Atmosphere Theme Report

Australia State of the Environment Report 2001 (Theme Report)
Lead Author: Dr Peter Manins, Environmental Consulting and Research Unit, CSIRO Atmospheric Research, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06746 9

Introduction (continued)

Australia's climate

Australia's highly variable climate needs to be taken into account when interpreting environmental trends. In assessing air quality, for example, observations from one or two years may be insufficient to assess trends, since climatic patterns such as the presence or absence of an El Nio event can have a marked effect, and patterns of change longer than decades are also evident.

Australia's location and size mean that it is subject to a wide range of climatic influences (Figure 2). These range from tropical climates in its northern third to temperate ones in Tasmania and the southern parts of the mainland (with a small alpine region occurring in the south-east of the continent and in central Tasmania) to Mediterranean in the south-west and south-central areas. More than 75% of the continent is classified as arid or semi-arid (see Australia's Antarctic territories).

Figure 2: Map showing Australia's climate zones.

 Map showing Australia's climate zones

Source: BoM

The surrounding oceans have an important effect on the Australian climate, as does topography in certain regions such as the south-east.

Most of Australia comes under the influence of a subtropical ridge of high atmospheric pressure. The air above the ridge is in the descending branch of a large 'cell' of air circulation that links the tropics and the middle latitudes (Figure 3). The air movement of the cell is driven by the temperature contrast between the warm tropical ocean to the north of Australia and the cold of the Antarctic regions. The other global-scale atmospheric feature exerting an influence is the east-west air circulation (the Walker Circulation), with air ascending over the warm western Pacific and descending over the colder waters off the west coast of South America.

Figure 3: Large-scale atmospheric circulation affecting our climate.

 Large-scale atmospheric circulation affecting our climate

Source: BoM (2003c)

Eastward moving high pressure cells dominate much of our weather and climate. During winter they shift northwards and are centred over the continent.

In the warm half of the year (Nov.-April), the highs become centred well to the south of the landmass. Easterly winds predominate and most of southern Australia experiences fine, warm, often heat-wave, conditions. During this time, northern Australia falls under the influence of monsoon lows associated with southward movement of warm, moist tropical air.

In the cooler part of the year (May-Oct.), the high pressure systems pass slowly across the continent, often remaining stationary for several days. Southern areas experience cooler, moist westerly flows, while northern Australia is influenced by mild, dry south-east trade winds. Frontal systems in the westerlies can cause intense rain and abrupt temperature changes, and snowfalls in the southern higher areas.

Australia receives less rain than any other continent except Antarctica. No continent has less runoff from its rivers than Australia. We have a highly irregular rainfall, linked to the ENSO phenomenon, a high evaporation rate and large temperature ranges.

About 80% of the continent has an annual average rainfall of less than 600 mm. Low rainfall combined with very high evaporation (particularly in inland Australia) leads to low surface water flows and seasonal river systems.

Australia experiences major droughts interspersed with wet periods. The frequency of heatwaves and bushfires, and the number of cyclones and their tracks are also associated with the Southern Oscillation.

Tropical cyclones develop over the seas to the north of the continent between November and April. The number of cyclones and their tracks vary greatly from year to year. About six tropical cyclones each season affect northern coastal areas, often producing much rain and strong winds. Some cause widespread heavy rainfall inland.

Variability of Australia's climate: It never rains but it pours.

On Sunday 7 June 1998, farmers throughout Gippsland in eastern Victoria awoke to an occurrence that they had not seen for months: rain.

'It's a great joy to us,' declared a sheep farmer, who had not had decent rainfall on his property for 18 months. One of the worst droughts on record had forced the farmer to sell 30% of his flock, as he could not obtain sufficient food for his animals.

The Commonwealth government had declared Drought Exceptional Circumstances in East Gippsland, and had announced an estimated $10 million in support to farmers in the region.

By 25 June, the farmer's delight that the drought was over had turned into despair. Up to 380 mm of rain in 48 hours had left the region in the grips of the worst floods for almost a decade. Hundreds of homes and farms were evacuated, the roads and highways were closed and power supplies cut.

The Victorian government declared a state of emergency for flood-affected areas. Many farmers now found themselves having to apply for drought and flood relief.

Australia is truly a land of droughts and flooding rains.

Australia's Antarctica territories

Figure 4: Map of Antarctica and bases

 Map of Antarctica and bases

Source: Australian Antarctic Division (http://www.aad.gov.au/stations/)

Australia claims 42% of Antarctica as sovereign territory. Macquarie Island (part of Tasmania), as well as Heard and McDonald Islands are Australian territories as shown in Figure 4.

Macquarie Island is located 1500 km south-east of Tasmania and 1300 km north of the Antarctic continent. It is 34 km long and 5 km at its widest point.

Heard and McDonald Islands are located 4100 km south-west of Perth, 1700 km from the Antarctic continent.

Antarctica has an area of 13 million square kilometres, nearly twice the size of Australia. The Antarctic continent is buried under an ice cap that is up to 4000 m thick. Antarctica presents one of the most hostile environments known, for the continent has the lowest humidities, coldest temperatures, strongest winds and is the largest desert on earth.

The further from the coast the lower surface temperatures become. In most coastal regions, the mean annual temperature is around -12C. At the highest locations, it falls to -60C. High on the polar plateau of Greater Antarctica, the temperature in summer rises to around -30C before the cold of winter returns with extremes of about -80C. The world's lowest surface temperature yet recorded was -89.6C at Vostok station in July 1983.

Low pressure systems, which bring adverse weather to the coastal regions of Antarctica, generally arise over the Southern Ocean and then move south-east towards the coast. These low pressure systems together with coastal katabatic winds can add their influence to strong winds originating at the heart of the continent to produce some of the strongest winds on the planet.