Long-range forecasts are hinting at a stint of summery weather closer to Christmas. What's in store for the rest of the season? Photo / Alex Burton
Long-range forecasts are hinting at a stint of summery weather closer to Christmas. What's in store for the rest of the season? Photo / Alex Burton
It’s El Niño: but not as we know it. Science reporter Jamie Morton explains why there’ll be plenty of weather wildcards mixed in with the big climate driver’s classic influences on our summer.
What is El Niño, anyway?
If you’ve been noticing a lot more pesky westerly winds these pastfew months, blame “The Little Boy” - El Niño.
We can think of it as one end of a three-phase, ocean-atmosphere see-saw that measures the movement of warm, equatorial water from one side of the Pacific Ocean to the other.
During El Niño, that warmth piles up in the eastern Pacific, while in La Niña, it’s typically concentrated in the west, in the tropics above New Zealand.
Each of these cycles tends to come with different summer circulation patterns for our part of the world.
As we know too well from our last three years of rain-soaked misery, La Niña often brings more north-easterly flows that helps ferry that subtropical moisture our way.
El Niño, however, traditionally serves up the reverse – with strong and frequent westerly and north-westerly flows in summer that can act like a blow-drier for eastern regions, and cooler south-westerly winds in winter, which helped deliver our mildest August in six years.
Right now, scientists aren’t just observing that ocean warmth east of the International Date Line, but right across the Pacific basin, where waters have been running several degrees above normal.
For New Zealand, this Pacific-wide warmth means our summer circulation patterns won’t be exactly like past classic El Niño events.
While a ridge of high pressure to our north will make it extremely unlikely that people in places like Auckland see another summer ruined by unending downpours, that lingering west Pacific warmth still poses a risk of the odd system reaching us at times.
Niwa forecaster Ben Noll said another patch of warm ocean water lying to the east of Australia may also prove a “focal point” for low pressure systems in the western Tasman Sea this summer – bringing us rain from another direction.
At the same time, our summer isn’t likely to feature widespread local marine heatwave conditions that’ve caused a range of dramatic knock-on impacts on land and at sea over recent years.
So, what can we say about summer at this point?
Niwa is picking near or above normal rainfall in the South Island’s West Coast – which may well see torrential rain at times this month – and near normal amounts for the north of the South Island.
Image / Niwa
For everywhere else, those totals will be near or below normal.
“In the lead up to Christmas, and potentially even the holiday period itself, long-range modelling is pointing to an increased chance of drier than normal conditions across many regions, which is good news for holidaymakers and beachgoers,” Noll said.
“For weather weary regions, this would hopefully go a long way toward making up for the challenging hand that Mother Nature has dealt over the last year.”
As for that summer heat, temperatures are most likely to sit above average in the east of both islands and the north of the North Island, and near average or above average elsewhere.
More frequent and occasionally strong north-westerlies are still on the cards – and it’s possible these could help push temperatures into the mid-to-high 30s, a similar setup that brought Wairoa record-setting heat in September.
“Although it’s unlikely to be a cool summer, because of the variability in pressure, there will be some cooler spells that will likely provide a little bit of relief.”
Noll said there’s also potential for long stretches of dry weather in places – but unlikely to the extent of horror El Niño summers like 1997-98.
“If you talk to someone who lived through the 1982-83 El Niño, they’d tell you how it blew south-westerly for a good four or five months in a row in that event, and it hardly broke,” he said.
“That’s clearly not what we’ve experienced to date and it’s not what’s shown as we go ahead, either.
“We might have dryness for three, four or five weeks – but then that might be followed up by a different flavour of weather that could last a week or two, and take the edge off.”
Image / Fire and Emergency NZ
The combination of frequent north-westerly winds and dryness also poses a problem for our rural fire authorities, who’ve been anticipating a season like 2019-20, when crews had to deal with more than 5700 blazes.
While weather variability will help negate that risk, fire potential for the next three months is still considered above normal across the east of the entire North Island – and in the south as far as north Otago.
Noll expected the event to peak at mid-summer, but it could carry on well into 2024.
“Compared with last month, the odds for this El Niño to continue through autumn have increased slightly, to 80 per cent – and there’s now even a 60 per cent chance that it could hang around through June,” he said.
“That speaks to the volume of warm water that’s built up in the Pacific, and because of this, it’s not out of the question that we could see a later peak.
“That potentially long tail means we certainly shouldn’t be getting complacent about the risk of dryness and drought.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
