On the Future of Extreme Rainfall in New Zealand

Abstract

Extreme rainfall is intensifying under climate change, but projected changes in the intensity, frequency, and spatial extent of extremes are not yet well constrained for smaller nations like New Zealand. Here, we use high-resolution (∼12 km) dynamically downscaled CMIP6 climate simulations to examine future changes in annual maximum one-day (Rx1d) and three-day (Rx3d) rainfall under moderate- (SSP2-4.5) and high-emissions (SSP3-7.0) scenarios across New Zealand. Projections show widespread increases in the intensity and frequency of extreme rainfall through the twenty-first century, with stronger intensification under SSP3-7.0 and more pronounced increases for Rx1d. Across models, average intensities increase by 7%–18% (8%–28%) for Rx1d and 4%–15% (5%–24%) for Rx3d under SSP2-4.5 (SSP3-7.0). While much of New Zealand is projected to experience increases in the most extreme rainfall events, high levels of internal variability result in 20%–30% of the country recording their wettest events within the historical experiment, rather than the final 30 years of the twenty-first century. Nevertheless, models consistently project 40%–50% (5%–10%) of locations around New Zealand will experience at least a doubling (trebling) in the frequency of historically extreme (once-a-decade) events by the late twenty-first century, albeit with no clear differences between regions as to who will experience more significant changes. Future record-shattering rainfall events resemble some of New Zealand's most devastating historical events, including Cyclone Bola (1988) and the May 1923 Canterbury storm, suggesting that the most impactful events of the future will manifest with familiar synoptic patterns but greater intensity under a warming climate.