They have fueled some of New Zealand’s biggest deluges in recent years, causing millions of dollars in flood damage – and another is expected to dump more than 100mm of rain on the Bay of Plenty early this week.
Now scientists have revealed startling new information about so-called “atmospheric rivers” that meander thousands of miles above the oceans and are the source of much of our extreme rainfall.
We can think of them as long, thin filaments of atmospheric moisture, capable of carrying twice the average flow of the Amazon River – or 200 times that of our largest river, the Clutha/Mata Au.
About 40 atmospheric rivers make landfall here each year, four or five of which are classified as strong and usually strike around summer.
But these giant rainmakers could also strike in winter: as dramatically demonstrated by a deluge that submerged swaths of Canterbury farmland last winter, and another that forced the evacuation of half of Westport .
An upcoming system, which is expected to sweep across the Bay of Plenty tomorrow, could bring 100mm to 120mm of rainfall between 8pm Monday and 3pm Tuesday, with peak rates of 15mm to 25mm per hour.
MetService issued an amber warning for the region, as well as the east coast north of Tolaga Bay, where between 100mm and 140mm could fall over the same period.
One of the biggest questions for scientists is what are the connections between atmospheric rivers and different types of local weather patterns across the country.
According to a study conducted by Niwa in 2000, there happened to be 12 major weather types in our region – each associated with different configurations of high and low pressure systems.
“While classifying weather patterns in this way can be really useful, the problem is that they can also oversimplify things, because weather doesn’t transition cleanly from one type to another – it’s continuous in time and in space,” the University of Otago said. said the speaker, Dr. Daniel Kingston.
In recent years, scientists have better defined these weather types to provide a more nuanced picture of temperature and precipitation variation across the country.
“But we also found that extreme precipitation can be caused by many different types of weather,” Kingston said.
“This suggests that weather patterns may not be the only factor causing extreme rainfall events.”
Their latest analysis, led by Ben Pohl of the University of Burgundy in France, explored which variables – including the strength of visiting highs and lows, such as a deep trough arriving next week – could help rivers form. atmospheric.
“In other words, we tried to find what is different, in terms of atmospheric conditions, between atmospheric river days and other days,” he said.
“But New Zealand’s landscape is complex, with rugged mountain ranges varying the effects of weather systems from west to east across the country.
“So we looked at what this interdependence between atmospheric and weather types is – and how it changes from one part of the country to another.”
One of the main discoveries was that the atmospheric rivers that affected the south were different beasts than those in the north.
“For the north, the humidity coming from the subtropical Tasman Sea is most important,” he said.
“For the south, the primary driver of atmospheric rivers is the strength of westerly winds across the Southern Ocean.”
The study also found that these events could occur in three or four weather types in each part of the country – but again the picture was somewhat messy.
“A key factor is the orientation of the coast and mountain ranges – an atmospheric river is more likely to cause extreme rainfall when it hits the coast or mountain range head-on, rather than from a vantage point. “
While weather types associated with generally wet conditions were most often associated with them, Kingston said that was not always the case.
“Surprisingly, they can also occur during weather types that are more often associated with average or drier than average conditions,” he said.
“This suggests that we have more to learn about how weather types and atmospheric rivers interact – and that we may need to develop a new weather classification system that has clearer links to extreme rainfall events. “
The effect of climate change was another big question – but there were some general patterns we could draw.
One was westerly winds moving further south – something that has already been caused by ozone depletion over Antarctica – which in turn would shift the location of systems high and low pressure in our atmosphere.
Another was the fact that for every additional degree of global warming, the atmosphere can hold 7% more moisture.
“That explains why, under climate change, extreme rainfall is expected to intensify,” Kingston said.
“The same will happen to atmospheric rivers: they are expected to hold more moisture under warmer conditions in the future.
“We therefore expect more frequent and intense atmospheric rivers in the future, and perhaps more concentrated in the south – but there are still many details to be resolved.”
Ultimately, he saw the potential to more accurately predict atmospheric rivers – something that is usually done well within days – using a combination of approaches and global timescales.
“Merging these time scales is a really exciting and developing approach in the wider meteorological and climatological community, and would be a great next step to take when it comes to atmospheric rivers.”
Meanwhile, Niwa forecaster Ben Noll said there was plenty of potential for heavy downpours across the country over the next fortnight – including the tropical plume of humidity arriving at the start of this week.
“It’s the North Island, and the upper and southern east that are most likely to see these rains.”
Noll said we can expect to see more of these systems over the next three months – partly due to a lingering La Nina signal, but also the distant influence of another weather phenomenon called the dipole. of the Indian Ocean.
“Overall, we will need to remain on our toes – particularly in the northern and eastern regions – as we look towards late winter and early spring.”