11.7.3.2 Mean Precipitation
A summary of projected precipitation changes from the MMD models is presented in Figure 11.17 and Table 11.1. The most robust feature is the reduction in rainfall along the south coast in JJA (not including Tasmania) and in the annual mean, and a decrease is also strongly evident in SON. The percentage JJA change in 2100 under the A1B scenario for southern Australia has an inter-quartile range of –26 to –7% and by comparison the same range using the probabilistic method of Tebaldi et al. (2004b) is –13 to –6% (Supplemental Material Table S11.2). There are large reductions to the south of the continent in all seasons, due to the poleward movement of the westerlies and embedded depressions (Cai et al., 2003a; Yin, 2005; Chapter 10), but this reduction extends over land during winter when the storm track is placed furthest equatorward. Due to poleward drift of the storm track as it crosses Australian longitudes, the strongest effect is in the southwest, where the ensemble mean drying is in the 15 to 20% range. Hope (2006a,b) shows a southward or longitudinal shift in storms away from south-western Australia in the MMD simulations. To the east of Australia and over New Zealand, the primary storm track is more equatorward, and the north/south drying/moistening pattern associated with the poleward displacement is shifted equatorward as well. The result is a robust projection of increased rainfall on the South Island (especially its southern half), possibly accompanied by a decrease in the north part of the North Island. The South Island increase is likely to be modulated by the strong topography (see Box 11.3) and to appear mainly upwind of the main mountain range.
Other aspects of simulated precipitation change appear less robust. On the east coast of Australia, there is a tendency in the models for an increase in rain in the summer and a decrease in winter, with a slight annual decrease. However, consistency among the models on these features is weak.
These results are broadly consistent with results based on earlier GCM simulations. In the CSIRO (2001) projections based on a range of nine simulations, projected ranges of annual average rainfall change tend towards a decrease in the southwest and south but show more mixed results elsewhere (Whetton et al., 2005). Seasonal results showed that rainfall tended to decrease in southern and eastern Australia in winter and spring, increase inland in autumn and increase along the east coast in summer. Moise et al. (2005) also find a tendency for winter rainfall decreases across southern Australia and a slight tendency for rainfall increases in eastern Australia in 18 CMIP2 simulations under a 1% yr–1 atmospheric CO2 increase.
Whetton et al. (2001) demonstrate that inclusion of high-resolution topography could reverse the simulated direction of rainfall change in parts of Victoria (see Box 11.3). In a region of strong rainfall decrease as simulated directly by the GCMs, two different downscaling methods (Charles et al., 2004; Timbal, 2004) have been applied to obtain the characteristics of rainfall change at stations (IOCI, 2002, 2005; Timbal, 2004). The downscaled results continued to show the simulated decrease, although the magnitude of the changes was moderated relative to the GCM in the Timbal (2004) study. Downscaled rainfall projections for New Zealand (incorporating differing results from some six GCMs) showed a strong variation across the islands (Ministry for the Environment, 2004). The picture that emerges is that the pattern of precipitation changes described above in the global simulations is still present, but with the precipitation changes focused on the upwind sides of the islands, with the increase in rainfall in the south concentrated in the west, and the decrease in the north concentrated in the east.