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A most unusual atmospheric river – Expert Reaction

A long-lived atmospheric river is landing on New Zealand shores today, which NIWA says could lead to “well over a month’s worth of rain” this week, especially in parts of the South Island.

Atmospheric rivers occur year-round, but NIWA says this one is unusual for this time of the year.

The SMC asked experts about what atmospheric rivers are, how this one stands out, and what future trends might occur.

Tristan Meyers, Meteorologist, NIWA, comments:

“Atmospheric rivers are huge plumes of moisture that move from the tropics to the mid-latitudes (where we are!). Atmospheric rivers are most common in the southwest of New Zealand, but can occur anywhere in New Zealand. New Zealand sees our peak atmospheric river activity during summer, and our lowest atmospheric river activity during winter. When these atmospheric rivers encounter other weather features or New Zealand’s mountainous terrain, the vast amount of water vapour within these atmospheric rivers can get ‘squeezed’ out, falling as heavy rain or snow. Atmospheric rivers are important to New Zealand’s water supply; for the West Coast of the South Island, atmospheric rivers bring 78% of total rainfall. However, they are also responsible for extreme rainfall events; up to 94% of extreme precipitation on the west coast of the South Island are from atmospheric rivers.

“This current atmospheric river is pretty exceptional; it’s long lasting (> 3 days) and has a very anomalously large moisture content. Current forecast models indicate that this would be classified as an ‘extreme’ to ‘exceptional’ atmospheric river – indeed, an analysis we undertook indicates that the amount of moisture in the atmosphere for this particular event is unprecedented for August in climatological data going back to 1959. As a caveat – we’d need to go back and analyse this to confirm whether or not that shapes out to be true after the event.

“In terms of impacts, we are anticipating very heavy rain over a prolonged period with this atmospheric river. In fact, it looks like the rain may occur in two separate ‘bursts’; specifically one for Tuesday – Thursday for the South Island, and then again on around Saturday – Sunday for a similar area. All up, over a month of rainfall could fall for large parts of the West Coast, Canterbury High Country, Tasman, Marlborough and Nelson, with the heaviest rain about the elevated ground.

“This will lead to slips, and possibly flooding for some South Island rivers. Additionally, because the air will be so warm with this event (it’s tropical moisture!), the heavy rain associated with this atmospheric river will melt a lot of snow, which may exacerbate riverine flooding in some areas. And of course – it’s already been wet over New Zealand for the last month. Although it’s not extremely windy, it’s wet and gusty enough where trees falling down could be an issue too. Our colleagues at MetService have issued number of warnings and watches in place for this significant weather event.

“The science is investigating whether or not atmospheric rivers are increasing for us, but I’d speculate that it’s in-line with what we would expect from climate change; for every degree of warming, the amount of water vapor in the atmosphere can increase by about 7%.

“Right now though, the current climate drivers at play are La Niña and a Negative Indian Ocean Dipole- two climate drivers that work in tandem to give us frequent sources of atmospheric rivers.”

No conflicts of interest

Dr Daniel Kingston, Senior Lecturer in Geography, University of Otago, comments:

“The heavy rain forecast for this week is associated with an ‘atmospheric river’. These are transient narrow bands of extremely high water vapour transport in the atmosphere and are an important feature of our weather in Aotearoa New Zealand, because of the huge amounts of rain associated with them. A number of locations around the country have broken their rainfall records this winter (wettest July on record nationally, with more than 40 locations experiencing record or near-record rain totals), and that is linked to a higher than usual number of atmospheric rivers occurring. Normally, atmospheric rivers are relatively rare during the winter months.

“The event this week is notable because of its duration – usually these systems pass over more quickly. A blocking high-pressure system is currently located to the east of New Zealand, which will essentially cause the atmospheric river to stay parked overhead for a number of days. The event might also be more damaging than otherwise because it comes on top of an already exceptionally wet winter, meaning that the ground is already very wet with limited capacity to absorb further rain.

“Although analysis hasn’t been performed on this specific event yet with respect to the influence of climate change, it is more than likely playing a role. Average air temperature has warmed by slightly more than 1 °C over the past century – and as the atmosphere warms it can hold more moisture, increasing the likelihood for extreme heavy rainfall events such as this. Sea surface temperatures around New Zealand are also warmer than average right now, which can further amplify these sort of events.”

No conflicts of interest

Dr Luke Harrington, Senior Lecturer in Climate Change, Te Aka Mātuatua – School of Science, University of Waikato, comments:

“The big unknown this time round is whether the intense high pressure system sitting to the east of the country – which has allowed this atmospheric river to stall over the country and rain to persist – is something that we’ll see more or less often during the winter in a warmer world. I’m not sure we have clear answer to that question yet.”

No conflicts of interest declared.