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Climate change not the full story when it comes to above average winter temperatures

By ABC Regional Weather Unit's Ben Deacon and Kate Doyle, Thursday August 17, 2017 - 09:33 EST
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The weather in Brisbane this week has been more like spring than winter. - ABC

Temperatures across southern Queensland and northern NSW have been up to 13 degrees Celsius above the August average on a number of days this week, and while climate change may be the words on everyone's lips, it is not the whole story.

Coffs Harbour, on the north coast of New South Wales, reached 32.8C on Wednesday, well above its average maximum of 19.8C.

It was a similar story across most of southern Queensland and northern New South Wales.

The last time Coffs Harbour saw a day this hot in August was in 2009 when temperatures reached 34C.

"So we're not as hot as 2009 but nonetheless, it's a warmer year than average by a long way," Bureau of Meteorology extreme weather desk senior forecaster Scott Williams said.

"In a place like Coffs Harbour where the records go back to the 1940s, I could only find four daily temperatures above 30."

Why so hot?

For starters there is always natural variability in temperature, or what climate scientists call "noise" — some days are just going to be hotter than others.

But this does not completely explain 13C above average.

For that we need to look at the big drivers of the Australian climate.

The consensus among climate scientists the ABC spoke to is that it is hot because of what has not been happening in the atmosphere.

The weather across the country has been very stable in the first half of winter.

With few cold air intrusions to mix things up, pressure and temperatures have been able to build.

El Nino Southern Oscillation (ENSO) and another ENSO-like driver called the Indian Ocean Dipole (IOD) have been neutral for a while, and that means they are not to blame for the hot weather.

This means other climatic systems that usually sit in the back seat are getting their chance at the wheel.

Storm track moves south, away from Australia

One of those other systems is the Southern Annular Mode (SAM).

In winter, SAM affects whether storms in the Southern Ocean impact Australia.

According to Melbourne University climatologist Andrew King, the kind of extreme warmth over central Australia could be partly due to storm tracks passing to the south of Australia.

SAM is designed to measure this.

"We know that the storm track is moving southwards, away from Australia, due to climate change," Dr King said.

"This means the kind of weather we've seen this winter will likely be more common in future."

SAM has been in a positive mode in late autumn and early winter, and the resulting unusually stable weather has caused all sorts of trouble.

Farmers right across the south of Australia have faced a , and wind energy providers have been plagued by a lack of wind.

Luckily, SAM now appears to have moved to a neutral mode, and there has finally been of the country.

For meteorologists like Mr Williams, events like this week's hot weather need to be seen as part of a bigger picture.

"It's obviously not the warmest year in history because 2009 seemed to set a lot of records," he said.

"But these two years are both pretty recent, and when you look at what the climate science is saying [about] those sorts of extremes, they're saying they will happen more commonly."


© ABC 2017

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