El Niño is a warming of surface waters in the eastern tropical Pacific Ocean. Together with, La Niña, these make up two of the three states of the constantly changing El Niño/Southern Oscillation (ENSO) that can affect weather patterns around the globe.
ENSO is just one of many oscillations in Earth’s ocean and atmosphere that occur naturally over different time and geographic scales. El Niño, Spanish for "the little boy" or "the Christ child," was named by Peruvian fishermen when they noticed changes in anchovy populations around Christmas more than 100 years ago caused by uncharacteristically warm surface water in the eastern tropical Pacific Ocean. Much later, scientists realized that El Niño was part of a much larger, recurring phenomenon that can bring about abnormal and often severe changes in temperature and precipitation throughout the tropics.
In a "normal," or ENSO-neutral year, a low atmospheric pressure center forms over northern Australia and Indonesia and a high pressure center forms on the other side of the Pacific over Peru. At the same time, the trade winds blow steadily east to west along both sides of the equator to move warm surface waters from the eastern to the western Pacific and cause cold, nutrient-rich bottom water to well up off the coast of South America.
La Niña, Spanish for "the little girl." As the name suggests, conditions this phase of ENSO are generally the reverse of El Niño. Where eastern tropical Pacific waters are warmer than normal during El Niño, they are much colder during a La Niña phase.
What are other oscillations?
Many other naturally occurring ocean-atmosphere oscillations in the Pacific, Atlantic, and Indian Oceans have been recognized and named. Some of them have much more of an impact on climate and weather patterns in the U.S. and elsewhere than ENSO. Many of these, as during ENSO, ocean and atmosphere interact as a coupled system, with ocean conditions influencing the atmosphere and atmospheric conditions influencing the ocean. However, not all exert as strong an influence on global weather patterns, and some are even less regular than ENSO.
Why are they important?
When ocean and atmospheric conditions in one part of the world change as a result of ENSO or any other oscillation, the effects are often felt around the world. The rearrangement of atmospheric pressure, which governs wind patterns, and sea-surface temperature, which affects both atmospheric pressure and precipitation patterns, can drastically rearrange regional weather patterns, occasionally with devastating results.
Because it affects ocean circulation and weather, an El Niño or La Niña event can potentially lead to economic hardships and disaster. The potential is made worse when these combine with another, often overlooked environmental problem. For example, overfishing combined with the cessation of upwelling during an El Niño event in 1972 led to the collapse of the Peruvian anchovy fishery.
Extreme climate events are often associated with positive and negative ENSO events. Severe storms and flooding have been known to ravage areas of South America and Africa, while intense droughts and fires have occurred in Australia and Indonesia during El Niño events.