Scientists have delved into decades of data to tease out global warming’s hidden hand in shifts in local rainfall patterns over New Zealand - offering a glimpse of what we can expect from storms of the future. Photo / Dean Purcell
Scientists have delved into decades of data to tease out global warming’s hidden hand in shifts in local rainfall patterns over New Zealand - offering a glimpse of what we can expect from storms of the future. Photo / Dean Purcell
We’ve long understood that climate change is packing more moisture into our visiting rain-makers, resulting in increasingly costly weather disasters.
Now, scientists have delved into decades of data to tease out global warming’s hidden hand in shifts in local rainfall patterns over New Zealand - offering a glimpse of whatwe can expect from storms of the future.
Some of the biggest deluges to soak the country over recent years – including last year’s monster Auckland Anniversary Weekend storm and 2021′s Westport floods - have been fuelled by what are called atmospheric rivers.
As such, whenever one of these long, snaking filaments of subtropical moisture are headed our way, we’re likely to hear about it in the news.
Yet these “rivers of the sky” are only part of the story - they account for just 10 per cent of the moisture transported over New Zealand - prompting scientists to look at the wider rainfall picture.
In a new study, they looked at the total amount of moisture carried through the entire depth of our atmosphere – or what’s called vertically integrated water vapour transport (IVT) - and whether this itself met the threshold for what we’d considered an atmospheric river.
About 40 atmospheric rivers make landfall here every year, with four or five classified as strong and usually hitting around summer. Image / Niwa
“We wanted to characterise the full spectrum of atmospheric moisture transport to better understand how it drives rainfall - especially extreme events,” said the paper’s lead author, Nithin Krishna, of Otago University.
After analysing records between 1981 and 2020, Krishna and colleagues were able to draw links between high-IVT conditions – when there’s intense volumes of vapour loaded right through the atmosphere - and the top 100 rainfall events at 15 different locations.
When it came to unusually rainy conditions around the country, they found a stronger association with extreme IVT than atmospheric rivers alone.
“While examining the top 100 rainfall events in Auckland, we found that 84 events were found to coincide with extreme IVT - whereas only 73 events occurred during atmospheric rivers.”
This pattern was more pronounced in certain locations, including Dunedin and east coast regions where atmospheric river events were less common.
The results also pointed to some interesting shifts. Since 1980, high-IVT conditions had become more common in the South Island, but less frequent in the north of the country, especially during the warmer months.
“These trends have been strongest in the period since 2000.”
The researchers saw two potential causes: increases in wind speed, which could move moisture around faster, or global warming.
“In the case of New Zealand, the increases we’ve seen are mainly from warming,” Krishna said.
“In fact, the windspeed component has actually decreased in places, essentially masking some of impacts of warming on vapour transport.”
Krishna said a changing climate came with plenty of complex factors.
The atmosphere’s water-holding capacity was growing as it warmed, at rate of 7 per cent per degree of global heating.
“For moisture transport, there is a complication though: it not only depends on the amount of water in the atmosphere, but how quickly and where it is transported.”
An atmospheric river helped put much of Westport underwater in July 2021. Photo / George Heard
Over recent decades, changes in wind – namely shifts in westerlies over the Southern Ocean – had helped offset impacts from the warming–driven heaping of moisture into the atmosphere.
“The combined effects of climate change and ozone depletion in recent decades have resulted in the westerlies being pushed more towards the poles,” Krishna said.
“We’d need to do further research to confirm, but this displacement is likely an important factor influencing the trends in IVT we have observed.”
Looking ahead, he said the trends observed in the study suggested rising potential for more frequent and extreme rainfall in the south, particularly over summer months.
“This points to the potential for increased flood risk, and further hazards associated with heavy rainfall, such as landslides and their impacts on infrastructure,” Krishna said.
“On the other hand, decreasing trends seen in the northern North Island may have the reverse effects.”
That included less rainfall for farmers, raising the need for smarter water storage options.
The Niwa-led project is also investigating potential changes in blocking high-pressure systems to the east of New Zealand, which have helped to stall atmospheric rivers over the country as they’ve dumped vast amounts of rain over days-long periods.
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.
