Antarctic vortex sends drought spiralling

The weather systems driven by the strong westerly winds of the Antarctic polar vortex curl over the southern continents in this satellite view of Antarctica (NASA, Galileo)

An alarming interaction between ozone depletion and global warming may explain why Australia's southern cities and farms have lost 20% of their rainfall in the last 30 years.

The claims are aired tonight on ABC-TV's Catalyst program tonight.

"It really is a revolution in the climate sciences," said Dr David Jones, chief analyst of the National Climate Centre at Australia's Bureau of Meteorology in Melbourne. "We can't just look at natural variability or greenhouse climate change in isolation -- we also have to factor in ozone."

Across many future climate projections, Australia in winter shows the largest reductions of rainfall of any region in the world. However, rainfall in south-western Australia has already decreased faster than predicted, suggesting factors other than those already identified are at work. According to Jones and colleagues, the clues lie 20 km high above Antarctica.

The Antarctic polar vortex is a natural, continent-wide 'tornado' of 200 km/h, super-cold winds surrounding the ozone 'hole' from the stratosphere to the surface. It is created by the movement of the globe interacting with temperature differences between the pole and the rest of the Earth's surface. The vortex delivers the winter rain-bearing westerly winds called the 'Roaring Forties,' which southern Australia relies on for its water supplies.

However, Jones and team have found that global warming and ozone depletion are interacting to shrink and accelerate the vortex, dragging crucial rainfall towards the South Pole, away from Australia's landmass.

The 'reliable' rainfall regions most affected by the loss of winter rains are coloured dark blue and purple (Bureau of Meteorology)

The researchers relied on three decades of data from Antarctica, U.S. and Australian research to show the operation of a vicious cycle. On the one hand, ozone depletion leads to cooler temperatures and lower pressures above Antarctica. On the other, greenhouse warming leads to higher temperatures and pressures over other parts of the globe. This steepens the temperature-pressure gradient between Antarctica and the rest of the Southern Hemisphere, resulting in the stronger and faster-spinning polar vortex which 'pulls' the rain-bearing westerlies southwards.

The last thirty years have recorded a dramatic 20% loss of the average rainfall along Australia's southern fringe, marked by sudden drops in southwestern Australia in the 1970s, and Victoria in the 1990s. Apart from providing much needed water to farmers, winter rains are crucial for topping up the reservoir supplies for more than six million people living in our southern capitals. The reservoirs of Melbourne, Adelaide, and Perth are all currently at low levels -- Perth has been at 18% capacity this year (2003).

Dramatic declines in winter rainfall over the last thirty years have resulted in even sharper falls in the streamflows that recharge city reservoirs (Mark Horstman)

Dr James Risbey, at Monash University in Melbourne, believes the 2002 drought [pdf report] was the worst in recorded history -- and the first in Australia with a global warming 'signature' -- because the temperatures were higher than in other drought years.

The mid-range prediction for temperature increase in Australia by the end of this century is 4 °C. This has a direct impact on the water balance because as warming continues the amount of evaporation increases.

Risbey fears a climatic 'double whammy': where the combination of less rainfall from a contracting vortex and less available moisture from increasing evaporation rates drags southern Australia into a state of permanent drought.

"With four degrees of warming across Australia, we'd need to see an increase in rainfall of some 30 odd percent to keep pace with that. This is most unlikely to happen, and so we're going into increasing water deficit across the continent."

"The worst case scenario is that we start to run out of water around the cities," he told Catalyst. "In that case we'd have to think seriously about moving some of the water out of agriculture and into urban uses."

Link to article: http://www.abc.net.au/science/news/enviro/EnviroRepublish_946924.htm