Future projections of winter rainfall in southeast Australia using a statistical downscaling technique |
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Authors: | B Timbal D A Jones |
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Institution: | (1) Bureau of Meteorology, Melbourne, Australia;(2) BMRC, GPO Box 1289, Melbourne, VIC, 3001, Australia |
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Abstract: | Much of southeast Australia has experienced rainfall substantially below the long-term average since 1997. This protracted
drought is particularly noticeable in those parts of South Australia and Victoria which experience a winter (May through October)
rainfall peak. For the most part, the recent meteorological drought has affected the first half of the rainfall season May–June–July
(MJJ), while rainfall during the second half August–September–October (ASO) has been much closer to the long term average.
The recent multi-year drought is without precedent in the instrumental record, and is qualitatively similar to the abrupt
decline in rainfall which was observed in the southwest of Western Australia in the 1960 and 1970s. Using a statistical downscaling
technique, the rainfall decline is linked to observed changes in large-scale atmospheric fields (mean sea level pressure and
precipitable water). This technique is able to reproduce the statistical properties of rainfall in southeast Australia, including
the interannual variability and longer time-scale changes. This has revealed that the rainfall recent decline may be explained
by a shift to higher pressures and lower atmospheric precipitable water in the region. To explore the likely future evolution
of rainfall in southeast Australia under human induced climate change, the same statistical downscaling technique is applied
to five climate models forced with increasing greenhouse gas concentrations. This reveals that average rainfall in the region
is likely to decline in the future as greenhouse gas concentrations increase, with the greatest decline occurring during the
first half of winter. Projected declines vary amongst models but are generally smaller than the recent early winter rainfall
deficits. In contrast, the rainfall decline in late winter–spring is larger in future projections than the recent rainfall
deficits have been. We illustrate the consequences of the observed and projected rainfall declines on water supply to the
major city of Melbourne, using a simple rainfall run-off relationship. This suggests that the water resources may be dramatically
affected by future climate change, with percentage reductions approximately twice as large as corresponding changes in rainfall. |
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