El Niño-Southern Oscillation (
ENSO; commonly referred to as simply
El Niño) is a global coupled ocean-atmosphere phenomenon. The Pacific ocean signatures,
El Niño and
La Niña are important temperature fluctuations in surface waters of the tropical
Eastern Pacific Ocean. The name El Niño, from the
Spanish for "the little boy", refers to the
Christ child, because the phenomenon is usually noticed around
Christmas time in the Pacific Ocean off the west coast of
South America.
[1] La Niña, similarly, means "the little girl". These effects were first described in 1923 by
Sir Gilbert Thomas Walker from whom the
Walker circulation, an important aspect of the Pacific ENSO phenomenon, takes its name. The atmospheric signature, the
Southern Oscillation (
SO) reflects the monthly or seasonal fluctuations in the air pressure difference between
Tahiti and
Darwin,
Australia. The most recent occurrence of El Niño started in
September 2006[2] and lasted until early 2007.
[3]. From June 2007 on, data indicated a weak La Niña event, strengthening in early 2008.
ENSO is associated with floods, droughts, and other disturbances in a range of locations around the world. These effects, and the irregularity of the ENSO phenomenon, makes predicting it of high interest. ENSO is the most prominent known source of inter-annual variability in weather and climate around the world (about 3 to 8 years), though not all areas are affected. ENSO has signatures in the Pacific, Atlantic and Indian Oceans.
During major warm events, El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the intensity of the Southern Oscillation. While ENSO effects are basically in phase between the Pacific and Indian Oceans, ENSO effects in the Atlantic Ocean lag behind those in the Pacific by 12 to 18 months. Many of the countries most affected by ENSO are developing countries that are largely dependent upon their agricultural and fishery sectors for food supply, employment, and foreign exchange.
New capabilities to predict the onset of ENSO events can have global socio-economic impacts. While ENSO is a natural part of the Earth's climate, an important concern is whether its intensity or frequency may change as a result of global warming. Low-frequency variability has been evidenced; interdecadal modulation of ENSO from the Pacific Decadal Oscillation (PDO) or the Interdecadal Pacific Oscillation (IPO) might exist. This could explain the so-called protracted ENSO of the early 1990s.
