2006), Attribution of the late 20th century rainfall decline in South-West

Abstract: There has been a dramatic decrease in rainfall in the South-West of Australia (SWA) since the mid 1960s. A statistical method, based on the idea of analogous synoptic situations, is used to help clarify the cause of the drying. The method is designed to circumvent error in the rainfall simulated directly by a climate model, and to exploit the ability of the model to simulate large-scale fields reasonably well. The method uses relationships between patterns of various atmospheric fields with station records of rainfall, to simulate the local rainfall variability. The original technique was developed in a previous study. Here it is modified for application to two ensembles (each of four members) of simulations of the past century performed with the Parallel Climate Model (PCM). The first, called "Natural", is forced with natural variations in both volcanic activity and solar forcing. The second, called "Full Forcing" also includes three types of human-induced forcing resulting in trends in greenhouse gases, ozone and aerosols. "Full Forcing" provides a better match to observational changes in sea surface temperature in the vicinity of SWA. However, it is not possible to discriminate between the two ensembles which one match the observed decline. There is a hint that the fully forced ensemble is more realistic, but it is nothing more than a hint. The downscaling approach, on the other hand, provides a much more accurate reproduction of the day to day variability of rainfall in SWA than does the rainfall simulated directly by the model. The downscaled ensemble mean rainfall in "Full Forcing" declines over the region with a spatial pattern that is similar to the observed. This contrasts with an increase of rainfall in the downscaled rainfall in the "Natural" ensemble. These results give the clearest indication yet that anthropogenic forcing played a role in the drying of SWA. Note, however, that ambiguity remains. For example, although the observed decline fit within the range of downscaled model simulation, the ensemble mean rainfall decline is only about one half of the observed estimate, the timing differs from the observations, drying did not occur in the downscaling of one of the four "Full Forced" ensemble members, and not all potential forcing mechanisms are included in "Full Forcing" e.g. land surface changes