El Niño-Southern oscillation or ENSO, commonly known as El Niño, is a recurring weather pattern affecting the climate of South America - and other parts of the world - over a period which varies from three to seven years. Floods, droughts and other weather disturbances are associated with it.
ENSO occurs over the tropical eastern Pacific ocean, with direct and stronger effects on the coastal communities of Peru and Ecuador, causing major floodings and threatening their economies, heavily dependant on agriculture and fishing.
It brings warm and very wet summers (December to March).
While Argentina and southern Brazil experience wetter than normal conditions, central Chile has a heavy rainfall. In the Amazon river basin, hotter and drier weather occurs.
ENSO has two components...
When winds slow, ocean circulation decreases and eastern Pacific surface temperatures warm, it's called El Niño or "warm event". Conversely, when strong winds appear, ocean circulation increases and eastern Pacific ocean temperatures cool, we are in front of La Niña or "cold event".
The name El Niño comes from Spanish, meaning "The little boy" - referring to Christ child - due to the fact that appears around Christmas time.
La Niña means "The Little Girl". It is sometimes called El Viejo (Old Man) or anti-El Niño. This cold phase occurred in 1904, 1908, 1910, 1916, 1924, 1928, 1938, 1950, 1955, 1964, 1970, 1973, 1975, 1988 and 1995.
El Niño was originally reconized by fishermen off the coast of Peru as the appearance of unusually warm water in the Pacific ocean, and initially thought to be a climatic condition of this part of the world. Warm waters during El Niño events reduce the number of fish along the Peruvian coast causing reduced fish catches.
However, in the 1960s, became widely acknowledged that it wasn't only a Peruvian occurence or an isolated phenomena, but part of a larger oscillation over the entire tropical Pacific ocean and beyond.
Between 1982 and 1983, a very strong El Niño caused havoc around the world, but an even stronger warmer event developed globally during 1997 and 1998, causing over 20,000 deaths, many millions of people displaced and US$34 billion of direct losses.
During the last several decades the number of El Niño events increased, and the number of La Niña events decreased. What remains unclear is whether this is is a random fluctuation or a normal instance of variation for that phenomenon, or the result of global climate changes towards global warming.