Large-scale Climatic Changes and their Attribution

During the last sixty years there has been a marked and continuing reduction (about 20% to date) in autumn and winter rainfall over SWWA. This negative rainfall trend has coincided with major shifts in the structure of the large-scale circulation of the Southern Hemisphere (see Frederiksen and Frederiksen 2005, 2007), which has in turn changed the nature of the prevailing weather systems, including their growth rates and likelihood of development. In IOCI Stage 2, this three-way relationship between changes in large-scale circulation, weather systems and rainfall was only investigated for July and over the period 1948 to 1994, when some of the largest changes occurred. In IOCI Stage 3, this research was extended to all months and into the 21st century. The issue of attribution (i.e., identifying the causes) of these changes was also addressed with the formulation of a new methodology to identify the forcing responsible.

This chapter summarises some of the IOCI3 research that links observed regional Western Australian rainfall changes to large-scale atmospheric changes. It also describes some of the consequences of these large-scale atmospheric changes for important weather systems, using dynamical models and mechanistic studies. The issue of attribution and future projections of changes are also discussed. The changes in the statistics of the different weather systems and their impacts on SWWA, as well as future projections of these systems, are discussed in Chapter 4.