{News} 081015! Climate's three-headed dog

Climate's three-headed dog
MATTHEW CAWOOD

Farmonline, 15/10/2008 2:10:00 PM
There's a "three-headed dog" savaging Australia's climate, according to CSIRO scientist Dr Wenju Cai, and two of the heads are eating away at rainfall in southern Australia.
The three climate influences referred to by Dr Cai are El Nino, the Southern Annullar Mode, and the Indian Ocean Dipole.
All are driven by ocean temperatures, and all are being intensified, to Australia's disadvantage, by global warming.
El Nino, driven by temperatures in the Pacific Ocean, is well studied and has had fairly predictable consequences for mid-latitude eastern Australia.
The other two influences are less well known, but Dr Cai said there is growing evidence that their fluxes are behind the drying-out of southern Australia.
One, Southern Annular Mode (SAM), appears to be driving rain-bearing winter fronts progressively out into the Southern Ocean.
To the west, the Indian Ocean Dipole (IOD) is blocking the generation of moisture-rich air that crosses the continent to bring Victoria, South Australia and southern NSW vital spring rains.
Dr Cai, a principal research scientist with CSIRO Marine and Atmospheric Research, believes that it's the IOD that is behind the failed spring rains currently causing stress to farmers in the south-east.
In a "good" year, the eastern Indian Ocean lying off Western Australia is warmer than the ocean off Africa.
The warmth produces convection, and humid pressure systems that stream across to south-eastern Australia where, on meeting cold air from the south, they produce rain.
When the eastern Indian Ocean is cool, convection dies away and south-eastern Australia (mainly Victoria and South Australia) can expect less spring rain.
According to Dr Cai, the IOD is now at work with rare savagery.
"During 2006-07-08, we've had a very unusual situation in that three Indian Ocean Dipoles occurred consecutively," Dr Cai said.
"The only time we've known this to occur before is in 1944-45-46. From 1939-46 were big drought years.
"The polar shift of the fronts [SAM] is also going on, but these consecutive dipole events are making that very much worse."
Modelling, and trends to date, indicate that as atmospheric carbon dioxide rises, warming patterns in the Indian Ocean will be "dipole-like", Dr Cai said — that is, the phenomenon is likely to be more frequent.
Unusually in the world of modelling, all nine major climate models agree on SAM.
The theory, according to Dr Cai, is that as the planet warms, the big rain-bearing fronts that deliver southern Australia much of its winter rain are progressively tracking further south, out to sea.
Frontal systems are always moving west-to-east across the upper reaches of the Southern Ocean, shifting around the globe in a broad band defined by ocean temperature.
SAM refers to the seasonal north-south movement in those fronts as oceans warm and cool with the seasons.
In summer, with the warming of the oceans that share a latitude with southern Australia, the weather systems move south, tending to track across the ocean.
In winter, as the cool temperature gradient moves north again, so do the fronts.
Historically, their winter track has been over southern Australia, delivering the south much of its characteristic winter rainfall.
But Dr Cai said that as climate change and ozone depletion warms the planet's oceans from the equator down, the ocean temperature gradient is changing.
As the temperature rise creeps south, it takes these rain-bearing systems south - not just in summer, but in winter too.
"This is the most consistent result we see in the computer models," Dr Cai said.
"The only possibility for stopping it is to stabilise levels of atmospheric carbon dioxide and stop the change in ocean temperature gradient."
And even then, he added, it might take hundreds of years for patterns to re-establish themselves to those of the recent past.
"I don't have any good news," Dr Cai confessed.

Source: http://nqr.farmonline.com.au/news/state/agribusiness-and-general/general/climates-threeheaded-dog/1334547.aspx