The Government has declared the flooding as an "adverse event" for Canterbury meaning $500,000 in funding can be unlocked for farmers and growers. Video / Pool
More deluges fuelled by "atmospheric rivers" from the subtropics could be on the cards, over a coming winter predicted to be warmer – and in some places wetter – than average.
As Canterbury begins its recovery from widespread flooding that forced hundreds of evacuations, Niwa today published its climate outlook for the next three months.
Along with continued warmth across the country, and more of the dry spells that have browned the southeast, Niwa also reported the potential for periodic heavy rain events.
Some of those downpours could come as a result of atmospheric rivers making landfall in New Zealand - particularly in eastern areas – which increased the risk for more flooding events.
Niwa forecaster Ben Noll said another rain event was coming this weekend – this time over the North Island – but there was no indication it'd compare to Canterbury's weekend soaking.
The outlook predicted near normal rainfall would be most likely in the north of the North Island, and north and east of the South Island, while near or above normal rainfall was equally likely in the east of the North Island.
In the west of both islands, there was an equal chance of near or below normal rainfall.
Noll said this represented "a bit more of a wetter trend" than had been seen in Niwa's recent three-month outlooks – and raised the potential for more variable weather.
That could be put down to a combination of background climate factors influencing New Zealand weather.
While the tropical Pacific was currently stuck in a climate state between La Nina and El Nino –called El Nino/La Nina Southern Oscillation (ENSO) neutral – the Western Pacific still packed some remnant warmth from a now-faded La Nina event.
That lingering ocean warmth effectively left the door open to more "atmospheric rivers" – which we could picture as long siphons sending moisture from the subtropics to New Zealand, on the back of visiting low pressure systems.
"I'm not sure I'd say we'll see more events similar to what we just had in Canterbury – that was obviously exceptional – but just more making landfall in New Zealand, because of all of that warmth sitting to our north," he said.
"These will be more likely, probably in the eastern areas of both islands, and maybe the north of the North Island as well, but a bit less likely in those western areas of both islands."
Source / Niwa
Another driver of one-off large downpours was the more pronounced influence of a big storm-making system called the Madden-Julian oscillation, or MJO.
Essentially a pulse of rain and thunderstorms that circled the globe every 30 to 40 days, the MJO was the largest element of the intra-seasonal variability in the tropical atmosphere and, as at the weekend, could drive deluges over New Zealand.
"When we're in an ENSO-neutral state, the MJO is moving around the Earth at a quicker pace," Noll said.
Source / Niwa
"Also, with that remnant warmth from La Nina in the west Pacific, that's going to make the MJO a bit more likely to be in our neighbourhood."
Yet the atmospheric set-up in the Pacific, with the regular impacts of the MJO, didn't tell the whole climate story for New Zealand this winter.
The long dry spells and sunny weather that much of the country had seen this year was partly down to another influencer – but to our south.
The Southern Annular Mode, or SAM, is a ring of climate variability that encircles the South Pole, but stretches far out to our own latitudes.
Some warm years had been put down to the SAM being locked in a mostly positive phase, which came with westerly winds farther south over the southern oceans but lighter winds and sunnier skies over New Zealand.
"We've had more positive SAM days overall this year than negative ones," Noll said.
"So that would kind of argue that those longer, dry spells will still be a piece of the climate puzzle as we go through winter – it's just that they'll probably be interrupted by these more potent low pressure systems."
The outlook predicted temperatures were most likely to be average in the west and east of the South Island, and about equally likely to be above average or near average in all other regions.
While cold spells and frosts would occur, the outlook said "they may be infrequent".
Air pressure was also predicted to be higher than normal to the south and east of the country – which could be linked with more westerly and northeasterly winds than normal through the season.
Overall, Noll said winter's wetter outlook boded well for restoring river flows and soil moisture.
"Obviously, there was a bit of a concern, with our dry summer and dry autumn," he said.
"So there's some hope on the horizon for getting the benefits of those more regular rainfall events as we work our way through the season."
