It is starting to be time to look at rainfall trends over extended periods as well as shorter (up to six months) outlooks.
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For example, rainfall in southern Australian can vary from year to year, but on top of this year-to-year variability, trends over longer periods of time are now becoming more apparent.
In recent decades there has been a generally drier climate than the previous long-term average, particularly in winter and especially in Victoria, southern NSW, south-eastern South Australia and in the south-west of Western Australia.
Research has shown that in south-eastern Australia, many catchments have experienced around a 50 per cent decline in streamflow in recent years (in the past 20 years compared to the previous 20-year period of 1975–1996).
While this recent period included the so-called Millennium drought, there is some evidence climate change has played a role in the decline in rainfall in recent decades across southern Australia.
This is because of variations in the band of westerly winds that circle the hemisphere south of Australia and influence the weather systems that affect southern Australia.
The expansion and contraction of this band is called the Southern Annular Mode (SAM).
Contraction of these winds closer to Antarctica often results in lower winter rain for southern mainland Australia and this contraction is probably linked to warmer sea surface temperatures (SSTs) caused by global warming.
Many long term models indicate that the SAM will become positive more frequently resulting in higher atmospheric pressures over southern Australia and therefore less favourable conditions for winter rainfall across the mainland.
Conversely, warmer SSTs in the north-east Indian Ocean can result in a positive Indian Ocean Dipole (IOS) which can bring increased rain to south-eastern Australia especially from April to August so this can counter the SAM on occasions but looking at the past 20 years, the SAM has been more influential more often. So maybe at least southern NSW and Victoria will see more frequent “dry” years in the coming decades.
The longer term trend for northern NSW and the east coast is not as clear. Warmer SSTs can increase rainfall potential as can variability in the IOD and the Pacific Ocean Southern Oscillation Index (SOI).
However, the associated increase in atmospheric energy is likely to result in more severe events occurring, but less frequently.
A dependence on these occasional events will result in enhanced rainfall variability but this will be needed if rainfall levels in the east and north of NSW and eastern Qld are going to be maintained in the coming decades.
