El Niño riding the climate roller coaster
This topic is sponsored by the Cooperative Research Centre for Southern Hemisphere Meteorology and the Australian Government's National Innovation Awareness Strategy.
It seems that El Niño and La Niña are here to stay. What are they and how do they affect Australia's climate?
El Niño events have been with Australia for a long time in 1791 an El Niño very nearly wiped out Australia's first European settlement. The phenomenon is not likely to go away, so it is very much in our interest to find out as much as we can about it and learn to live with its inevitable effects.
El Niño got its name from Spanish-speaking fishermen from Chile and Peru working in the Pacific. They noticed that their catches of anchovies sometimes suddenly declined around Christmas time (El Niño means the Christ child). We now know that the change in the availability of fish off the Pacific coast of South America is just one small part of an enormous series of changes with effects in both the southern and northern hemispheres. Atmospheric scientists refer to these changes as the El Niño Southern Oscillation (ENSO).
The Southern Oscillation refers to the see-sawing change in average atmospheric pressure between the mid-Pacific and northern Australia (measured at Tahiti and Darwin). The Southern Oscillation Index measures the difference in atmospheric pressure between Tahiti and Darwin. When the pressure is persistently low over the mid-Pacific, it is high over Australia and the Indian Ocean. A persistent below average atmospheric pressure in the mid-Pacific is associated with an El Niño and dry conditions. The opposite set of conditions to El Niño, known as La Niña, is frequently associated with heavy rains and flooding.
When an El Niño event occurs, eastern Australia, parts of Asia and southern Africa may be plunged into severe drought, while parts of South America and the west coast of the USA may suffer unusually heavy rain and floods.
How El Niño works
For most of the time, air over the Pacific Ocean circulates in a regular fashion. Hot, moist air rises over the wet, tropical Indonesian region and then travels eastwards at a height of about 10-15 kilometres. As it moves it cools and dries out, and finally it descends as cool, dry air near the Pacific coast of Peru. Consequently, this part of South America is usually dry. At the Earth's surface, the winds move in the opposite direction from east to west and this completes the circulation of air over the Pacific Ocean. This is known as the Walker circulation (Box 1: The Walker circulation and weather forecasting).
As the surface winds blow, they drag some of the surface waters of the ocean along with them. The result is that the Pacific Ocean is not quite level. The average sea level is usually slightly higher at the western (Australian) side than at the eastern (South American) side. The difference is very slight (less than a metre) but it can be detected.
For reasons that are still not well understood there is a breakdown of the Walker circulation that occurs every 2 to 7 years, leading to ENSO events lasting between 18 and 24 months. A possible trigger to the 1997 El Niño may have been the unusually long duration of cyclone Justin, which occurred off the northern Australian coast in March. The easterly flow of surface winds was interrupted by 'westerly wind bursts' from the cyclone which may have initiated the movement of warm water in the western Pacific across to the eastern Pacific. Warmer sea surface temperatures in the eastern Pacific Ocean indicate an El Niño; cooler temperatures, La Niña.
The flow of warm water from the western to eastern Pacific causes noticeable changes in sea-level (up to 0.5 metres) and in some locations, for example off the coast of Peru, sea surface temperatures can rise very rapidly. Monitoring the changes in sea surface temperature along the equatorial Pacific is now one of the key diagnostic tools in tracing the development of El Niño events.
Along with the temperature change, the moisture-laden east-west winds and their associated ocean currents slacken. The surface winds no longer blow from South America towards Indonesia carrying moisture picked up from the ocean. So the rains fail in Indonesia, Papua New Guinea and throughout much of eastern Australia too. The high altitude transport of air from Indonesia to Peru also decreases. The cool dry air doesn't arrive near Peru. The warm water now occurring there interferes with the cold, nutrient-rich Humboldt Current that normally travels northwards up that coast from Antarctica. As a consequence, the water is poorer in nutrients and fish numbers drop. But the warmer than usual water there means more evaporation and therefore a moister atmosphere. Heavy rain then occurs in that part of South America.
Impact of El Niño
El Niño events occur irregularly every 2 to 7 years and have major economic effects. In 1982-3 a major event occurred: the fisheries industry off the Pacific coast of South America lost about $290 million as catches declined. Countries like Peru and Ecuador had their heaviest ever recorded rains (northern Peru receiving about 340 times the average figure) and suffered considerable flooding, as did part of the western USA. Eastern Australia endured one of its worst ever droughts, resulting in a $2000 million loss in agricultural production, as well as bushfires and dust storms. Indonesia also had dry conditions, and the monsoon rains failed as far away as India.
Reliable forecasting of El Niños would help to minimise such devastating effects. Unfortunately, it is not yet possible to predict when an El Niño will start. Australian researchers are attempting to improve the forecasting of El Niño events. Climate modelling is just one component of forecasting (Box 2: Modelling climate).
Page updated October 2002.