Climate
The island continent of Australia features a wide range of climatic zones, from the tropical regions of the north, through the arid expanses of the interior, to the temperate regions of the south. Australia is the world's second-driest continent (after Antarctica), with mean annual rainfall below 600 millimetres (mm) per year over 80% of the continent, and below 300 mm over 50%. Summers are hot through most of the country, with average January maximum temperatures exceeding 30 degrees Celsius over most of the mainland except for the southern coastal fringe between Perth and Brisbane, and areas at high elevations. Winters are warm in the north and cooler in the south, with overnight frosts common in inland areas south of the Tropic of Capricorn; only at higher elevations do wintertime temperatures approach those found in much of northern Europe or North America.
Seasonal fluctuations in both rainfall and temperature can be large in parts of the country. In northern Australia, temperatures are warm throughout the year, with a wet season from approximately November through April, when almost all the rainfall occurs, and a dry season from May through October. Further south, temperature becomes more important in defining seasonal differences and rainfall is more evenly distributed through the year, reaching a marked winter peak in the south-west and along parts of the southern fringe.
Australia experiences many of nature's more extreme phenomena, including droughts, floods, tropical cyclones, severe storms, bushfires, and the occasional tornado.
Climatic controls
The dominant influence on Australia's climate is its latitude, with the mainland lying between 10�� south and 39�� south and Tasmania extending south to 44�� south. 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 the high pressure ridge.
The sub-tropical ridge takes the form 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 before moving on. The latitude of the sub-tropical ridge varies seasonally. During winter, the ridge is normally centred between latitudes 30�� and 35�� south, whereas in summer it moves south to between latitudes 35�� and 40�� south (although individual systems can form significantly further north or south than these characteristic latitudes).
As winds circulate counter-clockwise around anticyclones in the Southern Hemisphere, the flow on the southern side of the sub-tropical ridge tends to be westerly. This zone of westerly flow is 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.
North of the sub-tropical ridge the flow is generally easterly. In winter easterly to south-easterly flow is especially persistent over the northern half of the continent, bringing dry conditions everywhere, except along the east coast. In summer, the intertropical convergence zone moves southwards over northern Australia (the exact timing and location vary from year to year), allowing warm, moist monsoonal air from the north-west to penetrate into the northern reaches of the continent. Moist easterly flow from the Pacific Ocean and Tasman Sea brings summer rain to most of the east coast.
Australia's generally low relief means that topography causes less obstruction to 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 (as are found at similar latitudes off Africa and the Americas) off the west coast as a stabilising influence, allows the occasional penetration of tropical moisture deep into the continent, with the result that the Australian desert, while relatively dry, does not match the extreme aridity of deserts such as the Sahara where vast areas have mean annual rainfalls below 25 mm. 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 across the continent to the southern part of the continent (northwest cloud bands). These cloud bands can produce 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.
Topography does have a major influence on rainfall in Tasmania, where 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. Tropical cyclones can vary significantly in size, and once formed are classified as category 1 to 5 according to their intensity at any given time. Category 4 and 5 cyclones have wind gusts exceeding 225 kilometres per hour (km/h) and can be exceptionally damaging, as in the near-total destruction of Darwin by Tropical 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 Tropical 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. Warm water is required to maintain the strength of the winds and 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, bringing occasional 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 mean annual rainfall from these systems, and it is not unheard of for places to receive their normal annual rainfall within a one or two-day period.
On average, about three 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 Rockhampton on the east, but on occasions tropical cyclones have reached as far south as Perth and northern New South Wales.
Away from the tropics, 'heatwaves' can occur over many parts of Australia. In southern Australia, they are normally associated with slow-moving anticyclones. A large anti-cyclone remaining stationary ('blocking') in the Tasman Sea will result in northerly or north-westerly flow on its western flank, bringing hot air of continental origin over the south-east coastal regions (and sometimes to Tasmania). In south-western Australia heatwaves are more commonly associated with the characteristic north-south trough of low pressure along the west coast in summer moving offshore, suppressing sea breezes and causing hot north-easterly winds to blow from the interior to the coast.
'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 of Southern Ocean origin over the continent. These are most common in the south-east and can result in low temperatures and snow falling to quite 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.
Intense low pressure systems can 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 one in June 1967 off southern Queensland which caused major flooding and severe beach erosion in the Gold Coast region, and an intense low in Bass Strait that sank or damaged many yachts in the 1998 Sydney-Hobart race.
Interannual variability
The major driver of interannual climate variability in Australia, particularly eastern Australia, is the El Ni��o-Southern Oscillation phenomenon (ENSO). El Ni��o is an anomalous large warming of the central and eastern tropical Pacific Ocean. (La Ni��a, the reverse phase of the system, is an anomalous cooling.) The Southern Oscillation is a see-sawing of atmospheric pressure between the northern Australian-Indonesian region and the central Pacific Ocean. El Ni��o events are strongly associated with abnormally high pressures in the northern Australian-Indonesian region and abnormally low pressures over the central Pacific (the reverse is true during La Ni��a events).
The Southern Oscillation Index (SOI) is an index of pressure differences between Darwin and Tahiti and has traditionally been used as an indicator of El Ni��o events (which are very often, but not always, associated with a strongly negative SOI), although improved observation systems developed over the last 30 years now allow ocean temperature anomalies, both at and below the surface, to be monitored directly.
El Ni��o events characteristically develop during the southern autumn, and continue for about 9-12 months until the following autumn. The most recent El Nino followed this pattern, developing in May-June 2002 and dissipating in February-March 2003. On occasions El Ni��o events are followed immediately by La Ni��a events (or vice versa), but it is more common for them to be followed by near-normal (neutral) ocean conditions. Events lasting for more than one year are rare, but not unknown. There are typically 2-3 El Ni��o events per decade, but there is large variation from decade to decade in their frequency and the balance of El Ni��o and La Ni��a events; since 1980 El Ni��o events have been predominant, whereas La Ni��a events were frequent in the 1950s and 1970s.
El Ni��o events are generally associated with a reduction in rainfall across much of Australia, which can lead to widespread and severe drought in eastern Australia, particularly in winter and spring, as well as increased daytime temperatures and bushfire risk. Conversely, La Ni��a events are generally associated with wetter-than-normal conditions and have contributed to many of Australia's most notable floods. There is considerable variation, however, in the way each El Ni��o and La Ni��a event affects rainfall patterns from the time of onset through its developmental stages to eventual decay.
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 Ni��o 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 northwest cloud bands.
Climate change
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 2003 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 the continent as a whole, rainfall has increased over the 1900-2003 period, with the largest increases occurring over northern and north-western Australia. However, since 1960, 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).
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 only around 20 days per year. Very occasionally, 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.
The region with the highest mean 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 8,068 mm over 31 years of records, while at lower elevations, Topaz has an average of 4,401 mm over 24 years, and Babinda 4,236 mm over 92 years. The mountainous region of western Tasmania also has a high annual rainfall, with Lake Margaret having an average of 2,956 mm over 59 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 suggests 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.
Seasonal
The rainfall pattern of Australia 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:
�Ģ The marked wet summer and dry winter of northern and north-western Australia. In this region winters are almost completely dry, except near exposed eastern coastlines.
�Ģ The wet summer and relatively (but not completely) dry winter of south-eastern Queensland and north-eastern New South Wales.
�Ģ Fairly uniform rainfall in south-eastern Australia, including most of New South Wales, parts of Victoria and eastern Tasmania. The exact seasonal distribution can be influenced by local topography; for example, winter is the wettest season at Albury on the windward side of the Snowy Mountains, but the driest season at Cooma on the leeward side (e.g. Sydney, Melbourne, Canberra and Hobart).
�Ģ A marked wet winter and dry summer (sometimes called a 'Mediterranean' climate). This climate is most prominent in south-western Western Australia and southern South Australia, but there is also a winter rainfall maximum in some other parts of the south-east, particularly those areas exposed to westerly or south-westerly winds, such as western Tasmania and south-western Victoria (e.g. Adelaide and Perth).
�Ģ Low and erratic rainfall through much of the western and central inland. Rainfall events are irregular and can occur in most seasons, but are most common in summer (e.g. Alice Springs).
