A wetter winter is coming: why New Zealand can expect more ‘atmospheric rivers’


More downpours fed by “atmospheric rivers” from subtropics could be the order of the day, in a coming winter that is expected to be warmer – and in some places wetter – than average.

As Canterbury begins to recover from widespread flooding that has forced hundreds of evacuations, Niwa today released its climate outlook for the next three months.

In addition to continued heat across the country and more droughts that have browned the southeast, Niwa also signaled the potential for periodic heavy rain events.

Some of these showers could be the result of falling atmospheric rivers in New Zealand – especially in the eastern regions – which has increased the risk of further flooding.

Niwa forecaster Ben Noll said another rainy event was coming this weekend – this time on the North Island – but there was no indication it would compare to Canterbury’s weekend soak.

Outlook for near normal precipitation would most likely be in the north of the North Island and north and east of the South Island, while near or above normal precipitation was also likely in the east of the North Island.

In the western part of the two islands there was an equal chance of near or below normal precipitation.

Noll said this represented “a bit more of a wetter trend” than what had been seen in the recent three-month outlook from Niwa – and increased the potential for more variable weather conditions.

This could be attributed to a combination of background climatic factors influencing the weather in New Zealand.

While the tropical Pacific was currently stuck in a climatic state between La Nina and El Nino – referred to as neutral El Nino / La Nina Southern Oscillation (ENSO) – the Western Pacific was still teeming with residual heat from a now-extinct La Nina event. .

This lingering heat from the ocean has effectively left the door open to more “atmospheric rivers” – than we might imagine as long siphons sending moisture from the subtropics to New Zealand, due to visiting systems. at low pressure.

“I’m not sure to say that we will see more events similar to what we just had in Canterbury – it was obviously exceptional – but just no longer make landfall in New Zealand, because of all this sitting heat. in our north, “he said.

“These will be more likely, probably in the eastern areas of the two islands, and possibly also in the north of the North Island, but somewhat less in the western areas of the two islands.”

Source / Niwa
Source / Niwa

Another driver of large point showers was the more pronounced influence of a large thunderstorm-creating 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 most important element of the intra-seasonal variability of the tropical atmosphere and, like on weekends , could cause downpours over New Zealand.

“When we are in an ENSO neutral state, the MJO moves around the Earth at a faster rate,” Noll said.

Source / Niwa
Source / Niwa

“Plus, with this waste heat from La Nina in the West Pacific, it’s going to make the MJO a little more likely to be in our neighborhood.”

Yet the atmospheric configuration in the Pacific, with the regular impacts of the MJO, has not told the whole climate story of New Zealand this winter.

The long spells of drought and sunny weather that much of the country had experienced this year was in part attributable to another influencer – but in our south.

The Southern Ring Mode, or SAM, is a ring of climatic variability that surrounds the South Pole, but extends far to our own latitudes.

Some warm years had been attributed to the SAM lockdown in a predominantly positive phase, 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 days,” Noll said.

“So that would mean that those longer, drier periods will always be a piece of the climate puzzle as we go through winter – it’s just that they’ll likely be interrupted by these more powerful low pressure systems.”

Forecast temperatures were more likely to be average in the west and east of the South Island, and about as likely to be above average or near average in all areas. other regions.

As cold snaps and frosts would occur, the outlook indicated “they may be infrequent.”

Atmospheric pressure is also expected to be higher than normal in the south and east of the country – which could be linked to more westerly and northeast winds than normal throughout the season.

Overall, Noll said the wetter outlook for winter bodes well for restoring river flow and soil moisture.

“Obviously there was a bit of concern, with our dry summer and dry fall,” he said.

“So there is some hope on the horizon to take advantage of these more regular rains as we progress through the season.”

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