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| September 2005 | National Research Flagship | www.csiro.au/healthycountry/ |
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| September 2005 | National Research Flagship | www.csiro.au/healthycountry/ |
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The south-west of Western Australia has been becoming steadily drier since the 1960s, and researchers are being asked: is this part of greenhouse induced global climate change, or is it just a natural fluctuation? Can it be reversed? “Climate modellers take a very long view,” says CSIRO’s Dr Wenju Cai. “The southern annular mode or the Antarctic Oscillation (AAO, to distinguish it from AO, the Arctic Oscillation) is an index of air pressure difference at sea level between mid latitude (40S) and high latitude (65S). In our greenhouse warming experiments, we found that if there is increasing carbon dioxide in the atmosphere, the AAO rises, sea level pressure increases at midlatitudes, and there is a corresponding decline in rainfall in south west WA. “What we have also found is that this drying trend can indeed be gradually reversed if CO2 stabilisation is achieved,” says Dr Cai. “While this is desirable, it has to be remembered that our models predict that it will take a period of some five hundred years to recover.” Modelling is made more complex by differences in behaviour between the northern and southern hemispheres, and by the variation between higher and lower latitudes caused in part by the ‘sea ice albedo positive feedback’: in polar latitudes the retreat of sea ice and snow greatly reduces the surface albedo or reflection. This in turn leads to increased solar radiation being absorbed at the surface, and thus greater warming. Ocean circulations also play a significant role in the amount of rain which ultimately falls in south west WA. In recent years, a number of studies have examined Antarctic and Southern Ocean currents and ‘cells’ of colder water existing north of the Southern Ocean. As this oceanic behaviour interacts with atmospheric patterns, it can reinforce temperature anomalies. Researchers have observed that in south west WA the number and frequency of ‘extreme’ rainfall events has decreased, and there is once again a connection to the AAO. Using the data from five weather recording stations from 1930 to 2001, the researchers could pinpoint the change-point year as being 1965. For the purposes of this research, an ‘extreme’ rainfall event means one which is greater than a defined norm or threshold at the particular weather station; it causes a visible spike in the graph. Dr Cai says that although computer models show that the drying trend intensifies as atmospheric CO2 increases, there is still healthy debate about the extent of the link between the observed decreasing rainfall and greenhouse warming. There is also evidence that stratospheric ozone depletion is affecting the AAO and thus the region’s rainfall. “In the Southern Hemisphere, when the AAO is high, mid-latitude sea level pressure is greater, rainfall over south west WA decreases,” he says. “There is less cloud and fewer rain-bearing westerly winds. Average winter rainfall has decreased by nearly 20%, which causes a 40% decrease in dam inflow.” Dr Cai says that using the CSIRO Mark 3 Climate Model, he and his team have concluded that long-term natural climate fluctuation over decades or even centuries could itself be sufficient to account for the drying of south west WA. In particular, the decreasing pattern of ‘extreme’ rainfall events tallies with the model. Fewer ‘extreme’ rainfall events go hand-in-hand with a general reduction in rainfall. This is accompanied by a high AAO, and an increase in temperature in southern latitudes. This in turn maintains a southward movement of westerly winds. “The implication is a greenhouse warming-induced drying trend,” Dr Cai says. “This seems to be conspiring with a long-term natural drying trend to generate the most substantial rainfall decrease that we have ever observed.”
The work described here is part of a broader effort examining Australian climate drivers, understanding these drivers’ response to climate change, and reducing uncertainty in projections of future climate. Such climate information is essential to other activities of the Flagship in their endeavour to deliver a tenfold increase in the social, economic, and environmental benefits from water by 2025.
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IN THIS EDITION:
Water for the Future: Perth and South West WA A bird's eye view of Australia's biggest water user Greywater: Not everything is black and white! An eye to the future - Melbourne's climate study Meet some scientists from the Water for a Healthy Country Flagship. |
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Water for a Healthy
Country Flagship |
Phone: +61 02 6246 4565 Fax: +61 03 6246 4564 | editor.healthycountry@csiro.au www.csiro.au/healthycountry/ |
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How
the west has dried
