Extract from ABC News
An early-autumn heatwave spanning south-eastern Australia is finally nearing its end, with a cooler change expected to slowly take the edge off the scorching conditions from today.
The four-day heatwave saw several March records fall, with both day and night-time temperatures in some places rivalling, or even exceeding, those experienced during the peak of this year's summer.
Daytime temperatures across South Australia and Victoria climbed as much as 20 degrees Celsius above average throughout the course of the event — with some places inland reaching the low 40s.
But it was the night-time minimums' failure to provide any reprieve which really cemented the severity of the heatwave.
Bureau of Meteorology senior meteorologist Sarah Scully said they were also 8C to 14C warmer than normal.
An overnight low of 29C in Cleve, South Australia, and 26.2C at Melbourne Airport were both records for the region.
On top of the individual records, the heatwave also saw Adelaide record five consecutive days above 37C – the first time in autumn for 16 years, as well as two nights above 25C – also a first since 2008.
Drawn-out end to the heatwave
The autumn heatwave was the result of a "blocking high-pressure pattern" in the Tasman Sea, which helped fan hot winds from the north over southern districts, according to Ms Scully.
"So that's basically where you have a sort of strong and stubborn high pressure that sits in the one spot for multiple days," she said.
She said it was also being helped along by an upper-level high-pressure system over south-eastern Australia — a feature that's not able to be seen on the weather map – which further enhanced the heat.
The set-up is typical for heatwaves over southern Australia and has caused prolonged March heatwaves in the past including a spell of 12 days above 38C in Adelaide in 2008.
Thankfully, cooler weather is just around the corner – albeit breaking down much slower than normal.
Ms Scully said a southerly wind change would push into southern parts of Victoria this morning, breaking down the worst of the heatwave conditions.
Though she said it wouldn't quite make it over the Great Dividing Range, which meant northern Victoria would remain warm into Wednesday, as would Adelaide.
Then from Thursday, she said the blocking high-pressure system would be weak enough to allow the cooler air mass to push further north.
The danger of hot nights
Hot night-time temperatures are a key part of the bureau's heatwave warning calculation, with both day and night-time temperatures needing to be unusually hot for the location, for at least three days, for a warning to be issued.
There's a good reason these are taken into account.
Studies have shown days with hot nights can significantly increase the mortality risk, compared with days that don't have hot nights.
Ollie Jay, the director of the Heat and Health Incubator at the University of Sydney, said there wasn't a great deal of evidence to explain why.
But he said it was thought to be due to the extra strain it put on the body, making it work overtime.
"If you've got those higher night-time temperatures, you're less able to [lose heat passively], so you still have a sweating requirement to get rid of body heat," Professor Jay said.
"And that would then confer a greater physiological demand overnight, when usually you wouldn't have to, because it's cool enough for you to lose all of that extra heat without having to sweat."
Heatwave season changing
Though the recent temperatures have been unusually hot, autumn is no a stranger to heatwaves, particularly this early on, according to Australian National University professor Sarah Perkins Kirkpatrick, who specialises in heat extremes.
"It is hotter than we usually expect in autumn, but it's also not that unusual to have one last bit of summer before we start heading into cooler temperatures," she said.
Though it's not part of summer, March is also one of the hotter months on the calendar for southern Australia.
A look at the bureau's long-term temperature data from Melbourne shows December to February – the summer months — ranking as the hottest three months, by average maximum temperature.
But December and March are very close with only a 0.3C difference.
It's a similar situation for almost all other capital cities, outside of the tropics, including Sydney, Adelaide, Hobart and Brisbane.
Canberra has a slightly more substantial difference between the two months, with December averaging 26.2C, long-term, and March averaging a top of 24.5C.
In Perth, however, March is hotter than December.
This lingering warmth in autumn is often better reflected in the local Indigenous seasonal calendars for each region, several of which include March as part of the hottest season of the year.
But Perkins Kirkpatrick said the heatwave season was lengthening all across Australia – from both ends.
"We are seeing a lengthening of the heatwave season basically everywhere in the world, inclusive of south-east Australia," Dr Kirkpatrick said.
"Now, in Australia, we tend to measure heatwaves and forecast them generally between November to March, so we are still technically in a time of year where we might have extreme heat again.
"But we are certainly seeing the lengthening of the heatwave season on both sides."
February marks dire milestone
The burst of extreme heat comes after months upon months of broken global heat records.
The latest data from Copernicus Climate Change Service, run by the EU, shows it was the hottest February on record, globally.
It makes it the ninth month in a row to break such a record.
"I still struggle to find words around this – it's really depressing," Professor Perkins-Kirkpatrick said.
"I'm not wholly surprised that February was the warmest month on record, again.
"But it's really frustrating and disappointing, and it does kind of shake me a bit to my core, because things are happening faster than what we anticipated when we were making these predictions 10 to 20 years ago.
"And that's concerning because it's showing we're not doing enough, and things are getting away more quickly than we thought they would."
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