NZ swelters: The science behind our heatwave

How’s the heat, New Zealand?

If it feels warmer out there than your typical January, that’s because there’s been much more to this heatwave than a bit of classic summery weather.

On Sunday, Invercargill and Taumaranui registered their second hottest days on the books – 32.2C and 32C respectively – while many other spots sizzled with highs in the late 20s and early 30s today.

MetService forecaster Mmathapelo Makgabutlane said much of the heat was owed to a ridge of high pressure currently parked over the country.

“That’s just stagnant air that is not really going anywhere,” she said.

“There’s also a lot of subsidence as well. That’s air from the upper atmosphere sinking down toward the surface and, as it descends, it warms up.

“If that process has been happening for days, then one would expect temperatures to be quite warm at the surface.”

Climate scientist Professor Jim Salinger, who is leading a team of researchers observing conditions this summer, explained how these conditions were set against a backdrop of several major and influential climate drivers.

One was New Zealand’s second consecutive La Niña.

During these naturally-occurring systems, ocean water spanning from the coast of South America to the central tropical Pacific cooled to below average – a result of stronger than normal easterly trade winds, which churned cooler, deeper seawater up to the ocean’s surface.

This unusually cool water in the eastern Pacific then suppressed cloud, rain, and thunderstorms, as sea temperatures in the far west of the ocean warmed.

Here in New Zealand, we could usually expect more north-easterly winds that bring rainy conditions to North Island’s northeast, and drier conditions to the south and southeast of the South Island.

Thanks to the northeasterly winds, warmer temperatures also tended to play out over much of the country during La Niña – as had been delivered this season.

Salinger said another phenomenon that often accompanied La Niña – warmer seas – had similarly been strikingly apparent.

🌡What is "Significantly hot"?🌡On days like this (i.e. hot!), you may see information about temperatures presented a…

Late last year, coastal waters warmed to a state similar to that seen over the summer of 2017-18, which went down as New Zealand’s hottest ever.

Seas around the country were still running abnormally hot – particularly in Hawke’s Bay (3C above average), and Wairarapa, the Bay of Plenty, and Auckland’s west coast (2C).

When high-pressure systems like this one brought settled weather and little wind, that translated to less churn and mixing in our seas, and more warmth at the surface, which in turn helped drive heat on land.

Salinger singled out three further factors.

One was that the Southern Annular Mode, or SAM – a ring of climate variability that encircles the South Pole, but stretches far out to our own latitudes – had spent long stints in a positive phase.

When this happened, westerly winds blew farther south over the southern oceans, while New Zealand saw lighter winds and sunnier skies.

“It means we get blocking anti-cyclones across southern New Zealand – and that’s what we’re seeing at the moment.”

A separate climate indicator, called the tripolar index, had been in a negative phase, which encouraged La Niñas and sea surface temperatures around New Zealand to be above average.

“Plus, we’ve got global warming. Compared with the 1870s, temperatures right now are one and a half degrees warmer.”

Amid the hot weather, MetService has been issuing online alerts as part of a new pilot project to develop New Zealand’s first-ever early warning system for extreme heat.

Makgabutlane said the alerts – based on local temperature thresholds just developed by scientists – had already been noted by many Kiwis in sweltering spots this week.

“Most of it has been people noticing the amped-up wording that we’ve been using,” she said.

“The phrase ‘significantly hot’ got quite a lot of attention yesterday, which I guess is a good thing, because we were trying to let people know that it was going to be quite warm for most of the country.”

For those eager for respite from the scorching temperatures, MetService was forecasting a mid-week change brought on by a weak front moving up the South Island.

For eastern spots like Ashburton and Timaru, that could see the mercury dip from the mid-20s to highs of 16C on Thursday.

By week’s end, most places in the North Island could expect highs in the low to mid-20s.

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