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
Climate Variability and Change
This section reports on the environmental indicators listed below and are defined in Manton and Jasper (1998).
| Environmental Indicator | |
|---|---|
| A 1.1 | Southern Oscillation Index |
| A 1.2 | Average sea surface temperatures |
| A 1.3 | Average sea level |
| A 1.4 | Average daily rainfall |
| A 1.5 | a | b | | Average maximum and minimum temperatures |
| A 1.6 | Occurrence of rainfall extremes |
| A 1.7 | Occurrence of temperature extremes |
| A 1.8 | Occurrence of tropical cyclones |
| A 1.9 | Temperature of the free atmosphere |
| A 1.10 | Greenhouse gas atmospheric concentrations |
| A 1.11 | Annual total greenhouse gas emissions |
| A 1.12 | Greenhouse gas emissions by sector of the economy |
| A 1.13 | Insurance losses due to extreme weather events |
| A 1.14 | Cost of drought to Government |
| A 1.15 | National crop yield |
| A 1.16 | Government expenditure on climate related activities |
| A 1.17 | Rainfall intensity |
| A 1.18 | Average evaporation rate |
| A 1.19 | Average aerosol loading |
| A 1.20 | Development of carbon sinks |
| A 1.21 | National involvement in international processes on climate variability and change |
Introduction
Australia is a large, flat continent reaching from the tropics to mid-latitudes, with an arid interior and highly variable rainfall. Our continent has little effect on the global circulation system because of its low terrain, compared with the topography of Asia and North America. The climate of Australia is strongly influenced by the surrounding oceans. Weather systems such as tropical cyclones and cold fronts give each region of the country its own characteristic meteorology. Dominant weather systems that affect Australia include the monsoon, north-west cloud band, east coast low, mid-latitude westerlies, southerly buster and the subtropical ridge. The influence of these systems results in a highly variable pattern of rainfall, across the country and from season to season and year to year. Most of the variations to rainfall are linked to large-scale phenomena such as the tropical intra-seasonal oscillation, the ENSO phenomenon and mid-latitude and high-latitude weather systems.
This section gives both historical data and the latest available information about the indicators proposed by Manton and Jasper (1998). The structure of this section does not necessarily follow the order of the indicators, nor the pressure-condition-response implication model proposed by Manton and Jasper (1998). Rather, the section is organised to provide a more sequential structure for the document.
A brief description of the factors that control the climate of Australia is followed by a description of the spatial and temporal aspects of rainfall and temperature and their extremes and trends in the free atmosphere over the continent.
There is a short description of evaporation and aerosol loadings at some stations in Australia. The variability in ENSO, and its links to sea surface temperature, floods and droughts, and sea level is described, followed by trends in tropical cyclones and effects related to extreme weather events, such as cyclones, floods, storms, hailstorms, bushfires and droughts.
Information about long-term changes, greenhouse gases and national and international involvement in climate activities is included.
Monsoon
During summer, the Australian monsoon brings humid and rainy conditions to northern Australia. On average, the arrival ('onset') of the monsoon occurs between late December and early January. Strong westerly winds and heavy rainfall are associated with the onset. The monsoon trough, a low pressure area between the equatorial north-west winds and the tropical south-east trades, tends to dominate the weather during this season (Figure 11).
Figure 11: Typical winds and synoptic features in January.
Source: Atlas of Australian Resources, Vol. 4., Climate, Division of National Mapping, Canberra 1986
A fully developed monsoon circulation system is comprised of at least three distinct weather patterns: the active, moderate and break phases of the monsoon. Their occurrence is associated with variations of the entire monsoon system within seasons and from year to year. The active phase is more energetic than the moderate phase, with stronger westerlies, intense tropical monsoon trough and enhanced rainfall.
Tropical cyclones often form during the active phase. The break or dormant phase is associated with weak westerlies and no or little rainfall, while the moderate phase has average monsoon conditions. The monsoon rainfall is modulated by the tropical intraseasonal oscillation, which is linked to the ENSO phenomenon. Monsoon depressions and tropical cyclones are responsible for large amounts of rainfall during summer, particularly in northern Australia. The formation of tropical cyclones in the Australian region is highly seasonal and is largely confined to the summer months, although tropical cyclones do form in late spring and early autumn.
North-west cloud band
A weather system often associated with rainfall over southern Australia during winter is the north-west cloud band. When it occurs, it stretches from north-west Australia to the east coast. It derives a large amount of moisture from the tropical east Indian Ocean and feeds moisture towards south-east Australia, causing rainfall in the regions where the band is located. There is a link between north-west cloud bands and sea surface temperatures in the Indian Ocean. Occasionally, north-west cloud bands lead to the formation of cut-off lows over eastern Australia, which can also cause substantial amounts of rain. A cut-off low is a low pressure centre that becomes isolated from a more extensive region of low pressure generally located further south.
East-coast lows and mid-latitude systems
One of the important weather systems that affects eastern New South Wales, eastern Victoria and Tasmania is the east-coast low. East-coast lows are intense low pressure systems that usually develop over the Tasman Sea, east of New South Wales. They can bring torrential rain and strong winds to parts of the south-east coast of Australia. There is a strong seasonal variability in the occurrence of these systems, being most frequent in autumn and early winter.
Low pressure systems or frontal systems in the Southern Ocean also affect southern Australia. The path of the eastward travelling low pressure systems moves northwards during winter. As a result, they bring cold winds, rain and sometimes snow to southern States. In summer, the path of these systems is typically found far south of the continent, although cold fronts can still affect the southern parts of Australia.
Southerly buster
The southerly buster is a type of violent cold front that affects the New South Wales coast. The southerly buster moves rapidly up the coast and is sometimes associated with thunderstorms. Southerly busters bring a sudden 'cool change' to this area and occur predominantly during summer.
Subtropical ridge
The subtropical ridge is a broad belt of high pressure between 20 and 35S that separates the trade winds of the tropics from the westerlies of the mid-latitudes (Figure 12). The subtropical ridge is characterised by light winds, fine and sunny weather and an absence of storms. It moves southwards as summer approaches, bringing fine weather and sunshine to southern Australia, although cold fronts still occasionally affect the southern coasts.
Figure 12: Typical winds and synoptic features in July.
Source: Atlas of Australian Resources , Vol. 4., Climate, Division of National Mapping, Canberra 1986
Following the southwards movement of the subtropical ridge in summer, tropical air sometimes brings humid conditions to northern Victoria and New South Wales, accompanied by thunderstorms.