonal maximum daily rainfall by TCs (or TC-induced extreme rainfall) in the SWP region. Specifically, it shows how these contributions are modulated by the various modes of natural climate variability (such as the Madden-Julian Oscillation - MJO and the El Niño Southern Oscillation - ENSO) that operate in the region and whether TC-induced extreme rainfall has changed over the past decades – and if so, how? Second, a state-of-the-art-statistical model has been developed to evaluate the relationships between extreme rainfall (both TC- and non-TC-induced) in the SWP region and the combinations of climatic drivers that modulate them.
Daily rainfall station data for the SWP Island nations were used and only stations that have a near complete record of daily rainfall data (i.e. a minimum of 330 days in a year) since 1970 were included in the analyses. Since the emphasis is on extreme rainfall due to TCs, the two widely used measures of TC-induced extreme rainfall are utilised in this work: a block maxima approach and peaks-over-threshold approach. A probit regression model using Bayesian fitting were also developed for individual island nations to better understand the synergistic relationships between TC-induced extreme rainfall and combinations of various climatic drivers that modulate the relationship.
It is shown that TC contributions to extreme rainfall are substantially enhanced during active phases of the MJO and by El Niño conditions; this enhancement is primarily attributed to increased TC activity in the SWP during these event periods. There are also indications of increasing intensities of TC-induced extreme rainfall events over the past few decades. In addition, the Bayesian regression models are found to be very useful for not only assessing probabilities of TC- and non-TC induced extreme rainfall events but also evaluating probabilities of extreme rainfall for cases with different underlying climatic conditions. For example, TC-induced extreme rainfall probability over Samoa can vary from ~95% to ~75% during a La Niña period, if it coincides with an active or inactive phase of the MJO and can be reduced to ~30% during a combination of El Niño period and inactive phase of the MJO. Several other such cases have been assessed for different island nations, providing information that have potentially important implications for planning and preparing for TC risks in vulnerable Pacific Island nations. To ensure Pacific information is made available full details of the paper on TC is available online via https://doi.org/10.1007/s00382-021-05680-5
The second paper on Historical and future drought impacts in the pacific and atoll will be featured in the next article.
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