CLIMATE VARIABILITY AND EL NIÑO
Generations of Australian school-children learned a poet's phrase which described their country as a land of droughts and flooding rains. Today's scientists talk in terms of the continents large climate variability from season to season, and from year to year.
The impact of climate variability on Australia was highlighted by events during the early 1990s. In 1990-91 the wet season produced abundant rains, yet it failed almost completely the following year as drought set in across Queensland and New South Wales. While drought continued in some areas through 1992 and 1993, many people in southeast Australia will long remember the floods of spring 1992 and spring 1993, and the cool summers which followed. What causes these fluctuations? They are connected with the climate phenomenon called the Southern Oscillation, a major air pressure shift between the Asian and east Pacific regions whose best-known extremes are El Niño events. The Southern Oscillation (strength and direction) is measured by a simple index, the SOI, defined on the next page. Rural productivity, especially in Queensland and New South Wales, is linked to the behaviour of the Southern Oscillation. The graph opposite shows how Australia's wheat yield (trend over time removed) has fluctuated with variations in the Southern Oscillation. Negative phases in the oscillation (drier periods) tend to have been linked with reduced wheat crops, and vice versa. Tourism is another industry vulnerable to large swings in seasonal climate. Because climate variability can affect the Australian economy, Australians need the best possible understanding of the physical mechanisms controlling this dramatic feature of their climate.
Why 'El Niño'
El Niño's Global Effects
The Pacific Ocean's Circulation Features
The Walker Circulation
"I cannot help believing that we shall gradually find out the physical mechanism by which these (relationships) are maintained..."
The easterly trade winds are part of the low-level component of the Walker circulation. Typically, the trades bring warm moist air towards the Indonesian region. Here, moving over normally very warm seas, moist air rises to high levels of the atmosphere. The air then travels eastward before sinking over the eastern Pacific Ocean. The rising air is associated with a region of low air pressure, towering cumulonimbus clouds and rain. High pressure and dry conditions accompany the sinking air. The wide variations in patterns and strength of the Walker circulation from year to year are shown in the diagrams opposite.
The Southern Oscillation
This definition remains valid. We now say that the Southern Oscillation occurs because of the large changes in the Walker circulation closely linked to the pattern of tropical Pacific sea temperatures.
The Southern Oscillation Index (SOI)
Climate clues to El Niño
El Niño's Opposite Phase
When the Southern Oscillation Index sustains high positive values, the Walker circulation intensifies, and the eastern Pacific cools. These changes often bring widespread rain and flooding to Australia - this phase is sometimes called anti-El Niño (or La Niña). Australia's strongest recent examples were in 1973-74 (Brisbane's worst flooding this century in January 1974) and 1988-89 (vast areas of inland Australia had record rainfall in March 1989).
Forecasting El Niño
Scientists have made important advances in understanding El Niño/Southern Oscillation phenomena in recent decades. These led to the National Climate Centre's launch of the Seasonal Climate Outlook Service in 1989. The service offers medium-term (three-months ahead) outlooks of rainfall. Useful predictions of seasonal rainfall have the potential to contribute to the goals of sustainable development in the rural sector.
Ecologically Sustainable Development in Australia
A large proportion of Australia's natural environment is farmed, harvested or managed by farmers. Many renewable resources, from topsoil to wildlife, are broadly under rural sector management. Rural communities need the best climate advice to help them protect and sustain national ecological resources in the face of climate extremes. Improved understanding of climate variability, and application of appropriate management techniques, will be crucial to achieving sustainable development goals.
Sustainable development requires improved management in all climate ranges, especially during climate extremes, which bring the greatest risk of environmental degradation. The diagram below suggests how improved climate understanding and forecast skill may increase the range of low-risk conditions, and enhance our capacity to better manage high-risk periods.