Australia experiences considerable variation in itsrainfall. If we could take a `typical' ten-year periodwe would have about four years of above averagerainfall, three average years and three belowaverage years. These fluctuations in rainfall haveseveral causes, many of which are not fully understood.
Probably the main cause of major rainfallfluctuations in Australia is the SouthernOscillation, which is a major air pressure shiftbetween the Asian and east Pacific regions. Thestrength and direction of the Southern Oscillationis measured by a simple index called the SouthernOscillation Index (SOI). The SOI is calculatedfrom the monthly or seasonal fluctuations in airpressure difference between Tahiti and Darwin.
When there are `typical' pressure patternsprevailing, the SOI is close to zero. If the SOIbecomes strongly positive this means that thesea-level air pressure at Darwin is much lowerthan normal and a La NiÃÂ±a event occurs. LaNiÃÂ±a is a period of well above average rainfall ineastern Australia, which often brings floods.
SOUTHERN OSCILLATIONDuring an El NiÃÂ±o event, the SOI is stronglynegative and the sea-level air pressure atDarwin is higher than at Tahiti. An average yearoccurs when the SOI is between 10 and +10. ALa NiÃÂ±a event occurs when the SOI is above +10;an El NiÃÂ±o event occurs when it is below 10.
Probably the main cause of drought in eastern Australiais El NiÃÂ±o ÃÂ a warm ocean current in thePacific. At irregular intervals, it spreads furthersouth and the water in the central and easternPacific becomes much warmer, bringing heavy rainfalland floods to arid parts of South America.
At the same time, normally warm water in theoceans to the east and north of Australia arereplaced by much cooler water as the warm waterspreads east. As well, the easterly trade windsthat normally blow across the Pacific Oceanbringing warm, moist air to Australia reversetheir direction. There is an accompanying reversalof air pressure across the Pacific, resulting instrong high pressure systems building up overWeather conditions in (a) a typical year, and (b) an El NiÃÂ±o yearCold deep waterEl NiÃÂ±oTypicalOcean Cold upwellingceases.
Warm surface currents reverse.
AustraliaAustraliaTrade winds reverse direction.
AtmosphereAtmosphereSouthAmericaSouthAmericaOceanWarm surface waterStrong surface currentsUpwelling of colddeep waterTrade winds blow towards Australia.
Warm rising aircauses thunderstormsand floods.
Dry sinking aircauses droughts.
Dry sinking air causesdroughts.
Warm rising air causesthunderstorms andfloods.
Most of Australia. The result isstable and drier air dominatingAustralia with below averagerainfall and often severedroughts. El NiÃÂ±o brings signifi-cant climatic change, not only toAustralia but to other parts ofthe world.
In recent years scientists havemade great advances in understandingand forecasting ElNiÃÂ±o and Southern Oscillationevents. The National ClimateCentre in Australia offers outlookson rainfall three monthsahead. These outlooks are proving to be of greatvalue to farmers and especially valuable for ecologicallysustainable development in rural areas.