Greenhouse gases cut WA rainfall

About half of the long-term rainfall decline experienced in south-west Western Australia since the 1960s can now be linked to increases in greenhouse gases, say two CSIRO scientists.

Using the latest results from more than 70 experiments involving 21 climate models from around the world, CSIRO scientists Tim Cowan and Wenju Cai have traced a complex set of interactions between the circulation of the atmosphere and rainfall over the south-west of the state.

Mr Cowan says the region receives the bulk of its rainfall in the winter months as a result of the seasonal movement of a ridge of high-pressure in the sub-tropics.

"What happens is that the ridge of high-pressure moves northward passing beyond the southern tip of Western Australia, in turn allowing low-pressure systems to also move northward, bringing the rainfall," he says.

"Year-to-year variations in this seasonal cycle is a well-known feature of the climate of the southern hemisphere and is known as the Southern Annular Mode or 'SAM'. What seems to be happening is that, on average, this ridge of high-pressure is not moving as far north as in earlier decades of the 20th Century."

Using climate models to identify why this is happening, the scientists concluded that increases in greenhouse gases can explain half the rainfall trend. The usual natural variations in the SAM might contribute to the remainder of the reduction.

"One of the most consistent results from the climate models is that as carbon dioxide continues to increase, south-west Western Australian rainfall will continue to decrease," Mr Cowan says.

"This work is important because the 15 per cent reduction in south-west Western Australian winter rainfall since the 1960s has, on average, translated to a 40 per cent reduction of inflows to Perth dams."

The research was undertaken for the Indian Ocean Climate Initiative (IOCI), a multi-million dollar project established by the WA Government in partnership with CSIRO and the Bureau of Meteorology to support informed decision-making on climate variability and change in WA. IOCI scientists are now trying to develop a better understanding of the SAM, what could force it and how it could be shaping the world's climate. The research is delivered through the Water for a Healthy Country Flagship.

Reference: Cowan, T. and Cai, W. Geophysical Research Letters (Geophys. Res. Lett., 33, L24708, doi:10.1029/2006GL028037).

Contact: Tim Cowan, CSIRO Marine and Atmospheric Research

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