HR: 09:24h
AN: A51F-07    [Abstracts]
TI: Attribution of South West Australia observed rainfall trends
AU: * Timbal, B
EM: b.timbal@bom.gov.au
AF: Bureau of Meteorology Research Centre, 700 Collins St, Docklands, VIC VIC 3008 Australia
AU: Arblaster, J
EM: jma@cgd.ucar.edu
AF: National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, Col Col 80305 United States
AB: A statistical method, based on the idea of analogous synoptic situations, is used to study the observed rainfall trends in South-West of Western Australia (SWA). The method has been developed and optimized by relating patterns of atmospheric field (predictors) to station records of rainfall (predictands) in the SWA, for both rain occurrences and daily rain amounts. When tested on observational data, the analogue method is able to reproduce the observed trend and the inter-annual variability observed during the past 50 years, showing the sensitivity of the analogue approach to changed climatic conditions. Among the parameters considered, the set of predictors has one of the most important impacts on results. It is demonstrated that moisture fields explain an important part of the observed trend. The model is then applied to two ensembles of four simulations of the past century performed with the Parallel Climate Model (PCM) developed at NCAR. One ensemble is forced with natural forcings: volcanic eruptions and solar variability. The other ensemble is obtained adding human induced forcing to the natural forcings: greenhouse gases, ozone and aerosols. It is shown that the downscaling approach improve the reproduction of local rainfall compare with rainfall directly modeled to the point that realistic statistics of the local series are reproduced. A significant difference is observed between two sets of ensembles for the reproduction of the observed drying trend. With natural forcing only, the downscaling of the ensemble suggest an increase of rainfall in the SWA during the second half of the 20th century. When anthropogenic atmospheric forcings are added, the four-member ensemble gives an average reduction of the rainfall, which resemble what has been observed but with the magnitude is halved. One member of this ensemble was re-run with the effect of global land clearance taken into account. In this case, the drying trend is enhanced and resembles what was observed. These results suggest that the observed drying trend in the SWA is due to anthropogenic forcing and has been enhanced by global and local land clearance.
DE: 9330 Australia
DE: 3354 Precipitation (1854)
DE: 1803 Anthropogenic effects
DE: 1610 Atmosphere (0315, 0325)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting