How NZ could face twice as many big rain-making ‘atmospheric rivers’ by the century’s end

Atmospheric rivers are long, thin filaments of moisture that stretch thousands of kilometres from the subtropics to New Zealand. Image / Niwa

• Atmospheric rivers are long, thin filaments of moisture that stretch thousands of kilometres from the subtropics to New Zealand
• They’ve featured in a series of recent deluges, including 2021’s Westport flood and 2023’s Auckland Anniversary weekend storm
• Scientists who analysed a high-end emissions scenario found New Zealand could be hit by twice as many extreme atmospheric rivers by the end of the century – and in some places they could bring up to 20% more total annual rainfall

They’ve fuelled some of New Zealand’s most damaging deluges – and now scientists project that extreme “atmospheric rivers” could become twice as frequent by the century’s end.

Capable of carrying 200 times the flow of our largest river, the Clutha, these long, thin filaments of atmospheric moisture snake thousands of kilometres between the hot-and-humid tropics and the mid-latitudes, where New Zealand sits.

Along with driving a series of extreme deluges in Auckland’s record-wet summer of 2022-23, they powered monster storms that put large areas of Canterbury farmland underwater in 2021 and another that forced the evacuation of half of Westport the same year.

In a just-published analysis, Niwa scientists found that under a relatively high greenhouse gas emissions scenario, the country could face a doubling of these events by the close of the century.

By then, these “rivers in the sky” could also be contributing much more than the roughly 10% of New Zealand’s total annual rainfall they do today.

Study co-author Dr Peter Gibson said there were two key reasons for those trends.

“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,” he said.

“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.”

Gibson said the analysis, led by Niwa research assistant Felix Goddard, also suggested atmospheric rivers could eventually increase annual rainfall totals in some places by as much as 20%.

The flooding in Westport in 2021 caused tens of millions of dollars' worth of damage. Photo / George Heard

“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.”

The new projections drew on projections released last year by Niwa and the Ministry for the Environment, which provided detailed data for the country broken down to spaces of 5sq km.

“The latest projections give us a much more comprehensive and detailed picture of our future climate,” Gibson said.

The study comes amid a growing focus by scientists on atmospheric rivers – and how climate change is influencing them.

Researchers have also been learning more about their geographic drivers and variations, with one recent analysis showing that those affecting the south of the country were different beasts from those in the north.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.

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