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Australia's climate is largely determined by its latitude, with the mainland lying between 10 degrees south (°S) and 39°S and Tasmania extending south to 44°S. This places much of Australia under the influence of the sub-tropical high pressure belt (or ridge), which is a major influence on climate near, and poleward of, the tropics in both hemispheres. The aridity of much of Australia is largely a consequence of the subsiding air associated with this ridge of high pressure.
The sub-tropical ridge consists of areas of high pressure (anticyclones) which pass from west to east across the continent. Individual anticyclones, which can be up to 4,000 km across, can remain near-stationary for several days, bringing clear skies and fine conditions to large parts of the continent, before moving on. The latitude of the sub-tropical ridge varies seasonally. During winter, the ridge is normally centred between latitudes 30° and 35°S, whereas in summer it moves south to between latitudes 35° and 40°S (although individual systems can form significantly further north or south than these characteristic latitudes).
Winds circulate counter-clockwise around anticyclones in the Southern Hemisphere, and hence the flow on the southern side of the sub-tropical ridge tends to be westerly. This zone of westerly flow is generally strongest south of Australia (the so-called 'Roaring Forties'), but the northern part of the zone can affect southern Australia, particularly in winter and spring. Extensive depressions (lows) over the Southern Ocean have associated frontal systems embedded in the westerlies, which bring periods of rain and showers to southern parts of the country. Tasmania is under the influence of westerly flow for much of the year.
Australia's generally low relief (map 1.4) means that topography has less impact on atmospheric systems that control the climate than is the case in other more mountainous continents. This lack of topographic obstruction, and the absence of cool ocean currents off the west coast (as are found at similar latitudes off Africa and the Americas) as a stabilising influence, allows the occasional penetration of tropical moisture deep into the continent. As a result, the Australian desert, while relatively dry, does not match the extreme aridity of deserts such as the Sahara where vast areas have average annual rainfalls below 25 mm (see the article Australia's deserts in Year Book Australia 2006). There are also no barriers to occasional bands of moisture and cloud extending from the warm waters of the Indian Ocean off north-western Australia right across the continent to the southern states. These 'north-west cloud bands', which are most common in late autumn and early winter, can produce good rainfall in their own right, sometimes in significant amounts, but their major influence is to provide an additional in-feed of moisture into frontal systems traversing southern Australia, enhancing the rainfall produced by those systems.
One area where topography does have a major influence on rainfall is in Tasmania. Westerly winds are intercepted by the island's mountains, causing heavy rainfall on the western (windward) side, and leaving eastern and central Tasmania in a much drier so-called 'rain-shadow'. The interaction of topography with westerly winds in winter also plays a role in locally enhancing rainfall in regions such as the Australian Alps and the Adelaide Hills. The Great Dividing Range and associated ranges in eastern Australia enhance rainfall over the east coast hinterland during periods of easterly flow, and partially block moisture from penetrating further inland.
Episodic weather events
Tropical cyclones are the most dramatic episodic weather events to affect Australia. Tropical cyclones are strong, well-organised low pressure systems that form poleward of about 5° of the Equator, over water that is warmer than approximately 26°C. (The weak Coriolis force near the Equator, which is important in inducing the rotation required for the development of a tropical cyclone, accounts for the lack of cyclones in that region.) Tropical cyclones can vary significantly in size, and once formed are classified as category 1 (weakest) to 5 (strongest) according to their intensity at any given time. Category 4 and 5 cyclones have wind gusts exceeding 225 kilometres/hour (km/h) and can be exceptionally damaging, as in the near-total destruction of Darwin by Cyclone Tracy on 25 December 1974. The strongest wind gust instrumentally measured in a tropical cyclone on the Australian mainland is 267 km/h, at Learmonth (Western Australia) during Cyclone Vance on 22 March 1999, but it is believed that gusts in excess of 320 km/h have occurred away from instruments. The zone of most destructive winds associated with tropical cyclones is normally quite narrow, only about 50 km wide in the case of Tracy, and rarely more than 300 km.
Tropical cyclones bring heavy rain as well as strong winds, and are the cause of most of Australia's highest-recorded daily rainfalls (table 1.7). Warm water acts as the cyclone's energy source, and hence is required to maintain the strength of the winds. As a result, tropical cyclones rapidly lose their intensity on moving over land, although the rainfall with former cyclones often persists well after the destructive winds have eased, occasionally bringing heavy rains deep into the inland and causing widespread flooding. (Such flooding can also occur from tropical depressions that never reach sufficient intensity to be classified as cyclones.) Parts of inland Western Australia receive 30-40% of their average annual rainfall from these systems, and it is not unheard of for places to receive their average annual rainfall within a one or two-day period as a tropical cyclone (or ex-cyclone) passes by.
On average, about three tropical cyclones directly approach the Queensland coast during the season between November and May, and three affect the north and north-west coasts, but the number and location of cyclones vary greatly from year to year. The most susceptible areas are north of Carnarvon on the west coast and north of Rockhampton on the east, but on occasions tropical cyclones have reached as far south as Perth and northern New South Wales. The most intense cyclones (categories 4 and 5) are most common off the north-west coast, but can also occur off the northern and eastern coasts. Cyclone Monica (category 5), in April 2006, was the most intense cyclone ever recorded off the Northern Territory coast, while Larry (category 4 at landfall), in March 2006, was the most intense cyclone to make landfall in Queensland since 1918.
'Cold outbreaks' can occur over southern Australia when intense south to south-west flow associated with strong cold fronts or large depressions directs cold air from the Southern Ocean over the continent. These outbreaks are most common in the south-east of the country and can result in low temperatures and snow falling to low elevations. While principally a winter and early spring phenomenon, cold outbreaks can occur at other times of year, and the fact that the air originates over the Southern Ocean (where there is only about a 4°C change in temperature from winter to summer) means that they can also bring cold air and 'unseasonable' snowfalls at high elevations at any other time of year. On Christmas Day 2006 snow fell as low as 400 metres elevation in southern Victoria and Tasmania.
Intense low pressure systems can also form outside the tropics, most commonly off the east coast where they are known as 'east-coast lows'. These systems can bring very strong winds and heavy rain, particularly where they direct moist easterly winds on their southern flank onto the coastal ranges of southern Queensland, New South Wales, eastern Victoria and north-eastern Tasmania. Examples of systems of this type include two, a fortnight apart, in June 1967 off southern Queensland which caused major flooding and severe beach erosion in the Gold Coast region, an intense low in Bass Strait that sank or damaged many yachts in the 1998 Sydney-Hobart race, and a June 2007 system which brought flooding to the Hunter Valley in New South Wales and drove a large ship aground at Newcastle.
The article Natural disasters in Australia examines some significant climatic events that have impacted Australia.
Interannual and interdecadal variability
The major driver of interannual climate variability in Australia, particularly eastern Australia, is the El Niño-Southern Oscillation phenomenon. El Niño is an anomalous large warming of the central and eastern tropical Pacific Ocean, while La Niña, the reverse phase of the system, is an anomalous cooling. The Southern Oscillation refers to a see-sawing of atmospheric pressure between the northern Australian-Indonesian region and the central Pacific Ocean. El Niño events are strongly associated with abnormally high pressures in the northern Australian-Indonesian region and abnormally low pressures over the central Pacific, while the reverse is true during La Niña events.
The Southern Oscillation Index (SOI) is an index of the pressure differences between Darwin and Tahiti and has traditionally been used as an indicator of El Niño events (which are often, but not always, associated with a strongly negative SOI). However, with modern satellite and floating buoy observations developed over the last 30 years, ocean temperature anomalies, both at and below the surface, can be monitored directly and hence proxy measurements, such as the SOI, are less important than they once were.
El Niño events characteristically develop during the southern autumn, and continue for about 9-12 months until the following autumn. The 2002-03 El Niño followed this pattern, developing in May-June 2002 and dissipating in February-March 2003. In contrast, the 2006-07 event developed unusually late in August-September 2006 (although dry conditions were well established in many areas by then), before breaking down in February-March 2007. On occasions El Niño events are followed immediately by La Niña events (or vice versa), but it is more common for them to be followed by near-normal (neutral) ocean conditions. Events lasting more than one year are rare, but not unknown. There are typically two to three El Niño events per decade, but there is large variation from decade to decade in their frequency and the balance of El Niño and La Niña events; since 1980, El Niño events have been predominant, whereas La Niña events were frequent in the 1950's and 1970's.
Temperatures in the tropical Indian Ocean also have an influence on Australia's climate, particularly in the south-west of Western Australia, where the influences of El Niño and La Nina events are more limited. Indian Ocean conditions also have a bearing on winter rainfall in south-eastern Australia through their effects on the frequency of north-west cloud bands (see earlier section).
Many parts of Australia also have a high level of rainfall variability on decadal timescales. The drivers for this are unclear, although at least some of the variability is linked with variations on decadal timescales in the relative frequency of El Niño and La Nina events. Interdecadal variability is particularly high in the more arid areas of Australia. As an example, the 11-year average annual rainfall at Marree (South Australia) has fluctuated from around 100 mm in the 1960s to 250 mm in the 1970s.
The wide range of rainfall variability in Australia 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 1870s. In 1865, a boundary ('Goyder's Line'), based on surveys of native vegetation, had been defined by the Surveyor-General, GW 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 article Climate variability and El Niño in the Geography and climate chapter of Year Book Australia 1998 provides further details.
Temperatures in Australia were relatively stable from 1910 until 1950, and since then have followed an increasing trend, with an overall increase during 1910 to 2006 of approximately 0.7°C. Overnight minimum temperatures have warmed more quickly than daytime maximum temperatures, but both have increased over almost the entire continent, with the largest increases occurring in north-eastern Australia. In conjunction with this trend, the frequencies of frosts and other extreme low temperatures have decreased, while the frequency of extreme high temperatures has increased, although at a slower rate. Over Australia the observed warming has accelerated in recent years, and the late-20th century warming has been largely attributed to the enhanced greenhouse effect.
Over the continent as a whole, rainfall has increased over the 1900-2006 period, with the largest increases occurring over northern and north-western Australia. Since 1960, however, there have been substantial decreases in rainfall over three relatively small, but economically and agriculturally important, regions: south-western Western Australia; Victoria (particularly southern Victoria), and the eastern coastal fringe (particularly south-eastern Queensland).
Table 1.5 shows temperatures and rainfall averaged over Australia since the commencement of comprehensive national records. The article A hundred years of science and service - Australian meteorology through the twentieth century in Year Book Australia 2001 provides further details, including maps of temperature and rainfall trends to 1999.
While some temperature and rainfall data exist prior to the starting dates used in table 1.5, they have not been used in analyses of climate change. This is because large parts of the Australian continent had no observations before that time. In the case of temperatures, most pre-1910 data is also not comparable with post-1910 data, because the louvred, white-painted screen (the 'Stevenson screen') which is used for sheltering thermometers from direct solar radiation was only introduced as a national standard around that time. Many pre-1910 temperatures were measured in locations such as underneath tin verandahs or even indoors, and cannot be validly compared with more recent data (see the article Temperature measurement and the Stevenson screen in Year Book Australia 2005 for further details).
Rainfall and other precipitation
Map 1.6 shows average annual rainfall over the Australian continent.
The driest section of Australia, with an average of less than 200 mm per year, extends over a large area from the west coast near Shark Bay, across the interior of Western Australia and northern South Australia into south-western Queensland and north-western New South Wales. The driest part of this region is in the vicinity of Lake Eyre in South Australia, where average annual rainfall is below 150 mm. This region is not normally exposed to moist air masses and rainfall is irregular, averaging rain on only around 20 days per year.
On rare occasions, favourable synoptic situations (usually, but not always, disturbances of tropical origin) can bring heavy rains to many parts of this normally arid to semi-arid region, with falls of up to 400 mm over a few days being recorded in the most extreme cases. Such heavy rainfalls often lead to widespread flooding and a subsequent short-lived 'blooming' of the desert regions. Whilst such rain events are uncommon, the environment in Australia (both the lack of topographic barriers to moist air moving southwards from the tropics, and the presence of warm, rather than cold, waters as a potential source of moist air off the west coast) is more favourable to their occurrence than it is in some other arid zones. Rainfall in Australia's deserts is consequently higher than in some other deserts; the Atacama Desert on the west coast of South America has locations where no rain has fallen for centuries, whilst large parts of the Sahara and Arabian deserts, and parts of central Asia, have average annual rainfall of 25 mm or lower. There is only one recorded instance, at Mulyie (about 100 km east of Port Hedland, Western Australia) in 1924, of an Australian station being rainless for a complete calendar year.
The region with the highest average annual rainfall is the east coast of Queensland between Cairns and Cardwell, where mountains are very close to the tropical coast. The summit of Bellenden Ker has an average of 8,050 mm over 34 years of records, while at lower elevations, Topaz has an average of 4,438 mm over 27 years, and Babinda 4,246 mm over 96 years. The mountainous region of western Tasmania also has a high annual rainfall, with Lake Margaret having an average of 2,947 mm over 62 years, and short-term records suggest that other parts of the region have an average near 3,500 mm.
The Snowy Mountains area in New South Wales also has a particularly high rainfall. While there are no official rain gauges in the wettest areas on the western slopes above 1,800 metres elevation, runoff data suggest that the average annual rainfall in parts of this region exceeds 3,000 mm. Small pockets with averages exceeding 2,500 mm also occur in the north-east Victorian highlands and some parts of the east coastal slopes.
Australia's rainfall pattern is strongly seasonal in character, with a winter rainfall regime in parts of the south, a summer regime in the north and generally more uniform or erratic throughout the year elsewhere. Major rainfall zones include:
Rain days and extreme rainfalls
The frequency of rain days (defined as days when 0.2 mm or more of rainfall is recorded in a 24-hour period) is greatest near the southern Australian coast, exceeding 150 per year in much of Tasmania, southern Victoria and the far south-west of Western Australia, peaking at over 250 per year in western Tasmania. Values exceeding 150 per year also occur along parts of the north Queensland coast. At the other extreme, a large part of inland western and central Australia has fewer than 25 rain days per year, and most of the continent away from the coast has fewer than 50 per year. In the high rainfall areas of northern Australia away from the east coast the number of rain days is typically about 80 to 120 per year, but rainfall events are typically heavier in this region than in southern Australia.
The highest daily rainfalls have occurred in the northern half of Australia and along the east coast, most of them arising from tropical cyclones, or further south-east coast lows, near the coast in mountainous areas. Daily falls in excess of 500 mm have occurred at scattered locations near the east coast as far south as the Illawarra, south of Sydney, and falls exceeding 300 mm have occurred in north-eastern Tasmania, and the Otway Ranges and parts of Gippsland in southern Victoria. Most locations in temperate Australia away from the east coast have highest recorded daily rainfalls in the 75-150 mm range, although some locations have exceeded 200 mm. In these regions, extreme daily rainfalls are often associated with thunderstorms, for which rainfall recordings can vary dramatically over short distances.
The highest daily and annual rainfalls for each state and territory are listed in tables 1.8 and 1.9.
Heavy rainfall conducive to widespread flooding can occur anywhere in Australia, but is most common in the north and in the eastern coastal areas. There are three main flood types:
Drought, in general terms, refers to an acute deficit of water supply to meet a specified demand. The best single measure of water availability in Australia is rainfall, although factors such as evaporation and soil moisture are also significant and can be dominant in some situations. Demands for water are very diverse, and droughts therefore can be considered on a variety of timescales. Rainfall in a single year is important for unirrigated crop and pasture growth, while for large water storages and irrigation, variations on a multi-year timescale are more important, and a succession of relatively dry years that are not exceptional individually can cause severe water storages when aggregated over an extended period.
While droughts can occur in all parts of Australia, they are most economically damaging in south-eastern Australia (southern Queensland, New South Wales, Victoria, Tasmania and the settled parts of South Australia), an area encompassing about 75% of Australia's population and much of its agriculture. In south-western Western Australia, another economically and agriculturally significant area, interannual variability of rainfall is smaller than it is in the south-east and severe widespread droughts in individual years are a less important issue, although, in recent decades, this area has experienced a general decline in rainfall (see Climate change).
In terms of rainfall deficits over a 1-2 year period, the most severe droughts on record for eastern Australia have been those of 1901-02, 1982-83, 1994-95, 2002-03 and 2006-07, all of which were associated with El Niño. Occasionally, severe droughts are embedded within more extensive dry periods; the 1901-02 drought was contained within a persistently dry period from 1895 to 1903 (the so-called 'Federation Drought'). Droughts can have a severe economic impact. The direct effect of the 2002-03 drought on agricultural production is that it had a downward impact on gross domestic product growth of almost one percentage point between 2001-02 and 2002-03 (see the article in the National accounts chapter in Year Book Australia 2005). Other notable droughts on the 1 to 2-year timescale include those of 1888, 1914, 1919-20, 1940-41, 1944, 1946, 1965, 1967 and 1972.
Typically, these multi-year dry episodes are not ones of continuous below-normal rainfall, but rather periods of near-normal rainfall over several months, alternating with drier periods, and few, if any, times of sustained above-normal rainfall to offset the dry periods. Large water storages are particularly susceptible to such events, as they typically rely on a relatively small number of wet years to offset losses during drier periods. The Sydney water supply catchments provide an example of this, with about 40% of the total inflows into the Warragamba catchment since 1910 occurring in the wettest 10% of years.
The period since 2001 has been the driest on record over parts of eastern Australia, meaning that many large water storages did not fully recover from the 2002-03 drought prior to the onset of the 2006-07 drought. While rainfall returned to near-normal levels in the second half of 2003 following the severe drought of 2002-03, there have been no periods of sustained above-average rainfall in most of the region since early-2001. For eastern Australia as a whole (defined as the combined areas of Queensland, New South Wales, Victoria and Tasmania), the four-year period from June 2001 to May 2005 was the driest June to May four-year period on record, whilst the six-year period from June 2001 to May 2007 ranks second behind 1900-06. For Australia's cropping regions only the period 1911 to 1915 was drier. Conditions in the period 2001 to 2007 are comparable to those of the lengthy drought of the 1940s, although (to date) they have not persisted for as long.
Adding to the impact of recent dry conditions has been the accompanying increase in temperature. The period from July 2001 to June 2007 was clearly the warmest such period on record for eastern Australia. Maximum temperatures averaged over Australia were 1.00°C above the 1961-90 normal. In contrast, temperatures averaged through the driest periods of the 1940s were near the 1961-90 normal (see the article Averaging periods in climate in Year Book Australia 2007).
Drought definitions, and the area of coverage and length of droughts to that time, together with related information, may be obtained from the article Drought in Australia in Year Book Australia 1988.Thunderstorms, hail and tornadoes
Thunderstorms are most frequent over northern Australia. Thunder is heard at least once on 80 days or more per year near Darwin, largely as a result of convectional processes during the summer wet season. High frequencies (30-50 per year) also occur over the eastern uplands of New South Wales as a result of orographic uplift of moist air streams. Some parts of southern Australia receive fewer than ten thunderstorms per year, with eastern Tasmania receiving fewer than five. Through most of Australia thunderstorms are more common during the warmer half of the year, but along the southern fringe they also occur in winter as a result of low-level instability in cold air masses of Southern Ocean origin.
Thunderstorms are also relatively common over many parts of inland Australia, with most of the arid zone having at least 15 thunder days per year, and parts of interior Western Australia having 40 or more. These storms are often 'dry' with most or all rain evaporating before it reaches the ground - indeed, in a few locations there are more days of thunder per year than there are days of rain.
Some thunderstorms can become severe, with flash flooding, large hail and damaging winds. These storms can be very destructive. The Sydney hailstorm of 14 April 1999, in which hailstones up to nine centimetres (cm) in diameter were observed, was Australia's most costly natural disaster, with losses estimated at $1.7 billion. Flash flooding associated with severe thunderstorms has caused loss of life, notably when seven deaths occurred in Canberra on 26 January 1971, and thunderstorms have also been implicated in numerous air crashes, such as when a plane crashed into Botany Bay on 30 November 1961 with the loss of 15 lives. Wind gusts exceeding 170 km/h have been measured during severe thunderstorms, with one notable reading being 185 km/h at Brisbane Airport on 18 January 1985.
While thunderstorms in general are most common in northern Australia, the most damaging thunderstorms, in terms of hail and wind gusts, occur in the eastern halves of New South Wales and southern Queensland. Smaller hail (less than one centimetre in diameter) commonly occurs in southern coastal Australia in cold unstable air in the wake of cold frontal passages.
Tornadoes are also associated with severe thunderstorms, although they do not occur with the same frequency or severity as can occur in the United States of America. As tornado paths are narrow it is rare, but not unknown, for them to strike major population centres, with notable examples occurring in Brighton (Melbourne) in February 1918, the southern suburbs of Brisbane in November 1973, and several Perth suburbs in May 2005.
During most years, snow covers much of the Australian Alps above 1,500 metres for varying periods from late autumn to early spring. Similarly, in Tasmania, the mountains are covered fairly frequently above 1,000 metres in those seasons. The area, depth and duration of snow cover are highly variable from year to year. These areas can experience light snowfalls at any time of year. Small patches of snow can occasionally persist through summer in sheltered areas near the highest peaks, but there are no permanent snowfields.
Snowfalls at lower elevations are more irregular, although areas above 600 metres in Victoria and Tasmania, and above 1,000 metres in the New South Wales highlands, receive snow at least once in most winters, as do the highest peaks of Western Australia's Stirling Ranges. In most cases snow cover is light and short-lived. In extreme cases, snow has fallen to sea level in Tasmania and parts of Victoria, and to 200 metres in other parts of southern Australia, but this is extremely rare. The only major Australian cities to have received a significant snow cover at any time in the last century are Canberra and Hobart, although Melbourne experienced a heavy snowfall in 1849, and there are anecdotal reports of snowflakes in Sydney in 1836.
The heaviest snowfall in Australian history outside the alpine areas was that of 4-5 July 1900, when 50-100 cm fell around Bathurst and in the Blue Mountains, and 25 cm as far west as Forbes (only 240 metres above sea level). Other major widespread low-elevation snow events occurred in July 1901, July 1949 and July 1984. In August 2005, the heaviest low-level snowfalls since 1951 occurred in parts of southern Victoria, with snow falling to sea level in parts of south Gippsland and accumulations of 5-20 cm at elevations above 150 metres in the Strzelecki Ranges and Latrobe Valley.
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