Chart of ocean surface temperature anomaly [°C] during the last strong El Niño in December 1997

El Niño and La Niña are major temperature fluctuations in the tropical Pacific Ocean. They are Pacific signatures of the global ENSO phenomenon (El Niño-Southern Oscillation). Their effect on climate in the southern hemisphere is profound. Their role in global warming or cooling is an area of active research, with no clear consensus yet.

El Niño was originally recognized by fishermen off the coast of South America (Peru and Ecuador) as the appearance of unusually warm water in the Pacific Ocean. El Niño is the warming of the surface waters of the eastern equatorial Pacific Ocean that occurs at irregular intervals of 2-7 years, usually lasting 1-2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water of the Humboldt Current. Because this condition often occurs around Christmas, it was named El Niño (Spanish for boy child, referring to the Child Jesus). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous

El Niño's weather effects depend on the location, time of year, and the particular episode. Typically, winters are warmer than normal in the upper midwest states of the U.S., while central and southern California, and the southeastern U.S., are wetter than normal. The Pacific Northwest states, on the other hand, tend to be drier during an El Niño. During a La Niña, by contrast, the midwestern U.S. tends to be drier than normal. Often, an El Niño is associated with drier, hotter summers in parts South America and Europe, although the western coast of South America and parts of Central America can be much wetter than usual. There is often drought in both Australia and Africa.

Recent El Niños have occurred in 1986-1987, 1991-1992, 1993, 1994, 1997-1998, 2002-2003, and a new El Niño has been in effect since September 2004. The El Niño of 1997-1998, was particularly strong, while the period from 1990-1994 was unusual in that El Niños rarely occur in such rapid succession. The La Niña condition often follows the El Niño, and occurred strongly in 1988-1989 and 1998-2001, and weakly in 1995-1996.

ENSO

ENSO (El Niño, Southern Oscillation) is a set of interacting parts of a single global system of climate fluctuations that come about as a consequence of atmospheric circulation. ENSO is the most prominent known source of interannual variability in weather and climate around the world (~3 to 8 years), though not all areas are affected. The Southern Oscillation (SO) is a global-scale seesaw in atmospheric pressure between Indonesia/North Australia, and the southeast Pacific. Its measure is through the Southern Oscillation Index (SOI). Global ENSO has signatures in the Pacific, Atlantic and Indian Oceans. In the Pacific, during major warm events El Niño warming extends over much of the tropical Pacific and becomes clearly linked to the SOI intensity. While ENSO events are basically in phase between the Pacific and Indian Oceans, ENSO events in the Atlantic Ocean lag those in the Pacific by 12-to-18 months. Many of the countries most affected by ENSO events are developing countries within main continents (South America, Africa...), with economies that are largely dependent upon their agricultural and fishery sectors as a major source of food supply, employment, and foreign exchange. New capabilities to predict the onset of ENSO events in the three oceans can have global socio-economical impacts. While ENSO is a global and natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern. Low-frequency variability has been evidenced. Interdecadal modulation of ENSO might exist.

In the Pacific, La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the same area. La Niña usually comes after El Niño.

External links

• NOAA explanation (http://www.pmel.noaa.gov/tao/elnino/el-nino-story.html)
• ENSO events 1951 - present (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml)
• NOAA announces 2004 El Niño (http://www.noaanews.noaa.gov/stories2004/s2317.htm)

El Niño is also the nickname of Spanish golfer Sergio García.