Photo: Fairfax Graphics
The Roaring Forties, the Southern Ocean winds which once bore European sailors to Australia and the East Indies, are becoming more like the Furious Fifties as climate change triggers a shift in key weather patterns poleward, an Australian-led team of scientists has found.
Using data derived from Antarctic ice cores and other sources, the researchers found Southern Ocean winds are now stronger than at any time in the past 1,000 years.
Greenhouse gases are what are causing the winds to intensify now and that’s really moving the system beyond the natural range,” said Nerilie Abram of the Australian National University’s Research School of Earth Sciences and lead author of the research, published on Monday in Nature Climate Change.
In the past half century, the westerly winds have quickened 10-15% and moved 2-5 degrees closer to the South Pole – meaning fewer storms are reaching as far north as Australia.
“That isn’t good news for farmers in the southern parts of Australia who are reliant on the winter winds that come out of the Southern Ocean,” Dr Abram said. Winter rainfall has dropped 20% in southwest Western Australia since the 1960s, with cool-season rain tallies also lower in Australia’s southeast.
The stronger winds also help resolve a climate-change conundrum – why Antarctica is not warming as fast as other continents and the Arctic. “Over a large part of Antarctica we don’t get much warming at all,” Dr Abram said. [This may be changing - temperature anomalies over Antarctica have become very significant this year.]
The reason for the discrepancy is that cool air is being trapped over Antarctica, resulting in increased snowfall for some regions. However, areas exposed to stronger winds and warming seas, such as the Antarctic Peninsula, are heating up faster than anywhere else in the Southern Hemisphere.
“The West Antarctic Ice Sheet [adjacent to the peninsula] is probably the bit of the Antarctic ice mass that we’ve been most concerned about for the longest time,” said Matthew England, from the University of NSW’s Climate Change Research Centre, and a co-author of the paper. If it all melted, that ice sheet could lift global sea levels by 4-5 metres, he said.
Professor England said the changes to atmospheric variability that see the band of westerly winds oscillate north or south – known in the Southern Hemisphere as the Southern Annular Mode – are driven roughly equally by the effect of rising greenhouse gases and the ozone hole.
The relative contribution, though, should alter as internationally agreed constraints on the use of chemicals that destroy the protective ozone layer take effect, potentially slowing the pick-up in wind speeds.
“Going forward, the greenhouse aspect will dominate as the ozone hole starts to repair and, of course, greenhouse gases are going terrifyingly upwards in their concentration,” he said.
Wenju Cai, an atmospheric scientist at the CSIRO who was not part of the research team, said the findings would assist the study of other key processes, such as whether the rate at which the Southern Ocean absorbs heat and carbon dioxide is changing.
The faster winds “may have a lot of influences that we do not know now,” Dr Cai said. “We may even solve some of the big issues that have been puzzling scientists for many, many years.”
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