WEATHER is a local affair, but climate is a global matter. The distinction has been neatly illustrated in new research that shows an intriguing link between snowfall in Antarctica and a long decline in south-west WA’s rainfall.
Both changes, although thousands of kilometres apart, seem to stem from a change in atmospheric circulation that has no apparent parallels in the past 750 years.
Current evidence suggests the circulation pattern changed because ozone levels declined and greenhouse gas levels increased, altering the temperature of the stratosphere above the Earth’s southern pole.
Tas van Ommen, principal research scientist with the Australian Antarctic Division, said he and his colleagues had no thought of WA’s weather when they began investigating why snowfall at Law Dome in East Antactica has increased over the past 40 years.
When the team compared ice-core data with the long-term satellite record, they found a consistent pattern of warm moist air flowing down to Antarctica, which induced more snow—"more or less what we expected".
"But at that stage we realised that it wasn’t just a one-sided circulation. In the big picture, cold dry air was also being directed up towards Western Australia, toward the area that we instantly recognised as being subject to drought.
"We were surprised at the strength of the connection. This sort of thing is unusual in climate science."
South-west WA’s long-term average rainfall dropped about 10 per cent in the 1970s, and over subsequent decades has fallen another 10 per cent. Each 10 per cent decline in rainfall translates to a roughly 50 per cent decline in river inflows, playing havoc with the State’s water security.
Understanding the origins of this decline "is something climate scientists have been banging their heads against for 10-15 years", Dr van Ommen said.
The changed Southern Ocean pattern doesn’t explain everything, but Dr van Ommen believes it might be responsible for up to 40 per cent of changes in WA. Other factors are at work: rain-bearing fronts appear to be moving south, and ocean temperatures are changing, altering climate patterns.
There are two clues that Dr van Ommen believes suggest a human influence on the Antarctic-WA circulatory pattern.
The Law Dome ice cores currently go back 750 years. The past 40 years stand out as unusual across those seven centuries. "There’s something that just doesn’t look natural—a smoking gun," Dr van Ommen said.
Using climate models, the team evaluated this curiousity against the understanding that the greatest level of ozone depletion and greenhouse gas accumulation have occurred roughly over the same period.
Both influences are believe to cool the stratosphere over the south pole, and to influence the "Wave Three" lobe pattern of high and low pressure systems that circulate between Antarctica and the three continental land masses to the north.
"Climate models are imperfect, especially over the polar regions," Dr van Ommen acknowledged. "But the evidence we see when we introduce ozone depeletion and increased greenhouse gases to the models is this very neat signature of cold dry air moving toward south-west WA, and warm moist air moving toward the Antarctic coast.
"It supports the hypothesis, but the climate models will also benefit from the ice core data, which will help validate the models over longer time periods than currently possible."
If the hypothesis reflects reality, the good news is that the ozone hole is forecast to close in coming decades.
But at present, there are no signs of greenhouse gas concentrations falling.
Dr van Ommen said the WA-Antarctica link, and growing interest in the Wave Three pattern, will also influence future research on the "Big Dry" of south-eastern Australia.
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