Although originally named for a local warming of the ocean near the coast of Peru in South America, "El Niño" now refers to a sustained warming over a large part of the central and eastern tropical Pacific Ocean. Combined with this warming are changes in the atmosphere that affect weather patterns across much of the Pacific Basin, including Australia. These altered weather patterns often help promote further warming of the ocean because of the changes they cause in ocean currents.
Map showing departures from average ocean surface temperatures in December 1997 at the height of the 1997/98 El Niño.
El Niño events occur about every four to seven years and typically last for around 12 to 18 months. They are a natural part of the climate system and have been affecting the Pacific Basin for thousands of years.
Each El Niño event is unique in terms in terms of its strength (as measured by numbers such as the Southern Oscillation Index or changes in ocean temperature), as well as its impact in terms of altered rainfall patterns. Furthermore, El Niño events have a life-cycle during which the impacts vary, both in terms of spatial extent and timing.
El Niño is not a freak of climate, it's not a rogue weather phenomenon, and it isn't in any way abnormal. Furthermore it is not a scourge, and as far as Australia is concerned, it shouldn't be thought of as a synonym for drought, although it's often linked to reduced rainfall in eastern and northern Australia. Finally, and unfortunately, it's not regularly periodic so that predicting an event with more than about six to nine months warning is extremely difficult.
Thunderstorm approaching Bargara Beach, Bundaberg, Queensland.
Picture: CASSANDRA PRINCE, from the Bureau of Meteorology 2001 Calendar
More often than not, El Niño events result in reduced rainfall across parts of eastern and northern Australia, particularly during winter, spring and early summer. However, the precise nature (where and when) of the impact differs quite markedly from one event to another, even with similar changes and patterns in the Pacific Ocean. The progress of some events was punctuated by timely rains that made a significant difference to the season.
For example, the 1982/83 and 1997/98 events were both very strong as measured by changes in the Pacific, yet their impacts in Australia were completely different. Eastern and southern Australia was gripped by severe drought in 1982/83, but in 1997 average to above average falls were common in May, and a dry spell over winter was broken by widespread and heavy rains in September. Severe drought can sometimes result from a relatively weak event, as occurred in 2002/03.
Furthermore, changes in the Indian Ocean can enhance the general tendency for reduced rainfall in eastern Australia, or mask it by contributing to timely falls.
© Australian Government, Bureau of Meteorology