El Niņo and La Niņa are major temperature fluctuations in the Pacific Ocean. They are phases of the ENSO cycle (El Niņo-Southern Oscillation). Their role in global warming or cooling is controversial.
El Niņo was originally recognized by fishermen off the coast of South America as the appearance of unusually warm water in the Pacific Ocean. El Niņo 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 Niņo (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 Niņo conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Niņo 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. ENSO is the most prominent known source of interannual variability in weather and climate around the world, 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. In major warm events El Niņo warming extends over much of the tropical Pacific and becomes clearly linked to the SO pattern. Many of the countries most affected by ENSO events are developing countries 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 event can have a global impact. While ENSO is a natural part of the Earth's climate, whether its intensity or frequency may change as a result of global warming is an important concern.
La Niņa is characterized by unusually cold ocean temperatures in the Equatorial Pacific, compared to El Niņo, which is characterized by unusually warm ocean temperatures in the same area. La Niņa usually comes soon after El Niņo.
External link: NOAA explanation (http://www.pmel.noaa.gov/tao/elnino/el-nino-story)