Another long-running climate record might be about to fall in New Zealand – and for once, it’s not a bad thing. 

August has been so unusually chilly, compared with what we’ve come to expect in our climate-warmed winters, that it may prove the first month in more than six years to finish with a below-average nationwide temperature. 

This pattern break – still to be confirmed, with four days of August left to run – signals the arrival of our first El Niño in seven years, and the typically cooler winter flavours the big climate pattern brings us. 

But it also points to the fact a fast-heating planet has been shifting our climate state to a new normal, where the colder averages we recall from just a few decades ago become increasingly unlikely. 

The last time New Zealand happened to log a month that came in below Niwa’s 30-year average temperature baseline was May 2017 - some 74 months ago. 

Since then, New Zealand has recorded four of its warmest years on record (2018, 2019, 2021, 2022), a run of record-warm winters (2020, 2021, 2022), its wettest year (2022) and an extreme summer that smashed a slew of rainfall records across the North Island. 

An obvious culprit has been human-driven climate change: in just 111 years, New Zealand’s average has risen by more than 1C, and the starkest changes have come within the last three decades, when the pace of warming has tripled. 

We’ve already seen the effects of this warming in mountain glaciers that’ve lost a whopping third of their ice volume in just 40 years; a drop-off in annual frost days; and deluges that now involve more extreme rainfall than would occur in a world without climate change. 

Of course, it hasn’t been the only driving factor. 

Since around the start of the pandemic, and right through to the first months of this year, our weather patterns have been influenced by an unusual “triple dip” La Niña, bringing consistently warmer, wetter conditions to New Zealand’s north and east. 

At the same time, our land temperatures have been greatly affected by persistent marine heatwaves in our coastal waters – something projected to become more common with climate change – while subtropical disturbances have repeatedly ferried moisture-packed humid air down from a warmed-up West Pacific. 

Meanwhile, a key indicator of climate variability in our region - the Southern Annular Mode - has been running mostly positive over recent times, suggesting that fewer freezing storm systems have been able to make their way up from the pole. 

While early studies have suggested the 2022 eruption of the Hunga Tonga-Hunga Ha’apa volcano may have had some global climate impact, scientists have pointed out any influence on recent extremes around the world would’ve been small compared with that of background climate change and other drivers running alongside it. 

One of the largest of those was a forming-up El Niño - something which just contribued to the planet’s hottest recorded month in July, and which may ultimately help push the Earth’s surface temperature temporarily past the 1.5C mark in at least one of the next five years. 

For New Zealand’s winter, however, El Niño typically spelt not extra warmth, but a cooler regime favouring more southwesterly flows - often with showers chased by cold fronts. 

Wet, wintery weather in Auckland on August 8, 2023. Photo / Michael Craig 

If that weather sounds depressingly familiar after the last couple of weeks, Niwa has the data to show it. 

Up to August 26, the month’s average nationwide temperature came in at around 0.6C below the recently-introduced 1991-2020 baseline for August – and the result was only marginally different under the previously-used 1981-2010 normal. 

“The figure comes in at just a notch below the threshold of minus 0.5C that we use to denote below-average, so the next few days might prove to be a bit of a photo finish,” Niwa meteorologist Ben Noll said. 

As it happened, forecasters expect more frosty weather over what would be the final week of our meteorological winter, with temperatures expected to plummet in Auckland on Tuesday and Wednesday. 

“Year-to-year, we expect to see several months that are above or near average, but 73 months in a row? That’s a huge anomaly and describes Aotearoa’s changing temperature distribution,” Noll said. 

“The one that finally comes in below average, especially during winter, is going to be a stand-out and people are going to notice the chill.” 

Noll likened the pattern behind this picture to a brass band. 

“We can think of the conductor of this band as the high pressure that’s been over Australia and the Tasman Sea,” he said. 

“Around it, there’s been an anti-clockwise flow – a key feature of El Niño in late winter and springtime here – which has kept all of these systems coming up from the south and sweeping across New Zealand.” 

Noll also pointed to a subtle, recent shift in the location of the polar vortex that flows around Antarctica, and a long-overdue drop-off in local sea surface temperatures, brought on by the ocean-churning effect of those winter winds. 

Whether El Niño would also mean a dramatically hotter, drier summer for our rain-soaked north and east remained to be seen – but the menu for spring, at least, would be laid out this week in Niwa’s next seasonal climate outlook. 

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.