Recent rainfall, drought and southern Australia's long-term rainfall decline
Issued
Introduction
This article describes recent rainfall patterns in the context of the Millennium Drought, record high rainfall of the 2010 to 2012 La Niña years, and longer-term drying trends.
The past three years have seen the return of widespread rainfall deficiencies across southern and eastern Australia (Figure 1). Below-average rainfall across large parts of Australia since the end of the 2010 and 2011 La Niña events, and the failure of the northern wet season in much of inland Queensland over the past three years, have contributed to these deficiencies (see the latest issue of the Bureau's Drought Statement for more information).
These deficiencies re-emerge against a background of significant longer-term rainfall decline over southern Australia which has now persisted for decades. The southwest of Western Australia has experienced a 10 to 20 per cent drop in winter rainfall since around 1970—that has been expressed as a step-change or series of step-changes, rather than a gradual decline. This period also includes an absence of high rainfall years that were common prior to that period. The southeast of the continent has experienced a similar decline in late autumn and early winter rainfall since around the mid-1990s (see the State of the Climate 2014 for more information). These rainfall changes have been accompanied by much larger reductions in streamflow, particularly in the southwest.
The Millennium Drought
From late 1996 to mid-2010, much of southern Australia (except parts of central Western Australia) experienced a prolonged period of dry conditions, known as the Millennium Drought (Figure 2). The drought conditions were particularly severe in the more densely populated southeast and southwest, and severely affected the Murray-Darling Basin and virtually all of the southern cropping zones.
While this episodic dry spell contributed to a long-term statistical decline in southern cool-season rainfall, it is also partially distinct from those drying trends—most particularly in the southwest, where winter drying has persisted for more than four decades.
During the Millennium Drought, Perth, Adelaide, Melbourne, Hobart, Canberra, Sydney and Brisbane were all affected by persistent or periodic drought episodes.
While southern areas experienced dry conditions during the Millennium Drought, the north (particularly northwest) experienced unusually heavy rain. Indeed, while large parts of Australia were experiencing drought conditions, Australia as a whole saw above-average rainfall, driven by heavy monsoonal rainfall in the north.
This pattern can be understood in terms of the seasonal rainfall differences (Figure 3). Rainfall in northern Australia is dominated by the summer monsoon with thunderstorms and the occasional tropical cyclone bringing much of the annual rainfall. In contrast, the winter period is almost rain free.
In southern Australian, rainfall mainly tends to come as a result of cold fronts and associated low pressure systems. These systems peak between the months of April and October (the 'cool season') when the lows in the Southern Ocean tend to move closest to Australia. Summer sees some rainfall, but usually much less than the cool season in the southeast (and with reduced effectiveness because evaporation rates during summer are very high). The southwest has a more typical Mediterranean climate, with normally dry summers.
The Millennium Drought was essentially a cool season feature in southern mainland Australia (Figure 4a), with warm season (November to March) rainfall above average in most parts of Australia, and anomalously high in tropical areas (Figure 4b). Note that for Tasmania, the dry conditions were present in both seasons, but even stronger during the warm season. The effects of poor rainfall during the cool season was cumulative, meaning that the drought took hold through a series of increasing hydrological impacts over a series of years, with long-term drying of vegetation, the landscape and a drawdown on water resources. For example, Melbourne water storages dropped from almost full in October 1996 (97.5 per cent) to only one third full by June 2010 (33.0 per cent).
The La Niña years
2010 and 2011 were dominated by La Niña conditions and heavy rainfall for most areas except the far southwest (Figure 5). The rainfall was particularly heavy during summer with large areas experiencing record or near record rainfall. This rainfall was sufficient to give Australia its wettest two year period on record (PDF) (2010 to 2011) and set many records at individual locations, and effectively ended the extremely dry hydrological conditions that were a signature of the Millennium Drought.
While the very wet La Niña events may have suggested a return to more normal rainfall patterns for Australia, the seasonality of rainfall during and since these years suggests otherwise. The two La Niña years saw excessive rainfall during the warm season, but did not reverse the pattern of poor cool season rainfall that was the major feature of the Millenium Drought. The early cool season rainfall during these two years (April to July) shows the pattern of below average rainfall in the southwest, southeast and parts of the east still persisted.
Recent and upcoming rainfall in perspective
Looking at the rainfall for the past three years (Figure 6), we see that the rainfall pattern shows some of the characteristics of the long-term rainfall trends—with dry conditions in southeast and southwest Australia, contrasting with unusually wet conditions in the north and northwest.
There is also the addition of rainfall deficiencies across much of Queensland and New South Wales, similar to some periods of the Millennium Drought. Rainfall deficiencies further north reflect the rather poor northern wet-season rainfall in recent years in inland eastern Australia.
For southern parts of Australia that are largely dependent on the cool season rainfall cycle for agriculture. Dry conditions in the last three years have been most pronounced during the cool season, consistent with the longer-term pattern of reduced rainfall from frontal rain-bearing systems that has persisted.
While natural rainfall variability in Australia is large, and influenced strongly by the El Niño-Southern Oscillation, based on current research it seems likely that drying across southern Australia cannot be explained by natural variability alone. A shift in atmospheric circulation characterised by a contraction of mid-latitude storm tracks towards higher southern latitudes, and movement of the subtropical and polar jetstreams, has very likely contributed to the cool season rainfall declines in southern Australia. A contraction of these weather systems toward the pole is at least partly explainable by anthropogenic warming and potentially also contributed to by anthropogenic reductions in stratospheric ozone. Global warming also reduces the temperature gradient between the equator and pole, reducing the energy available to mid-latitude weather systems (see the Climate Change in Australia website and Technical Report (PDF) for more information).
However, natural variability still dominates Australian rainfall variability from one year to the next, and its influence is large enough to mask the impact of underlying rainfall trends in the short to medium term. In this way, high autumn and winter rainfall can still occur across southern Australia, and there is a chance that some cool season rain will come in the next three months. The most recent seasonal outlook (Figure 7) indicates that it is likely to be wetter than normal over much of the Australian mainland. Above average rainfall is not likely across the whole country—unfortunately for parts of Queensland that are currently in drought, the chances of getting substantial rainfall to provide long-term relief to these conditions is diminishing as the northern wet season is coming to a close.
It should be noted, however, that this high chance of above-median rainfall is relative to the median for 1981 to 2010, a period in which April to June has been relatively dry in many parts of the country. When compared to the average rainfall over the full period of record (1900 to 2014) in southeast Australia, there have only been five above-average April to June seasons in the last 20 years.
In Australia, we gladly take the rain when it occurs. Hopefully we get some good rainfall across parts of the continent in this outlook period. For the second half of 2015, the chance of an El Niño has increased, which typically means drier-than-average conditions for eastern Australia (see ENSO Wrap-Up for more information).
In the longer term, large year to year rainfall variability is expected against a background state which is expected to further change through time.
The Bureau will continue to monitor rainfall across Australia at all time and spatial scales to note both episodic drought events and the longer-term trends in rainfall.
Product Code: IDCKGMTA00
