Photo: Hamish Prince
New research suggests the West Coast of the South Island is most at risk of more extreme atmospheric rivers than anywhere else in the country.
A study by the National Institute of Water and Atmospheric Research (NIWA) shows the phenomenon may double in New Zealand by the end of the century.
It found that not only could the narrow bands of concentrated water vapour increase, but it is likely to make up a much bigger proportion of the 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 suggests the West Coast of the South Island could be the most vulnerable.
"Particular regions that this study has pulled out, where the most extreme atmospheric rivers these really large events are likely to increase is particularly over the West Coast of the South Island, where we've already seen some of our biggest deluges in in recent years...2021, 2022, and 2019," he told Morning Report.
"They have started to stack up in recent years."
Gibson said atmospheric rivers can cause very large totals of rainfall in New Zealand.
He said it acts as long pathways of moisture through the lower atmosphere, and it often taps in our warmer, wetter and subtropical regions and pull moisture down towards the country.
"The atmospheric rivers interact with our unique topography and mountainous regions," he said.
Atmospheric rivers aren't a new phenomenon, but have occurred frequently, which inspired their research, Dr Gibson said.
The study looked at two mechanisms of atmospheric rivers.
"The first part of the picture is quite simple physics," Dr Gibson said. "It's that a warmer atmosphere with warming can hold more water vapour, so that fuels these storms - that part was already sort of known.
"The new part and the interesting part that this study has added to is that we could also see changes in the wind patterns which pull the moisture down towards New Zealand and interact with our topography."
Their supercomputer also allowed the research team to dial up projections at high resolution.
"This is where this new understanding and this new study is sort of coming from, and because our mountains are so unique in the topography of New Zealand, how this interacts with these atmospheric rivers is something we need to simulate."
Dr Gibson said their supercomputer at NIWA ran hot for about a year to determine their projections.
It allowed them to look at the long-term simulations, such as mid-century and the end of the century, he added.
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