Climate change science
The El Niño-Southern Oscillation (ENSO) is a variation in normal sea surface temperatures in the equatorial Pacific Ocean. Pacific Ocean trade winds propel surface water in a westerly direction along the equator. As a result warm water accumulates in the western equatorial Pacific, to the north-east of Australia, heating air in contact with it. The warm, moist air produces clouds and rain.
During the El Niño phase of ENSO the Pacific trade winds and tropical currents weaken, and the warm water in the western Pacific is displaced to the central Pacific. Clouds disappear and parts of Australia may experience drought. Simultaneously, parts of Northern and Southern America experience above-average rainfall.
In the opposite phase of ENSO, La Niña, the ocean surface in the equatorial eastern Pacific Ocean cools. Meanwhile, the western equatorial Pacific warm pool, north of New Guinea, warms. This in turn warms the air in contact with it which rises, lifting tonnes of moisture that condenses in the atmosphere forming massive cloud banks. These clouds bring rain to eastern Australia and parts of South-East Asia. La Niña's warmer seas usually generate more tropical cyclones around Australia.
Typically, El Niño and La Niña events occur every two to seven years.
The Southern Oscillation Index (SOI) is a measure of the strength of ENSO. Visit the Australian Bureau of Meteorology's website for the
latest SOI data (www.bom.gov.au) . CSIRO has a graph showing SOI monthly values from January 1866 through to the present
(www.cmar.csiro.au).
Source: Bureau of Meteorology
