11 Feb 2021

Study links climate change to severe rainfall

5:58 pm on 11 February 2021

Climate change could cause larger and more frequent extreme weather events, a new study by University of Otago has found.

The Waiho Bridge on State Highway 6.

The Waiho Bridge on State Highway 6 washed away in April 2019 after torrential rain battered the West Coast for two days. Photo: Wayne Costello / DOC

The research aimed to find the causes and impact of atmospheric rivers - thin concentrated corridors of airborne moisture which could produce severe rainfall.

Lead author of the study Hamish Prince said as the climate warmed these may become more common and intense.

"In very basic terms, one of the results of a warmer climate is a wetter atmosphere. With more moisture in the atmosphere the frequency and magnitude of atmospheric rivers making landfall in New Zealand are expected to increase.

"The increase in extreme precipitation is not expected to occur equally throughout the nation however, with indications that the landfalling location of atmospheric rivers is shifting southward in New Zealand," Prince said.

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A diagram of atmospheric rivers in March 2019. Photo: Hamish Prince

The study found the West Coast faced the biggest impact of atmospheric rivers which were responsible for much of the heavy rainfall there, with the 2019 deluge in the region being an example of this.

Prince said that storm dropped over a metre of rainfall in 48 hours (the highest ever total over that period in New Zealand) which resulted in a death, millions of dollars of damage, and a state emergency declaration.

"This flood was produced by one of the most extreme atmospheric rivers observed over New Zealand, moving incredible amounts of moisture very quickly through the atmosphere, extending over 4000km and covering much of the South Island of New Zealand.

"Our study has shown that atmospheric rivers are responsible for the vast majority of extreme rainfall in New Zealand - along with over 50 percent of all precipitation - so understanding what drives these events is fundamental to planning for extreme weather and the management of freshwater resources in New Zealand," Prince said.

He said the most impactful rivers occur about once every five years - although that frequency may increase with climate warming.

Hamish Prince, the lead author of an Otago Unviersity study about the causes and impact of atmospheric rivers.

Hamish Prince, the lead author of an Otago Unviersity study about the causes and impact of atmospheric rivers. Photo: Supplied

Co-author of the study and NIWA hydrological forecasting scientist Dr Jono Conway said understanding these events allowed planning to soften their effects.

"The ability to automatically detect and understand the severity of atmospheric rivers before they make landfall is a promising tool for reducing the impacts of these storms, particularly on the West Coast of the South Island where these events are common but not always hazardous," Conway said.

Prince said he aimed to study the positive effects of atmospheric rivers on hydro-power and agriculture in New Zealand as well as their threat to life and property.

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