La Nina set to bring warm weather and humidity
Kiwis can expect more aboveaverage temperatures for the rest of summer — and plenty of sticky humidity — amid a very atypical La Nina climate system.
Niwa’s just-released outlook for the next three months predicted air temperatures were most likely to be hotter than normal in all regions, at a time the north of the country was sweltering through unusually dry conditions.
But that didn’t mean the coming months wouldn’t be wet — or muggy, with spells of high humidity could be expected from time to time, especially in the north.
More sub-tropical air flows could fuel spates of localised heavy rainfall that could drive flooding — similar to what was seen in places in late December and earlier this month.
“It is not possible to pinpoint exactly which regions may experience extreme weather months in advance — hence the need to keep an eye on day to day weather forecasts through the season,” the report said.
More broadly, there was near to above normal chances of rainfall everywhere except for the west of the South Island.
There, rainfall levels would be either near or below normal — fitting with the set-up of a traditional La Nina.
However, this weather-influencing system — the strongest in nearly a decade — wasn’t behaving like previous ones.
A clear example was another dry that had set in across the upper North Island — Auckland, Northland and northern Waikato were all seeing abnormal conditions, according to Niwa’s drought index.
“That is certainly not what we’d expect to see in a classic La Nina,” Noll explained.
Caused by a build-up of coolerthan-normal waters in the tropical Pacific, La Nina traditionally brought warmth everywhere in New Zealand over summer — but with stark differences in regional weather patterns.
“La Nina sometimes has different flavours — you have can have a classic, east Pacific La Nina, and then a more non-traditional, central Pacific La Nina,” he said.
“The basic difference between these two is, in an eastern La Nina, the coolest ocean temperatures are in the eastern Pacific, whereas in a Central Pacific La Nina, the coolest waters are in the Central Pacific.
“That little subtlety can have an influence on global patterns — and here in New Zealand its impacts aren’t aligned with the kind of climatology we’ve seen with La Nina in the past.”
Under classic La Nina trends, rainy weather became a pattern over the North Island’s northeast, while drier conditions tended to dominate the south and southeast of the South Island.
Waters around New Zealand, too, were heating up again after sea surface temperatures cooled last month, dampening down ocean conditions that were previously on track for another marine heatwave.
Since the start of the year, temperatures in the north of the North Island, along the west and east of the South Island, had risen to 0.6C above average.
While Niwa last year warned New Zealand could have a slightlyheightened risk of ex-tropical cyclones this season, none for the next few weeks, at least, were expected to form in the Southwest Pacific.
Nonetheless, that risk remained “elevated” until the end of the season in April.
Each season, an average one former cyclone typically drifted within 550km of New Zealand, causing deluges, gale-force winds and coastal inundation — and there were suggestions the country might receive two this time.
Meanwhile, Niwa forecaster Ben Noll said the summery weather much of the country has enjoyed this week is set to take a colder turn.
After a spell of more warmth yesterday — Gisborne could see a high of 31C, and Hanmer Springs 28C — a southerly change was forecast to sweep up the country.
Showers were on the cards for most of New Zealand late yesterday — with some potentially heavy and thundery around the lower South Island.
“As we look further ahead, things will turn quite a bit warmer as we go toward the middle of the month,” Noll said.
“So we’re looking at perhaps five to seven days of mixed temperatures around New Zealand — and then a warm trend smack dab in the middle of January.”