Broadly, Australia can be divided into 2 seasonal rainfall regimes: the north and the south. The northern half of the continent typically experiences a monsoonal, summer wet season, with rainfall falling from October through April, and the rest of the year being dry. The southern half of the continent—particularly southern parts of Western Australia and South Australia, and the whole of Victoria and Tasmania—experiences its highest rainfall during the cooler months from April through November.
Rainfall in Australia is highly variable, largely because of large-scale atmospheric and oceanic drivers that affect the region. The most important driver of Australian rainfall variability is ENSO, particularly for rainfall across eastern Australia, excluding Tasmania. This leads to very high year-to-year rainfall variability, such that long-term trends because of climate change are harder to distinguish than for temperature. For example, a trend towards higher rainfall in association with the northern monsoon since the 1970s is not statistically significant when compared with natural rainfall variability. However, a drying trend in south-western and south-eastern rainfall during the first half of the southern growing season is broadly consistent with expected patterns of change because of global warming (Whetton et al. 2015).
Rainfall in Australia has displayed both large natural variability and large trends during the past 2 decades. Large parts of Australia suffered an extended period of low rainfall during the millennium drought of 2000–10 (which in some areas began as early as 1997 and ended as late as 2012). This was followed by drought-breaking rain associated with Australia’s wettest 2-year period on record in 2010 and 2011, which were the result of 2 strong La Niña events (Figure ATM20). This record rainfall fell during the summer or northern monsoon seasons, and was not associated with a recovery in southern wet-season rainfall.
Since the 2011 La Niña, drought has re-emerged across large parts of Australia, especially western Queensland, northern New South Wales and western Victoria (Figure ATM21). The opposite ENSO phase to La Niña, El Niño, occurred in 2015 and is associated with below average rainfall across large areas of eastern Australia. This was especially challenging for the areas of south-western Western Australia, Queensland into northern New South Wales, and central to western Victoria, across the border into South Australia, that had seen the re-emergence of drought conditions after the 2 La Niña years. However, for some areas, near-record temperatures in the Indian Ocean are likely to have moderated the drying influence of the 2015 El Niño on Australia. The Indian Ocean has been at near-record temperatures for close to 10 years, with significant impacts on the climate of Australia.