IT RAINED so hard across the Southern Hemisphere in 2011 that ocean levels fell by six millimetres, temporarily reversing a long-term rising trend.
The burst of growth across Australia’s ‘dead heart’ sucked up more than a fifth of the carbon produced globally by the annual burning of fossil fuels.
It was an exceptionally wet year, but the 21st Century has also brought exceptionally dry years to Australia. A team of researchers puzzling over the boom-bust cycles of Australia’s semi-arid vegetation think they know why.
The usual suspect, the El Niño-Southern Oscillation (ENSO), doesn’t fully explain the strength of the 2011 La Niña; nor does the Indian Ocean Dipole (IOD) or the Southern Annular Mode (SAM).
But when all those big ocean cycles reinforce each other, weather in Australia and other countries around the Indian Ocean rim starts to move off the charts. That’s the conclusion of a team of researchers led by James Cleverly of the University of Technology Sydney.
The three oceans surrounding Australia are all-pervasive influences on the continent’s climate. Usually, they just make the weather. Dr Cleverly’s team found that when normally asychronous ocean cycles come into sync, as they have this century, then Australia can batten down for a long spell of extreme weather.
Dr Cleverly wasn’t originally looking at how oceans drive extreme weather cycles. He and Derek Eamus, also of UTS, were investigating Australia’s perplexingly large role in driving atmospheric carbon cycles.
In 2011, there was a dramatic increase in the drawing-down of atmospheric carbon by the planet’s vegetation. The 2011 “terrestial sink” was 54 per cent higher than in the previous decade, equivalent to more than 40pc of global emissions from burning fossil fuels.
More surprisingly, Australia accounted for about 60pc of the extra carbon uptake – because the dead heart was in full bloom.
Australia received about 55pc above its long-term average in 2010-11. With millions of square kilometres of mulga and spinifex grasses flourishing the inland drew down massive amounts of atmospheric carbon – only to return much of it in 2012-13, when rainfall over much of the semi-arid zones was half the long-term average.
Investigating these extreme shifts in weather led the researchers to ocean cycles.
The Pacific’s El Nino cycle is a global phenomenon. The Indian Ocean Dipole (IOD) influences weather around the ocean’s rim, and the Southern Annular Mode (SAM) drives seasonal rainfall in Australia’s Mediterranean climate zones.
Each are influential in their own right, but Dr Cleverly’s team, drawing on climate data and satellite records of vegetation growth, found that in the 21st Century, the cycles have often reinforced each other. In 2010-11, their “wet” phases were in sync; in 2008-09, their dry phases synced.
How often this occurred historically is difficult to assess, because satellite measurements of sea-surface temperatures only began in the late 20th Century.
