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1301.0 - Year Book Australia, 2006  
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AUSTRALIAN DESERTS, CLIMATIC ASPECTS OF AUSTRALIA'S DESERTS

Dr Blair Trewin, National Climate Centre, Australian Bureau of Meteorology, Melbourne.

Australia’s position makes it a prime location for the occurrence of deserts. Around the world, a belt of high pressure exists in the subtropics based on about latitude 30 (north and south), leading to dry conditions in the region near this latitude. Dry conditions also occur in the general easterly flow to the north of this high-pressure belt, except near the east coast of mainland Australia where this flow contains moisture from the Pacific Ocean. The continental interior is also a long way, in any direction, from potential sources of moisture. All of these factors combine to make rainfall a rare occurrence over much of central and western Australia.

THE CLIMATIC CLASSIFICATION OF THE AUSTRALIAN DESERTS

The first question to consider in describing the climate of the Australian deserts is - where are they? In most of Australia, no sharp boundary exists between arid regions and those with ample moisture (at least in part because of the lack of large mountain barriers which act as a sharp divide), and therefore any attempt to define a boundary line on a map is somewhat arbitrary.

The most simplistic definition used for a desert is an area where mean (average) annual rainfall is less than 250 millimetres (mm) or, alternatively, 10 inches. This takes no account of the differences in evaporation (and hence the effectiveness of rainfall) between hot and cold regions and is, therefore, inadequate.

A number of more sophisticated classification schemes have been developed. The best-known is that devised in 1918 by Dr Wladimir Kppen (University of Graz, Austria). A modified version of this is used by the Australian Bureau of Meteorology (BoM). (Endnote 1)

Most of the southern part of the Australian arid zone has rainfall fairly evenly distributed through the year, with summer rainfall becoming increasingly dominant as one moves north. As a general rule, using the BoM classification scheme this would equate to a boundary of about 250 mm mean annual rainfall on the southern boundary of the Australian deserts, and 350-400 mm on the northern boundary.

The Australian desert region using the BoM classification scheme is shown in map S1. This shows that desert climates (or arid areas) occupy most of the western and central interior of the continent. It also shows that some areas traditionally thought of as deserts are, in fact, semi-arid under this classification, notably the area around Alice Springs (Northern Territory) and the coastal fringe of the Nullarbor Plain (Western and South Australia). Both these areas are cooler than surrounding regions (Alice Springs because of its high elevation, the Nullarbor because of its proximity to the moderating influence of the coast), and around Alice Springs the mountains also play a role in increasing rainfall relative to the surrounding plains.

THE RAINFALL REGIME OF THE AUSTRALIAN DESERTS

Australian deserts are dry places, with average annual rainfall below 250 mm over large areas (see map S1). Depending on the averaging period used, the driest parts of Australia, around the Lake Eyre basin (South Australia), have somewhere between 100 mm and 140 mm per year.

While these rainfalls are low, they are not as low as those found in many other deserts. The world’s driest desert, the Atacama in coastal Chile and southern Peru, has locations where rain has not fallen for hundreds of years, while much of the eastern Sahara has an average annual rainfall below 10 mm, and parts of central Asia have as little as 25 mm.

In common with most deserts, rainfall in the Australian deserts is highly erratic from year to year. There is only one recorded instance, at Mulyie (about 100 km east of Port Hedland in Western Australia) in 1924, of an Australian station being rainless for a complete calendar year, but years with less than half the long-term average are relatively common everywhere. In contrast, occasionally the annual average will fall in a single month; on rare occasions, it can fall in a single day.

Graph S2 shows the actual annual and 11-year averaged annual rainfall for Marree, near Lake Eyre in the north of South Australia, over the past 120 years.

There are three major sources for the occasional rains that affect the Australian deserts. These are:

  • Southward extension of the monsoon trough

    The northern fringe of the desert zone receives occasional summer rain as a result of the monsoon, which brings heavy summer rains to northern Australia. The position of the monsoon trough varies from year to year. In some years, such as 1974, it is unusually far south, allowing moist north-westerly flow originating over the waters north of Australia to penetrate deep into the continent. This type of erratic rainfall on the equatorward fringes of a desert is common to other arid regions, notably the Sahel on the southern side of the Sahara in West Africa.
  • Tropical cyclones

    Tropical cyclones occur several times in a normal year off the north-western coast of Australia. Some of these cross the coast and penetrate well inland. While their winds rapidly lose strength over land, the circulation, with associated rain, often penetrates deep inland. This is a particularly important rain-producing mechanism over Western Australian segments of the desert, with some parts of the Pilbara receiving 30-40% of their annual rainfall from cyclones. In the western Pilbara, cyclone-related rainfall is common enough for the area to be classified in the semi-arid category.
  • Northwest cloudbands

    These are bands of moisture that originate over the warm waters of the Indian Ocean north west of Australia and move south east across the continent at middle levels of the atmosphere (several kilometres above the surface). They are most visibly apparent on satellite images as bands of cloud stretching across the continent from north west to south east (hence the name). They most commonly occur in late autumn and early winter, and can be associated with widespread rain along their length.
While all of these moisture-producing mechanisms in the Australian deserts are relatively rare, it is Australia’s relatively flat terrain that allows them to occur at all, as there are no high mountains to block moisture from the north or north west on the occasions when the atmosphere’s circulation is favourable to bringing it in from that direction. In addition, the waters of the Indian Ocean north west of Australia are warm, allowing it to be a source of moisture, whereas the eastern Pacific, bordering the Atacama Desert (north Chile) and Atlantic, bordering the Sahara (north Africa), are relatively cool and serve to stabilise air passing over them, as well as preventing tropical cyclone formation.

S1 THE AUSTRALIAN DESERT REGION

S1: THE AUSTRALIAN DESERT REGION

Note: Based on a modified Kppen classification system (see Endnote 2).

Source: Australian Bureau of Meteorology.


VARIABILITY IN AUSTRALIAN DESERT RAINFALL

The rainfall in Australia’s deserts is highly variable from year to year, but it is also highly variable from decade to decade. This is evident from graph S2, which shows that the 11-year average rainfall at Marree has fluctuated from around 100 mm in the 1960’s to 250 mm in the 1970s.

S2: ANNUAL AND 11-YEAR AVERAGED ANNUAL RAINFALL, Marree (South Australia)


This wide range of variability has had many consequences. Perhaps the most famous occurred on the southern fringe of the South Australian desert, in the Flinders Ranges region, in the 1870’s. In 1865, a boundary (‘Goyder’s Line’), based on surveys of native vegetation, had been defined by the Surveyor-General, G.W. Goyder, as the northern limit of the region where cropping was feasible. The years immediately following were particularly wet and many farms were established north of Goyder’s Line. They prospered for a few years, but when rainfall returned to more normal levels, the farms became unviable and were largely abandoned. Many of the ruined homesteads are still visible today.


The most severe sustained dry period since European settlement in much of Australia’s interior (and in most of eastern and central Australia generally) occurred between 1895 and 1903 – the so-called ‘Federation Drought’. Another particularly dry period occurred between about 1958 and 1968. In each of these protracted dry spells, periods of near-normal rainfall were punctuated by periods with little or no rain, but there were very few episodes of sustained above-normal rainfall.

In marked contrast, there have been a number of episodes of sustained above-normal rainfall across Australia’s desert region. The period from 1995 to 2003 was especially wet over the western half of the Australian arid zone. At its peak, between 1999 and 2001, some locations received nearly three times their normal rainfall (map S3), a remarkable anomaly over such a long period. Giles, near the point where Western Australia, South Australia and the Northern Territory meet, received more rain in the period between 1997 and 2003 than some locations in the western suburbs of Melbourne!

The 1973-76 period was also extremely wet, with parts of central Australia receiving more than double their total normal rainfall for the four years (map S4). Lake Eyre filled during this period, a rare event, and there was widespread flooding across many parts of the interior, particularly during the summers of 1973-74 and 1975-76. January 1974 was, by a substantial margin, Australia’s wettest month on record, due largely to an exceptional southward penetration of the monsoon trough.

S3 RAINFALL - 1 January 1999 to 31 December 2001

S3: RAINFALL - 1 January 1999 to 31 December 2001

Note: All means are based on a standard 30-year climatology (1961-1990).

Source: Australian Bureau of Meteorology.


S4 RAINFALL - January 1973 to December 1976
S4: RAINFALL - January 1973 to December 1976

Note: All means are based on a standard 30-year climatology (1961-1990).

Source: Australian Bureau of Meteorology.


TEMPERATURES IN AUSTRALIA'S DESERTS - MYTHS AND REALITY

Heat is a central part of the popular image of Australia’s deserts. The summers are hot everywhere in the region. Most areas, except for the southern fringe and the highest parts of the central Australian ranges, have average January maximum temperature exceeding 35 degrees celsius (C), and parts of the Pilbara region of Western Australia have averages exceeding 40C (see Australia's climate in the Geography and climate chapter). The hot conditions are also prolonged over much of the region, particularly its northern part, with Marble Bar (Western Australia) having average maximum temperatures above 35C in every month from October to April.

Winter daytime temperatures vary more across the region than summer ones do, increasing fairly evenly from south to north, with averages ranging from below 15C in the Flinders Ranges (South Australia) to 25-27C along the arid zone’s northern boundary.

Extreme high temperatures are also not unusual in the desert region, although possibly not as high as many people believe. The highest extreme temperatures occur in the Pilbara region in Western Australia (most of which is in the semi-arid zone), and in a belt stretching from south-western Queensland through the north of South Australia to the Nullarbor Plain of Western Australia. Many stations in these two regions have exceeded 48C, and the three instances of 50C being reached with standard instruments in Australia all occurred here, at Mardie (Western Australia), Oodnadatta (South Australia) and Wilcannia (New South Wales). Elsewhere, particularly in the Northern Territory and the interior of Western Australia, maxima over 45C are rare or unknown, due in part to the moderating effect of elevation on the most extreme temperatures.

Hot conditions can be extremely prolonged in much of the desert region, most of which has experienced spells of ten or more consecutive days over 40C. The most famous long hot spell in Australian history was that at Marble Bar in the summer of 1923-24, when there were 160 consecutive days above 37.8C (100 degrees Fahrenheit). Even in those areas where the most extreme heat is rare, there are many hot days; for example, at Giles, where the all-time record high is a relatively modest 44.8C, there are an average of 100 days per year of 35C or above, including 69 in succession during the summer of 1964-65.

In most of the desert region in summer, cool days are rare and normally associated with major rain events - a particularly exceptional example occurred in February 1949, when many stations failed to reach 20C on one or more days, and the maximum at Boulia (western Queensland) was a remarkable 14.4C, or 23C below normal.

The Nullarbor coast is one region where cool days in summer, influenced by sea breezes from the cool Southern Ocean, are relatively common. Fluctuations of more than 20C in maximum temperature from one day to the next can occur there, as northerly winds bring very hot and dry continental air from central Australia in place of the cool marine layer (or vice versa) - a more extreme version of the fluctuations for which Melbourne’s summers are well known. (Endnote 3)

As in most deserts, the dry air and clear skies lead to large ranges in temperature between day and night, with ranges of 15C being typical and 20C not unusual. Light overnight frosts in winter are not unusual over much of the southern half of the arid zone, where mean July minimum temperatures are mostly in the 3-6C range. They become increasingly rare as one moves north, with mean July minima around 10C on the northern boundary. The lowest extreme minima occur at high elevations, especially around Alice Springs, where the temperature has fallen as low as -7.5C.

The coldest nights are normally associated with near- or below-normal daytime temperatures - while frosty nights and days well in excess of 40C may occur at the same place, they do not occur on the same day. Overnight temperatures in summer, while normally well below daytime levels, are still quite warm - January averages range from around 17C in the south to 25C in the north - and extremely warm nights can occur during heatwaves, especially if there is enough wind to prevent cool air pooling near the surface overnight. Arkaroola (South Australia) and Wittenoom (Western Australia) have both recorded overnight minima as high as 35.5C, and Oodnadatta (north east South Australia) had nine successive nights above 30C in February 2004.

EVAPORATION, HUMIDITY AND OTHER CLIMATE VARIABLES

Not surprisingly, Australia’s deserts are regions where point potential evapotranspiration (which can be considered as the evaporation that would be measured from a pan) is very high, being more than ten times the average annual rainfall in most of the region (map S5). Average annual totals range from over 3,000 mm in the north to around 1,800 mm in the south. In summer, the region of peak evaporation moves south to be centred on about latitude 25S, as cloud associated with the tropical wet season reduces evaporation further north.

S5 ANNUAL AVERAGE POINT POTENTIAL EVAPOTRANSPIRATION

S5: ANNUAL AVERAGE POINT POTENTIAL EVAPOTRANSPIRATION

Note: Based on a standard 30-year climatology (1961-1990).

Source: Australian Bureau of Meteorology.


Actual evaporation is much lower than this, as there is very little water to evaporate. It matches, or very closely matches, rainfall throughout the region. In other words, virtually all the rain that does fall evaporates, with almost no runoff.


The air over the desert region is usually very dry. Annual average relative humidity in the mid-afternoon (3 pm) is below 30% over most of the area, and falls as low as 20% in parts of inland Western Australia. Days with humidity falling below 10% are not unusual, especially in late winter and spring. The main exception occurs along the Pilbara coast of Western Australia, where sea breezes can bring shallow, very humid layers of marine air from the ocean. Dew point temperatures in summer (the temperature at which dew will start to form if the air mass is cooled) regularly exceed 25C at Port Hedland and can reach as high as 30C, among the highest values observed anywhere in Australia.

Thunderstorms are more common over Australia’s deserts than one might expect. Except in South Australia, most of the area has more than 15 days of thunder per year, and some parts of the Western Australian interior have more than 40. These storms are often ‘dry’ storms with most or all rain evaporating before reaching the ground - indeed, in a few locations there are more days of thunder per year than there are days of rain. While reports are limited because of the extremely sparse population, evidence from observing sites indicate that these storms can be accompanied by very strong winds (Alice Springs has recorded a gust of 174 km/h, one of the strongest non-cyclone gusts ever measured in Australia), and it is likely that other severe thunderstorm phenomena such as large hail and tornadoes also occur from time to time, particularly in southern areas.

Occasional dust storms are a feature of the desert climate. They are most common in the south and east of the region, as widespread strong winds (usually associated with frontal systems) are more frequent there and there is less ground vegetation than there is in much of the west. The area around Birdsville (near the border of Queensland and South Australia) has an average of 6.5 dust storms per year, although the number can be much higher in severe drought years. On the northern fringe the number falls to less than one per year.

While the coldest air masses to affect the desert are usually far too dry for any precipitation by the time they get there, very occasionally, snow flurries may fall in a few parts of the region. Snow was observed falling at Uluru (Ayers Rock), in the Northern Territory, in July 1997 and east of Norseman (Western Australia) in June 2005, and probably occurs more frequently (possibly a couple of times per decade) on the highest peaks of the MacDonnell (Northern Territory) and Musgrave Ranges (northern South Australia) (image S6). On the southernmost fringe of the desert, the higher parts of the Flinders Ranges get snow heavy enough to settle once or twice per decade, with a particularly significant fall occurring, somewhat out of season, in October 1995.

S6: The MacDonnell Ranges in central Australia are one of several major ranges systems in the Australian arid zone. Photograph by Mike Gillam.

S6 The MacDonnell Ranges in central Australia are one of several major ranges systems in the Australian arid zone. Photograph by Mike Gillam.


CLIMATE CHANGE AND AUSTRALIA'S DESERTS

Global temperatures are expected to warm over the next century as a result of increased levels of greenhouse gases in the atmosphere, and Australia’s deserts are no exception. Projections made by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) indicate that the warming rate in the arid zone is likely to be on a par with global average levels, which are expected to be between 1.4C and 5.8C higher than today by 2100. A warming of this magnitude will substantially increase heat discomfort in the arid zones.

Projections of future rainfall changes are more uncertain. While rainfall averaged over the globe is likely to increase slightly, the regional distribution of changes is highly uncertain. CSIRO projections suggest no clear tendency towards either significantly drier or wetter conditions in the northern half of the arid zone, although it is worth noting that much of this region has had a substantial rainfall increase since 1960. On the other hand, rainfall is more likely to decrease than increase in the southern part of the arid zone, particularly in the Western Australian sector where a decrease in rainfall is highly probable.

Considering only the climatic definition of a desert (and not considering issues of land degradation, which are often part of what is meant by the term ‘desertification’), mid-range CSIRO projections suggest that by 2070 the southern boundary of the Australian desert would be expected to move south by 100-200 km, with the northern boundary moving less than 50 km.

Endnotes

1. Under this scheme, the desert boundary is defined by the formula:
    • If rainfall evenly distributed through year - Rain < 10 (T + 7)
    • If rainfall has a definite summer maximum - Rain < 10 (T + 14)
    • If rainfall has a definite winter maximum - Rain < 10 (T)

      where T is the mean annual temperature (in C). Semi-arid areas are defined as other areas where rainfall is less than double the above limits. < Back
    2. Arid and semi-arid areas derived from 0.025 x 0.025 degree resolution mean rainfall, mean maximum temperature and mean minimum temperature gridded data. All means are based on a standard 30-year climatology (1961-1990). < Back

    3. The south coast of Australia is one of the few parts of the world where daily temperatures are more variable in summer than in winter, as it is one of the few places in mid-latitudes with a poleward-facing coast and a hot land/cold ocean boundary in summer. < Back

    References

    The Bibliography in the Geography and climate chapter lists a number of references relevant to the description of the climatic aspects and characteristics of Australia's deserts.

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