With just over a week of January left, more than than 40 locations were now tracking toward a record or near-record warm month.
Places where temperatures had so far averaged more than 2C hotter than normal included Kaitāia, Dargaville, Whangapāraoa, Matamata, Whakatāne, Kawerau, Taupō, Hamilton, Te Kūiti, Masterton, Dannevirke, Martinborough, Napier, Wairoa, Paraparaumu, Palmerston North, Ōhakune, Whanganui, Christchurch and Balclutha.
On Sunday, Whanganui reached an overnight high of 21C – breaking a record set back in 1978 – while Wellington Airport yesterday registered its hottest day-time maximum since 1971, with a scorching 29.5C.
Auckland registered a similarly-high 29.7C yesterday, making for the city’s warmest day in four years – and its fifth-hottest ever.
New Zealand saw its hottest January – and month - on the books in 2018, with a toasty 20.3C mean which finished up 3.1C above the 30-year baseline average.
Why has it felt much hotter than it is?
For meteorologists whose business was watching the weather, this month’s warmth was all the more notable because these extremes were occurring even at the climatological peak of sea and air temperatures.
“When we see these extreme-extremes occurring at the time that is already our hottest time of the year, that’s when we really start to pay attention.”
For many Kiwis, meanwhile, the heat would have felt like more than “just summer”.
MetService just yesterday issued Heat Alerts for 18 centres – spread from Kaitāia to Kaikōura - reflecting temperature thresholds developed for dozens of locations.
Alerts were included with local forecasts if temperatures were likely to come within record values - or if there was an imminent run of very hot or humid weather.
They meant people should consider precautions like staying hydrated and under shade, check in on vulnerable people and children, and look after pets and livestock.
Many health risks could come with temperatures that people weren’t acclimatised to - and sudden shifts could disrupt our body’s normal biological cycle.
“Another factor of the heat we’ve felt this month has been its persistence: this has been ongoing for a couple of weeks now,” Noll said.
“When you’re not getting any relief from it at night – minimum temperatures further up the country have been hovering at about 20C – that’s going to take its toll after a while.”
What’s behind the heat?
Noll pointed to a mix of drivers that had combined to lift mid-summer’s heat to new heights.
One factor was flows that’d been carrying warm air into the country – whether westerly and northwesterly winds funnelling heat from Australia, as we’d normally expect in El Niño, or northerlies carrying down warmth from the tropics.
“A lot of these flows have originated up near the Cook Islands area, and they’ve followed a streamlined path down toward New Zealand,” he said.
“It’s almost like New Zealand has been transported to a lower latitude, and we’re a country that feels like we’re further into the subtropics right now.”
Summer had felt particularly muggy when those flows had come with northeasterlies – a feature that dominated the North Island’s last three warm-and-wet summers under La Niña.
Added to the equation was yet another element that forecasters had been surprised to see here in the thick of El Niño: marine heatwaves.
“This marine heatwave has again exceeded expectations for the summer, especially around the North Island, where sea surface temperature anomalies of 2C to 3C are widespread,” Noll said.
“We’re now actually seeing temperatures as high as 23C to 25C in the coastal waters around Auckland, which is about as warm as you can really find it within the records.”
And in the background, of course, was a warming planet.
Scientists recently calculated that the hottest days of the year had warmed by more than half a degree over the last 20 years for many populated regions of New Zealand, which has seen its three warmest years since 2021.
Is this peak heat for summer?
The good news for Kiwis beat by the heat was a southerly change expected to bring a welcome reprieve over the next few days.
“An active cold front, followed by cool southwesterlies affects Aotearoa New Zealand today and into tomorrow, bringing rain or showers to most, and sweeping out all that muggy, sticky air,” MetService meteorologist Dom Barry said.
“Temperatures tonight are set to plummet in many areas, with Alexandra forecast to drop to 3C, and Queenstown and Wanaka to 4C.
“Hastings will see a drop of 20C, from 32C to just 12C tonight.”
Looking further ahead to the rest of summer, Noll saw more opportunities for cooler spells.
“It does look like, as we go into February, that we see a little bit more variability.”
What’s called the Madden-Julian Oscillation – a pulse of rain and thunderstorms that circled the globe every 30 to 40 days - was expected to be more active next month.
“So I wouldn’t be shocked if we’ve seen the peak of the heat, but as we go into February, there are definitely going to be some hot days and hot periods,” Noll said.
“Whether it lasts 15 to 20 consecutive days as we’ve just seen, that would seem a bit of a stretch, given the variability we’ve got coming in.
“At the same time, I wouldn’t entirely rule it out: the marine heatwave we have now is very impressive, and it may well endure into late summer and early autumn, which will increase the odds of warmth on land.”
Niwa has projected temperatures to be above average in the west of the South Island and very likely to be above average elsewhere over the next three months, owing to more frequent northwesterly quarter winds.
The dominant but decidedly non-traditional El Niño pattern, meanwhile, could linger on through to winter.
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.
