Our changing climate Southern Australia rainfall: long-term trends and future projections Based on projections of future climate, the general drying trend over southern Australia over the past 50 or so years is likely to continue in the future. The seasonal cycle of rainfall is also Observed rainfall trends operating since 2006, now provides likely to change because trends in 18% of Perth’s water supply, with a warm and cool seasons differ. These Rainfall in southern Australian can second desalination plant completed factors have important implications vary greatly from year to year. This in 2011 at Binningup capable of for many sectors including water variability is influenced by a range of providing 33% of Perth’s total water management, industry, transport, climate drivers. On top of the strong needs. Two further desalination plants infrastructure planning, agriculture year-to-year variability, trends over time for Perth are under consideration. and natural resource management. are apparent. In recent decades there has been a generally drier climate than Similarly, in south-eastern Australia, Building on Australia’s national climate the long-term average, particularly many catchments have experienced change projections released in 2015, in winter, generally tracking at the a 50% decline in streamflow in recent we have continued to develop our lower end of the range of climate years (1997–2014 compared to understanding of the processes driving projections in key agricultural regions. 1975–1996) (Hope et al. 2017), which southern Australia’s rainfall, so we now has had serious implications for urban know more about the causes of our In south-west Western Australia, the water supply, environmental flows, declining winter rainfall, the seasonality trend to drier conditions has been and agriculture/horticulture including of rainfall, and the occurrence of accompanied by large reductions in dairy, stone fruit, and grapes. While this extreme rainfall in southern Australia. inflows into the main storage systems, recent period included the Millennium seriously impacting the total amount of drought, there is some evidence With this information, we are in a water held in Perth’s major dams, and that climate change has played a better position to use projections reducing groundwater levels in some role in the decline in rainfall in recent of future rainfall as a tool to help catchments (Smith and Power 2014). decades across southern Australia. identify and minimise exposure to The desalination plant near Perth, climate-related risk and make climate- smart decisions for the future. Trend in rainfall 1950–2017 Spring Summer mm/decade 50 40 30 20 10 5 Winter Autumn 0 -5 -10 -20 -30 A reduction in seasonal rainfall totals -40 from 1950-2017 is seen in much of south-eastern Australia and Tasmania -50 in spring and autumn. Reductions in winter rainfall are evident for many parts of southern Australia. (Source: Australian Bureau of Meteorology) 1. https://www.watercorporation.com.au/water-supply/our-water-sources/desalination What influences rainfall in southern Australia? The main rain-bearing weather systems for most of southern Australia, North Australian sea Natural decadal particularly the west-facing regions, surface temperatures variability are cold fronts and troughs coming from the west. These regions are often dominated by cool-season rainfall, Indian Ocean Dipole El Niño/La Niña with drier summers. Cut-off lows, including east coast lows (intense lows that develop periodically along Global warming the southern coast of NSW), are also important, particularly for eastern parts of Victoria and Tasmania. During summer, for Western Australia, the west coast trough (a long north- south band of coastal low pressure) is important for sea-breezes and showery rainfall, but tropical influences such Negative phase as the southward passage of ex- Southern Westerly Jet tropical cyclones can bring stronger Annular Mode downpours. In the south-east, moist Positive phase tropical airmasses can interact with other weather features such as an upper-level trough or cold front to bring Rainfall influences on southern Australia. heavy summer rainfall. Thunderstorms are also important in bringing rainfall (Dowdy and Catto 2017). of these winds closer to Antarctica rainfall. An El Niño event is often (positive SAM) is linked to reduced associated with drier conditions during These rain-bearing weather systems are winter rainfall for southern mainland winter and spring across eastern influenced by large scale atmospheric Australia, though in summer positive Australia, while La Niña generally brings circulations that act, and interact, on SAM is often linked to higher than wet conditions, sometimes extending various space and time scales. A major average rainfall. In its negative phase, into summer. In contrast, for south- feature is the seasonal progression when the strong westerly winds shift western Australia La Niña can mean very of the subtropical ridge, a band of equatorward, SAM is associated with dry conditions due to linkages with SAM high surface pressure that marks the wetter than normal conditions in winter, (Lim et al. 2016), although overall the boundary of mean westerly winds to the but drier in spring and summer. influence of ENSO is weaker in the west. south and easterly winds to the north. The subtropical ridge ranges in latitude Large-scale circulations in the tropics Each of the above climate ‘drivers’ from around 40°S in summer to 30°S in also influence southern Australian rainfall interact with each other and are winter, and has intensified (i.e. pressures variability. The Indian Ocean Dipole influenced by global warming. For have increased) in recent decades (IOD) is a basin wide ‘see-sawing’ of instance, SAM is projected to shift due to an increase in the number of temperatures across the Indian Ocean. towards its more positive phase, high pressure systems (Pepler et al. In its ‘negative’ phase, where there are resulting in higher atmospheric 2018). These changes are contributing anomalously warm waters off the north- pressures over southern Australia and to the observed cool-season drying west of Australia, the IOD is associated thus less favourable conditions for winter (Timbal and Drosdowsky 2013). with more rainfall than average over rainfall across the mainland. Tasmania, south-eastern Australia, primarily in being further south, will be less affected. To the south of Australia, the band of late winter through spring. A ‘positive’ Very severe ENSO and IOD events are strong westerly winds that encircle the IOD is associated with relatively expected to become more common hemisphere impact the behaviour of the reduced rainfall over this period. and result in greater impacts, including weather systems that affect Australia. enhanced rainfall variability (Power and The expansion and contraction of In the Pacific Ocean, the El Niño- Delage 2018; Wang et al. 2017ab). this band is called the Southern Southern Oscillation (ENSO) also has Annular Mode (SAM). Contraction a significant influence on Australia’s More drying to come... Rainfall projections for the end of the Winter rainfall trend century (2080–2099) show that as the urr asin North Australian sea Natural decadal Summer Autumn surface temperatures variability concentration of global atmospheric greenhouse gases increases, we Spring Winter can expect an increasing decline in Indian Ocean Dipole El Niño/La Niña winter rainfall across much of southern Australia. This is particularly evident West ictoria ast ictoria outwest Summer Autumn Summer Autumn for south-western Western Australia Western ustralia Global warming where there is very high agreement Summer Autumn Spring Winter Spring Winter among climate model simulations of a substantial decline in winter and Spring Winter West asmania ast asmania spring rainfall. This may be as great Summer Autumn Summer Autumn as 50% by the end of the century (compared with the reference period Spring Winter Spring Winter 1986–2005) under a high greenhouse limate moel areement Medium gas emissions scenario (RCP 8.5; note High roecte rainfall cane that the progression of greenhouse gas Very high Substantial increase Substantial decrease ncertain change Negative phase emissions will determine the extent of Southern the rainfall change, and our future might Westerly Jet The map illustrates the long-term (1950-2017) winter rainfall trend, while the boxes Annular Mode follow a different emissions pathway). show the regional projected changes in seasonal rainfall by 2090, relative to a Positive phase An exception is Tasmania, where there reference period of 1986-2005 under RCP8.5. Colours show direction of change, is medium model agreement of an where “Substantial” indicates the projected change is outside the 10-90% range increase in winter rainfall (of up to 20%) of model natural variability, and fill shows the extent of climate model agreement in association with projected increases (medium: >60% of models agree; high: >75%; very high: >90%). Source: data obtained from CSIRO and Bureau of Meteorology (2015) in the strength of the westerlies. Projected rainfall changes for summer and autumn are less clear, with less For short-term projections (to 2030s), variability or that the projections model agreement, which we attribute observed winter rainfall declines are underestimate the observed rainfall to the complex interplay between tracking at or below the dry end of the decline. Thus, climate models that tropical and mid-latitude rain-bearing winter projections for many regions of project a drier future would agree processes at that time of year. southern Australia (see below). This better with the recent observed Western Tasmania is an exception, suggests either large multi-decadal rainfall trends in these regions. with most climate models projecting a decrease in summer rainfall. Winter rainfall observations and projections As both observed and projected Murray Basin South-west Western Australian region trends in winter can differ from those 300 350 in summer for any given location, the 250 300 seasonal cycle is likely to change over 200 time, with implications for ecosystems, 250 150 agriculture and water supply.