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AUSTRALIAN DESERTS, CLIMATIC ASPECTS OF AUSTRALIA'S DESERTS
S1 THE AUSTRALIAN DESERT REGION Note: Based on a modified Köppen 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. 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 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 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 40°C (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 35°C 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 15°C in the Flinders Ranges (South Australia) to 25-27°C 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 48°C, and the three instances of 50°C 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 45°C 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 40°C. 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 37.8°C (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 44.8°C, there are an average of 100 days per year of 35°C 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 20°C on one or more days, and the maximum at Boulia (western Queensland) was a remarkable 14.4°C, or 23°C 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 20°C 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 15°C being typical and 20°C 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 3-6°C range. They become increasingly rare as one moves north, with mean July minima around 10°C on the northern boundary. The lowest extreme minima occur at high elevations, especially around Alice Springs, where the temperature has fallen as low as -7.5°C. The coldest nights are normally associated with near- or below-normal daytime temperatures - while frosty nights and days well in excess of 40°C 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 17°C in the south to 25°C 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 35.5°C, and Oodnadatta (north east South Australia) had nine successive nights above 30°C 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 25°S, as cloud associated with the tropical wet season reduces evaporation further north. 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 25°C at Port Hedland and can reach as high as 30°C, 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. 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 1.4°C and 5.8°C 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:
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. Document Selection These documents will be presented in a new window.
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1301.0 - Year Book Australia, 2006
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 20/01/2006
This page last updated 16 January 2008