New Zealand could be under as much as 20% more rainfall in some places by the end of the century as atmospheric rivers become more intense and frequent, according to new research.
The National Institute of Water and Atmospheric Research study – led by NIWA research assistant Felix Goddard – examined projections for the frequency and intensity of atmospheric rivers under a relatively high greenhouse gas emissions future.
The one scenario investigated found the narrow bands of concentrated water vapour could increase and the “rivers in the sky” were likely provide a much greater proportion of the country’s total annual rainfall.
The findings were published yesterday in the Journal of Geophysical Research: Atmospheres.
NIWA climate scientist Dr Peter Gibson, who co-authored the study, said atmospheric rivers “have been responsible for some of our biggest downpours in recent years, such as the West Coast flooding that took out the Waiho Bridge in March 2019, the widespread flooding in Westport in July 2021, and the exceptionally wet Auckland summer of 2022-2023”.
“As an island nation surrounded by oceans, we experience atmospheric rivers when huge amounts of water vapour are transported down from the tropics and sub-tropics and make landfall.
“These ‘rivers in the sky’ cause heavy precipitation and flooding when they make landfall in New Zealand, producing extremely large rainfall totals, especially when they interact with mountainous terrain.”
Gibson said there were two factors behind the likely increase in atmospheric rivers.
“The first reason is that an overall warmer atmosphere can hold more water vapour, while the second reason why we may see an increase in atmospheric rivers is because of changes in wind patterns.
“Climate projections show changes to atmospheric circulation, with an intensification of the westerly jetstream over New Zealand with strong winds flowing west to east in the South Pacific region, and a shift of the jet towards the South Pole.”
As well as finding the most intense atmospheric rivers were likely to be more frequent and more intense, the researchers said the phenomena may add up to 20% more to annual rainfall totals in some places.
“Overall, the biggest hotspot for these future changes is over the west coast of the South Island, where atmospheric rivers already often produce the largest impacts,” he said.
The study relied on climate change projections released last year by NIWA and the Ministry for the Environment, which provides detailed information for every 5 square kilometres across the country.
“The latest projections give us a much more comprehensive and detailed picture of our future climate,” Gibson said.
He said the updated projections – from a “mammoth science and technology effort” involving “huge supercomputing power” – was helping Aotearoa “better understand their specific climate-related risks”.
“In turn, this will improve decision-making for climate adaptation and support people to build resilience to extreme weather events.”
Further research was being carried out by NIWA into specific climatic features in the projections, to provide clarity on the future climate New Zealanders were likely to face in the ensuing decades.
