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Climate index explains dryness

24 March, 2010

CSIRO and Chinese scientists have developed a new climatic index which provides an answer to a riddle that has puzzled researchers for decades: 'Why has south-west Western Australia experienced dramatic declines in rainfall since the 1970s?'

The Southwest Australian Circulation Index (SWACI) shows how much of the ‘'blame' can be attributed to the weakening of a major atmospheric circulation over the Indian Ocean.

In a paper published in the Journal of Climate, CSIRO statistician Dr Yun Li and climate physicists Professor Jianping Li and Juan Feng from the Chinese Academy of Sciences, say the SWACI also provides researchers with a new tool for predicting annual patterns of wet season rainfall.

"Using this new index we have found that the Southwest Australian Circulation is becoming weaker early in the winter wet season (May to July) causing the drier conditions observed in south-west WA," Dr Li said.

Since the mid-1970s south-west Western Australia has seen a 15-20 per cent decrease in average winter rainfall, from 323 mm in 1925-1976  to 276 mm from 1976-2003.

Dr Li says the SWACI is similar to the more commonly known Southern Oscillation Index (SOI) used to indicate the strength of the El Nino/Southern Oscillation (ENSO) in the Pacific Ocean, which influences rainfall patterns in eastern Australia. It is a number determined from the intensity and direction of seasonal winds over south Western Australia that helps researchers get an at-a-glance picture of the local climate.

South-west WA – a vast area which includes Perth, the Margaret River wine region and the West Australian wheat belt – receives most of its annual rainfall during winter from passing cold fronts and storms. However, since the mid-1970s, the number of storms in the region have decreased leading to less rainfall with the drier conditions being exacerbated due to more high pressure systems entering the area.

"The atmospheric circulation over south-west WA is a monsoon-like system," Dr Li said. "It has wet and dry seasons like northern Australian monsoons but they happen at different times. The wet season rains fall in winter rather than in summer.

"We've found a strong correlation between the Southwest Australian Circulation and the winter wet season. In years when the circulation is weaker, the SWACI has a value of less than -1, and there are more high pressure systems over the southern Indian Ocean, delivering weaker westerly and northerly winds and dry conditions. When the index is stronger (above 1), low pressure systems are more common in winter and rain falls more often."

He said further investigation using global climate models would be required to confirm whether warming oceans or natural fluctuations are responsible for the weakening circulation.

"The SWACI will help identify rainfall patterns throughout the year as well as long-term drying trends for south-west WA. It will also help increase the accuracy of global circulation models in predicting future climate scenarios," Dr Li said.

According to Dr Ian Foster from the Department of Agriculture and Food WA, the department will adopt the new index to help forecast seasonal rainfall and to assist farmers with agricultural management decisions such as, which crops to plant and when.

"We will be using the SWACI to see if it can improve our skills in forecasting seasonal rainfall and crop growth potential in south-west WA," Dr Foster said.

Source: CSIRO

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