El Niño–Southern Oscillation

El Niño-Southern Oscillation (ENSO in short) is a term for a natural event that takes place in the Pacific Ocean. It is also called El Niño and La Niña. In Spanish, they mean "The boy" and "The girl."  

El Niño happens when the sea water temperature rises in surface waters of the tropical Pacific Ocean. Every two to five years, the Pacific Ocean has this event. A weak, warm current starts around Christmas along the coast of Ecuador and Peru. It lasts for only a few weeks to a month or more. Every three to seven years, an El Niño event may last for many months. This can change the weather and have important effects around the world. Australia and Southeast Asia can have drought, but the deserts of Peru can have very heavy rainfall. East Africa can have both. In a La Niña event, the weather patterns are reversed.

The Southern Oscillation was discovered by Sir Gilbert Walker in 1923. It is a "seesaw" of atmospheric pressure between the Pacific and Indian Oceans. There is an inverse relationship between the air pressure measured at two sites: Darwin, Australia, in the Indian Ocean and the island of Tahiti in the South Pacific. The Southern Oscillation Index (SOI) is the difference in sea-level pressure measured at Tahiti and Darwin. The Cold Tongue (CT) Index measures how much the average sea-surface temperature in the central and eastern Pacific near the equator varies from the annual cycle. The two measurements are anti-correlated, so that a negative SOI is usually together with an unusual warm ocean wind known as El Niño.

By the early 1980s, it was clear that El Niño and the Southern Oscillation were related, and the acronym ENSO is used to describe this large-scale event.

The 2010–2011 Queensland floods were caused by a La Niña event which brought very heavy rain to the east coast of Australia. Other big floods in Australia have also happened during a La Niña, 1916, 1917, 1950, 1954-1956, and 1973-1975. The cost of the 2010-2011 Queensland floods has been worked out to A$30 billion.

Central Pacific El Niño
Not all El Niños happen in the Eastern Pacific. During recent decades, Central Pacific El Niños have been discovered. When these happen, the effects from Central Pacific El Niños are very different from traditional El Niños. Central Pacific El Niños took place in 1986–88, 1991–94, 2002–03, 2004–05, 2006–07, 2009–10 and 2015–16. In fact, multiple pieces of information say that the moderate-to-strong El Niño events began in January 1991 and lasted until about the mid-summer in 1992, a similar El Niño between spring and fall 1993 and the final Central Pacific El Niño from spring to late summer 1994, after which there was a neutral 1994–1995 season and then the 1995–1996 La Niña period. In other words: There was not one single extended Central Pacific El Niño between the 1991 through 1994 period. Rather, it was several such El Niños over the Central to Western Pacific.

References

Category:Weather events
Category:Climate change