A rare event that took place 30 kilometres above the South Pole last week is expected to impact upon Australia’s rainfall outlook.
The upper atmosphere above Antarctica warmed by as much as 40 degrees Celsius in the course of a few days – and it is continuing to warm.
This rare phenomenon, known as sudden stratospheric warming (SSW), could deepen one of the worst droughts in Australian history.
The Bureau of Meteorology’s Harry Hendon warned of dry weather ahead.
“We will typically see conditions across most of Australia, but primarily concentrated in the eastern part of Australia, become warmer and drier through spring and into early summer,” Dr Hendon said.
SSW is rare in the southern hemisphere with only one major event ever identified, in 2002 – one of Australia’s driest years on record.
Dr Hendon said similar, less intense stratospheric warmings had been linked to other dry years in Australia.
“In the past 30 years we probably have had five or six occurrences that didn’t quite qualify as a sudden stratospheric warming,” he said.
We looked at what happened over that period and we’re pretty confident that we will see an increase in temperatures and a decrease in rainfall in central-eastern Australia in the following months.
Not climate change but ‘natural phenomenon’
SSW is not thought to be caused by global warming.
“We view SSW as a natural, internally-generated phenomena,” Dr Hendon said.
“It just happens to be very rare in the southern hemisphere.”
It is much more common in the northern hemisphere where SSWs are associated with cold weather.
Last February, the ‘beast from the east’ storm that covered much of Europe in snow was triggered by an SSW over the North Pole.
Antarctic temperatures affect Australia
Sudden stratospheric warming over Antarctica causes westerly winds south of Australia to track further north, a pattern meteorologists refer to as a ‘negative SAM’.
In spring and summer, this negative SAM pattern brings warmer, drier air into southern Queensland and New South Wales.
“Unfortunately, these are areas already in drought,” said a lead author of the BOM’s spring climate outlook, Andrew Watkins.
Dr Watkins said cooler than normal water in the Indian Ocean, a phenomenon meteorologists call a ‘positive IOD’, has led to a lack of moisture drifting over the continent.
“This has certainly been a big factor in why winter has been so dry in virtually all of Australia,” he said.
On top of that, we have the likelihood of prolonged periods of negative SAM, which also brings drier conditions to New South Wales and southern Queensland.
“So it’s a bit of a double whammy in those locations.”
Dr Watkins said the impact of the SSW may be felt in Australia through to the end of the year.
“These sudden stratospheric warming events and the patterns that we see from them can go from September [to] October, sometimes persisting through to January,” he said.
Dr Hendon said he was gratified the Bureau of Meteorology’s computer models were able to predict the event.
“In 2002 we didn’t even know about it until after it happened, and we didn’t know if we would ever be able to predict it,” he said.
“It’s exciting for us now that we have predictive capability that we didn’t have in 2002.”