Climate of Australia

Australia: The Land Where Time Began

Climate Climate cycles Climate in Aboriginal Australia RealClimate: Climate Science from Climate Scientists

Australia is the driest continent on Earth (if Antarctica is excluded because its millions of tonnes of water are in the form of ice or snow). Its climate is very erratic, often moving from one extreme directly to the other. It can have years of drought that is broken by devastating floods. This erratic climate has been found to be influenced very strongly by the El Nino-Southern Oscillation (ENSO) atmosphere-ocean system and Indian Ocean Dipole (IOD).

It has been said that the only thing predictable about the climate of the arid areas of Australia, 3/4 of the continent, is that it is unpredictable. In such a place the very flatness of the majority of the continent turns out to be of importance to the survival of many plants and animals in times of long droughts, which occur often and irregularly. Because of this overall flatness, any high points, from rock outcrops to desert mountain ranges, tend to collect and store more water for longer periods than the surrounding flat land, where it tends to run off, infiltrate to the deep water table or evaporate within a short time of the end of the rainfall. From rock outcrops to gorges in mountain ranges, the water collected is at least partially protected from the worst of the conditions of the surrounding flat land where any surface water rapidly evaporates.

The plants and animals of Australia have been very strongly influenced by the climate. Both have adapted to survive, as a species if not always as individuals, in very harsh environments in many parts of the continent. The plants of a particular area may have to cope with poor soils, often low and unpredictable water availability, variable climate including droughts of variable and unpredictable length, and occasional floods, and a wide range of temperatures.

The geological feature that influences the climate of Australia most strongly is the absence of sufficiently high mountain ranges west of the Great Dividing Range running north-south near the east cost of the continent. It is not high as mountain ranges go, but is high enough to force winds crossing it to rise high enough to lose some of their moisture as rain, or in the southern states in winter, snow.

Climate change as Australia broke from Antarctica

Prior to the separation of South America and Australia from Antarctica, cold currents flowing along the Antarctic coast were diverted north to the tropics when they struck the west coasts of South America and Australia, returning south to Antarctica after they had been heated by their passage through the equatorial regions, taking that heat south to warm Antarctica. These warming currents were disrupted by the opening of the ocean between Antarctica and South America and Australia, allowing the polar regions to become a progressively colder closed climatic system. The southern parts of Australia became cooler, and the latitudinal temperature gradients steepened, and the climatic zones became more pronounced. The movement of Australia north gradually moved the central and northern parts of the continent away from the moist westerly winds, and into the region of the drier, warmer subtropical high pressure systems (Bowler, 1982; Bowman, 2000).

The zone is dominated by the belt of high pressure around the Earth, composed of series of high pressure systems that move from west to east near the latitude of 30^o S that is about 3000 km wide. In summer these high pressure systems cover the southern parts of the continent and by winter they have moved north to the central regions. The area they cover at any particular time experiences mostly clear skies, the descending air being dry. The increasing aridity of the Australian continent as it moved north is a result of this dry air.

A band or westerly winds is located immediately to the south if the high pressure zone. Fronts and depressions in this band of westerly winds are areas where the air pressures are locally lower, the air in the lower atmosphere converging and ascending, any contained water vapour then condenses as the rising air mass cools, which occurs as it rises, until the water coalesces into rain drops which fall as rain when they reach a sufficient mass to overcome the updraft tending to push them higher. In the winter rain parts of southern Australia it is these mid-latitude systems that bring the rain, usually moving from west to east.

The southeast trade winds occur immediately to the north of the high pressure belt. These winds converge with the northeast trades of the Northern Hemisphere to form the inter-tropical convergence zone (ITCZ), a belt where the rising warm air containing large amounts of moisture lead to the heavy rain of the tropics. The ITCZ moves from north to south of the equator in the Australian summer and back again in the Australian winter, as it tracks the movements of the sun in relation to the Earth, being over the latitudes of northern Australia in the Southern Hemisphere summer and over the areas to the north of the Equator in the Australian winter. During summer there is a low pressure trough that remains continuously over northern Australia, the monsoon season that is hot and wet. The southeast trade winds can bring rain to the east coast of the continent at any time of the year, the moist air rising to cross the Great Dividing Range, which runs the fuul length of the continent, from Cape York to Tasmania, the moisture being condensed into rain as the air rises. The amount of rain brought by these winds is the result of the water temperature along the east coast of the continent, the warmer the water the higher the evaporation rate and the warmer the air the more water vapour it can hold, hence the potential problems when the oceans are warming.

Both the tropical and mid-latitudes are subject to substantial variability, many rainy weather types being recognised. In the tropical north cyclones often bring heavy rain to the northern coast, in the south, in addition to the normal frontal systems, low pressure systems that originate in the mid-latitudes can be cut-off from the westerlies, often moving slowly while dumping large amounts of rain. The presence of hills or range in an area influences the rain it receives from rain-bearing winds, depending on the direction of flow of the winds in relation to the high ground. In southeastern Australia the western slopes of high ground receive much of their rain from band sof cloud ahead of northwesterly fronts.

The moist southwesterly streams following fronts bring rain to western Tasmania, southern Victoria and the far southwest of Western Australia. Low pressure systems, that can originate in either tropical or mid-latitude regions, situated off the east coast can bring heavy rain to the east coast of New South Wales via the associated onshore easterlies.

In the arid interior of the continent rainfall is usually connected with strong tropical systems that penetrate southward, or the passage of strong fronts, and in winter by 'cut-off' lows.

Variability of the climate

The Byrd Ice Core, the first ice core to be drilled to bedrock in Antarctica, through 2164 m of ice, 99 % of the core being recovered, was drilled through the ice at Byrd Station, Antarctica . It contained a record of atmospheric concentrations of methane that proved a picture of the fluctuations of global climate through much of the last glacial cycle (Blunier & Brook, 2001). The high concentrations are believed to have been from warm, wet periods, when methane is believed to have been produced in tropical wetlands that would be expanding at these times. The evidence from this core confirms the results from other lines of evidence that during the last glacial cycle there were many short-term fluctuations on scales of about 1,000 years. These fluctuations were most dramatic in the middle part of the last cycle, settling down as the LGM (last glacial maximum) approached, so that the climate was very stable between about 28,000 to 20,000 years ago. Sediment cores from the lake bed that formed on the Carpentaria Plain at times of low sea level also show a climatically calm period at the LGM, the fine structure of the sediments indicating that there were few intense storms to disturb them as they were being deposited (De Dekker, 2001). Reconstructions of sea surface temperatures of the ocean around Australia indicate there was little variation of temperature between seasons, seasonal variability being less than at present (Barrows & Juggins, 2005).

The Climate Now

The Australian climate is influenced by several main weather systems related to regular patterns in the oceans and the atmosphere.

Rainfall is brought to northern Australia by the north-west monsoon, and in most years, by cyclones that deliver large amounts of water to mostly coastal areas. Along the east coast rain increases when La Nina is active, when the trade winds in the Pacific ocean push warm water towards the Australian coast, and decreases when El Nino events slow or stop the trade winds, sending the warm water east away from the Australian coast. This is why El Nino brings drought to eastern Australia. It has been assumed that the La Nina events would bring drought-breaking rain to the southern parts of Australia as well. But it has now been realised that this hasn't been happening. While the coasts of New South Wales and Queensland were being deluged by rain brought by La Nina, large areas of Victoria were still in drought.

Queensland weather forecasters have been watching what they believed to be the return of El Nino, but an unexpected finding recently has been that unlike previous El Nino events in which the warm water moves to the east towards South America, it appears that the accumulating warm water stretches across the equatorial Pacific, a confusing situation for forecasters, being unable to predict with confidence what the weather is likely to be, wet or dry.

Indian Ocean Dipole - IOD

Researchers are finding that the trade winds in the Pacific seem to be weakening. If they do weaken further that would lead to a lowering of pressure in the western Pacific, so less power to force the warm water that feeds the Leeuwin Current. They are also seeing what they believe is a change in the IOD that could be leading to a state in which the cool positive phase could be the dominant condition. As if that wasn't enough, the temperatures over Australia have risen by 1^o C, drying the continent out even more.

Whether part of a natural cycle or man-made, climate change is beginning to bite in Australia, and the continent is set to be affected more significantly and earlier than other continents. Yet another first for Australia.

Climate Cycles

Tropical Cyclones

Looking at the past to see the future

Climate scientists have been studying ocean sediments searching for C¹³/C¹² isotope ratio anomalies which indicate times when increased amounts of C¹²-rich carbon is added to the oceans/atmosphere. The correlation of such an anomaly with the Late Palaeocene Thermal Maximum about 55 Ma has been found. It is known that there are many places on the continental shelves around the world where there are large deposits of methane clathrate that has the potential to cause catastrophic climate change if released in sufficient quantities over a sufficiently short time.

The Southern Annular Mode (SAM)

This is a weather system based in Antarctica that has been found to affect the climate of southern Australia, by controlling the strength of the westerly winds that cover the ocean to the south of the Southern Hemisphere continents, including Australia. The SAM determines how far north the westerly winds reach, the further north, the greater the winter rainfall over southern parts of Australia.

The belt of strong westerly winds contracts towards Antarctica in the positive part of the cycle, expanding north, hopefully over southern Australia, in the negative mode. In the positive mode there is reduced autumn and winter rainfall over southern Australia, especially the southern part of Western Australia. The resulting higher pressures over the southern parts of Australia lead to fewer storm systems reaching Australia.

This weather pattern has been found to be conspiring with the IOD to bring the long severe drought to southern Western Australia and western Victoria.

Sources & Further reading

Mary E. White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006
Peter Whetton in Webb, Eric K, (1997), Windows on Meteorology, Australian Perspective, CSIRO Publishing.

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