An example of an extreme atmospheric river impacting New Zealand from a NIWA climate model simulation (white shading showing elevated atmospheric water vapour; red shading showing extreme rainfall). Photo: Supplied / National Institute of Water and Atmospheric Research
New Zealand could face twice as many of the most extreme atmospheric rivers by the end of the century, according to new research by the National Institute of Water and Atmospheric Research (NIWA).
It found that not only could the narrow bands of concentrated water vapour increase, but in the future rainfall from these 'rivers in the sky' is likely to make up a much bigger proportion of New Zealand's total annual rainfall.
The research, led by NIWA research assistant Felix Goddard, analysed the latest climate change projections released last year by NIWA and the Ministry for the Environment, and was recently published in the Journal of Geophysical Research: Atmospheres.
The co-author, NIWA climate scientist Dr Peter Gibson, said the study examined projections for the frequency and intensity of atmospheric rivers under a relatively high greenhouse gas emissions future.
"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 reasons why atmospheric rivers were likely to increase in the future.
"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 that the most intense atmospheric rivers are likely to be more frequent and more intense, the researchers say that the phenomena may add up to 20 percent 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," Gibson said.
He said the latest climate projections for New Zealand, available on the Ministry for the Environment's website, provide detailed information for every five kilometre square across the country.
"The latest projections give us a much more comprehensive and detailed picture of our future climate.
"These updated climate change projections are based on refining the projections from six global climate models from the projections provided in the most recent report by the United Nations body that assesses the science related to climate change, the Intergovernmental Panel on Climate Change."
The more detailed information would enable better evidence-based discussion and choices, Gibson said.
"Development of these updated, high-resolution climate projections was a mammoth science and technology effort and involved huge supercomputing power.
"The dataset is now helping New Zealanders - including councils, companies and communities - to 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."
More research is being undertaken by NIWA into specific climatic features in the projections, to provide clarity on the future climate New Zealanders are likely to face in the decades to come.
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