El Ni�o

El Ni�o and La Ni�a are major temperature fluctuations in the Pacific Ocean. They are Pacific signatures of the global ENSO phenomenon (El Ni�o-Southern Oscillation). Their role in global warming or cooling is controversial.

El Ni�o was originally recognized by fishermen off the coast of South America as the appearance of unusually warm water in the Pacific Ocean. El Ni�o 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. Because this condition often occurs around Christmas, it was named El Ni�o (Spanish for boy child, referring to the Christ child). In most years the warming last only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Ni�o conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Ni�o has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.

ENSO

ENSO 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 Ni�o 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 Ni�a is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Ni�o, which is characterized by unusually warm ocean temperatures in the same area. La Ni�a usually comes usually after El Ni�o.

External link: NOAA explanation