4 Mar 2021

Massive rivers in the sky account for NZ's costliest floods

From Afternoons, 1:24 pm on 4 March 2021

New research has found nine of the country's 10 costliest floods have coincided with a phenomenon known as atmospheric river events.

Storm damage along the Esplanade, Owhiro Bay, Wellington, 2013.

Storm damage in Ōwhiro Bay in Wellington in 2013. Photo: NIWA / Dave Allen

The rivers carry more water than the Amazon and Nile rivers, are up to 2000km long and can be found up to five weeks before they make landfall.

Lead author Kim Reid, from the Australian Research Council Centre of Excellence for Climate Extremes, told RNZ's Afternoons that while a lot is known about the impact of the phenomenon in the Northern Hemisphere, the research is more limited in the Southern Hemisphere.

Reid said they had been studied more in the Northern Hemisphere because it was more highly populated.

In the last 10 years there had been more research in this part of the world, and it has been found that places like New Zealand get a lot more of these systems than the Northern Hemisphere.

"They're basically just huge narrow streams of water vapour so water in its gas form that flow through the atmosphere thousands of kilometres long and when they hit land they rain out all this water and can cause extreme flooding, extreme rainfall and landslides and a lot of trouble for people."

While Reid has been mainly studying the impact in Australia, her colleagues at NIWA have sent her data from this country which showed that nine of the 10 most expensive floods in New Zealand between 2007 and 2017 coincided with an atmospheric river event over the location where the flooding occurred.

She also studied data from 11 weather stations from around the country and looked at how many of the heaviest rainfall days at those stations coincided with an atmospheric river event.

She found that anywhere from seven to 10 out of 10 heavy rainfall events coincided with an atmospheric river event over a 40-year period through to 2019. It even applied to the east coast of the country, she said.

"You can basically see a stream of really narrow strong wind and really humid warm air."

She said conditions over Northland today which included thunderstorm warnings fitted the pattern.

Information from satellites, shipping, planes and weather stations could be pieced together to get an accurate picture.

"Heavy rainfall is really hard to predict but because atmospheric rivers are much bigger than say an individual thunderstorm the weather models can pick those up a lot easier.

"So it's kind of an extra step going from the model to the river to the rainfall is a lot easier than going from the model straight to the rainfall... we can basically make a lot stronger prediction of extreme rainfall based on what we know about the bigger weather systems like the atmospheric rivers."


Flooding in Napier last November. Photo: RNZ / Dom Thomas

Impact of climate change

Asked if the atmospheric river events were affected by climate change, she said the consensus is that as the atmosphere gets warmer it holds more water.

"So that means that these atmospheric river events are likely to be more intense with climate change. Scientists have also found that they're shifting south so places like New Zealand might actually get a lot more atmospheric rivers over the next few decades."