Explained: What caused the heatwave?

The mercury is being pushed up to astonishing levels around the country. Photo / Doug Sherring

Where did all of this heat come from?

One only needs to take a look at heat maps over the past few days to see all of those tongues of red and orange poking out of Australia and being yanked across the Tasman Sea.

That's the jet stream: a river of fast-flowing air being pushed well to the south of New Zealand, blocking the cooler air coming up from the Southern Ocean.

The heatwave riding this atmospheric highway across the ditch has already broken records in Australia, where dozens of horses have died, and is now pushing the mercury up to astonishing levels here.

WeatherWatch NZ has reported recordings of 37C in parts of Napier and Hastings today, along with 36C in the Marlborough Sounds.

Analyst Philip Duncan said some of those higher temperatures were recorded in more sheltered areas and not across the whole regions.

However, generally temperatures in eastern parts of the country were getting up around 33C to 34C.

Niwa said already in the Nelson region the record for the warmest minimum temperature had been broken, with 24.3C recorded in Richmond.

This was the warmest minimum temperature for the region since records began 157 years ago.

The previous record was 23.7C.

So what fired the Australian heatwave in the first place?

Climate scientist Professor Jim Salinger explained that a rain-making trough which typically sat over inland Australia had been parked over inland New South Wales and western Queensland.

"This has prevented normal thunderstorm activity in inland Australia, so central Australia and southern New South Wales have really heated up."

Hot, dry, northern winds flowing from the continent's centre had been allowed to push through to South Australia and Victoria, generating a full-blown heatwave.

The blocking systems had moved over New Zealand and to the east, allowing warm air to shoot out of Australia and over here, Salinger said.

"And what you have here is a thicker atmosphere, which traps the heat."

MetService meteorologist Tui McInnes said the visiting heatwave had also been combined with calm and settled conditions due to a high over New Zealand, which meant the sun was "able to knock those temperatures up a few more degrees".

Another part of the picture was sea surface temperatures that had climbed to more than 1C above the long-term average.

Waters had warmed particularly around the South Island and to the west, south, and east of the country.

Such marine heatwaves were expected to grow in frequency and intensity under climate change.

The warming trend had already been borne out by temperatures over recent times: Salinger said average temperatures had risen by some 0.8C in the past four decades.

Salinger pointed out last summer's record-hot conditions.

Temperatures as at 3pm today. Source / MetService

"Last summer's heatwave was unparalleled, for several extreme events and statistics," he said.

"The average land surface air temperature was 2.2C above the 1981-2010 normal of 16.7C."

The peak month was January 2018 - 3.2C above normal and the warmest month on record – and a marine heatwave that lasted reached 2C above normal at its peak.

"As the country enters another heatwave period for January, land air temperature is running at 1.4C and sea surface temperature is 1.8C above average," he said.

"Global warming means temperatures are now about 1C warmer than a hundred years ago.

"With further global warming, what we are seeing now and last year's summer could be the norm in the years 2081-2100."

Meanwhile, those bearing the brunt of the heat were being urged to keep hydrated.

"With hot temperatures and scorching sun, it is really important for people to be sun smart and stay safe," McInnes said.

"Keep hydrated and look out for vulnerable members of your community, and don't forget about pets."

Dr Karin Schutz, a senior scientist at AgResearch, said animal welfare was a major concern for New Zealand farmers during hot summer months.

"Cows will change their behaviour to cope in the warm conditions, including drinking more, eating less, seeking out micro-climates in the shade or close to water, and orienting themselves differently from the sun," she said.

"If you want to keep up production, you need to keep your animals cool. That can mean providing shelter such as trees, increasing access to drinking water, reducing walking distances, and preventing stress."

If it really heated up, farmers could use sprinklers at their milking sheds to cool the cows as they waited to be milked.

"Given a choice, however, we have found the cows will seek shade over the sprinklers, and from our research we know the cows can tell the difference between different degrees of shade and will choose shade that protects them more from solar radiation," Schutz said.

"Research shows that when the air temperature reaches 21C and humidity more than 75 per cent, it can affect the cow's behaviour and milk production could decline."

Unfortunately, it wasn't just the days that were set to be hot, with overnight temperatures set to stay in the mid to high teens for most, but a few will see overnight minimums of around 21C.

But McInnes said some respite would be on the way, with a relative cold change expected closer to the weekend.