Atmospheric rivers: What does climate change mean for NZ’s biggest rainmakers?

They helped make for Auckland’s wettest summer in history - but what will moisture-laden “atmospheric rivers” look like at the end of this century?

That’s a key question facing scientists in a freshly-funded study drawing on the latest modelling technology to investigate what a warming climate means for the giant rainmakers.

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 summer, they fuelled monster storms that put swathes of Canterbury farmland underwater in 2021, and another that forced the evacuation of half of Westport the same year.

Recent studies have shown their enormous influence on normal rainfall here - contributing as much as three-quarters of what the South Island’s West Coast receives - and how those that soak the north often have different driving factors to those which reach the south.

But there was still much more to learn about how - and where - they would drive extreme rainfall in decades to come.

Modelling has indicated they’ll increase substantially over certain “hotspots” - the greatest of which spanned a large area of the Southern Ocean centred around New Zealand.

Atmospheric rivers fuelled monster storms that put swathes of Canterbury farmland underwater in 2021 and another that forced the evacuation of half of Westport (pictured) the same year. Photo / George Heard

In a first-of-its-kind study, just awarded a $360,000 Marsden Fund grant, Niwa scientist Dr Peter Gibson and colleagues will investigate how well these global models really capture changes over New Zealand.

“We’ve recently completed very high-resolution climate change simulations at Niwa, and we will use them to delve into things like how atmospheric rivers interact with our terrain to produce extreme rainfall,” Gibson said.

They’d also be looking at what changes could be expected in blocking high-pressure systems to the east of New Zealand, which helped to stall atmospheric rivers over the country as they dumped vast amounts of rain over days-long periods.

Broadly, studies have suggested the atmosphere’s water-holding capacity increased at a rate of 7 per cent per 1C of warming.

Given the planet has already warmed by about 1.3C since pre-industrial times - and that could rise to 2C to 4C by 2100 - the associated extra moisture meant today’s disastrous extreme rainfall events would only grow more intense.

For New Zealand, Gibson said the research would ultimately give more reliable and precise estimates - something that could aid local climate adaptation efforts - while also contributing valuable new data to the global picture.

The study is among 123 research programmes allocated a total of $83.5 million in this year’s Marsden Fund round, exploring everything from seismic activity in the Southern Alps and new chemical compounds for next-generation lithium-ion batteries, to how true crime podcasts inform our opinions about pressing social issues.