El Niño is a naturally occurring event in the equatorial region which causes temporary changes in the world climate. Originally, El Niño was the name used for warmer than normal sea surface temperatures in the Pacific Ocean off the coast of South America. Now, El Niño has come to refer to a whole complex of Pacific Ocean sea-surface temperature changes and global weather events. The ocean warming off South America is just one of these events.
Fishermen off the west coast of South America were the first to notice appearances of unusually warm water that occurred at year's end. The phenomenon became known as El Niño because of its tendency to occur around Christmas time. El Niño is Spanish for "the boy child" and is named after the baby Jesus.
ENSO is the "El Niño-Southern Oscillation," the name scientists use for what is often called El Niño. The Southern Oscillation is a see-saw shift in surface air pressure between the eastern and western halves of the Pacific. When pressure rises in the east, it falls in the west and vice versa. In the 1950's scientists realized that El Niño and the Southern Oscillation were parts of the same event.
In normal, non-El Niño conditions, trade winds blow in a westerly direction along the equator. These winds pile up warm surface water in the western Pacific, so the sea surface is as much as 18 inches higher in the western Pacific than in the eastern Pacific. These trade winds are one of the main sources of fuel for the Humboldt Current. The Humboldt Current is a cold ocean current which flows north along the coasts of Chile and Peru, then turns west and warms as it moves out into the Central Pacific. So, the normal situation is warmer water in the western Pacific, cooler in the eastern.
In an El Niño, the equatorial westerly winds diminish. As a result, the Humboldt Current weakens and this allows the waters along the coast of Chile and Peru to warm and creates warmer than usual conditions along the coast of South America. As far as we know, other forces, such as volcanic eruptions (submarine or terrestrial) and sunspots, do not cause El Niños.
El Niños occur irregularly approximately every two to seven years. Warm water generally appears off the coast of South America close to Christmas, and reaches its peak warmth in the eastern Pacific during the late fall of the following year. After peaking, the waters will tend to cool slowly through the winter and spring of the next year. Effects can be felt continually around the globe for more than a year, though this is generally not the case in any one place.
A strong El Niño is often associated with flooding rains and warm weather in Peru, drought in Indonesia, Africa, and Australia, torrential downpours and mudslides in southern California, a mild winter in the northeast, and fewer hurricanes in the southeast. Keep in mind that these effects aren't guaranteed, but an El Niño makes these conditions more likely to happen.
El Niños occur irregularly approximately every two to seven years. Warm water generally appears off the coast of South America close to Christmas, and reaches its peak warmth in the eastern Pacific during the late fall of the following year. After peaking, the waters will tend to cool slowly through the winter and spring of the next year. Effects can be felt continually around the globe for more than a year, though this is generally not the case in any one place.
In non-El Niño years, upwelling of deep, cold ocean water brings up nutrients that lie near the bottom. Fish living in the upper waters feed plankton that are dependent on these nutrients. Kelp forests also depend on cool, nutrient-rich water for survival and growth. An El Niño reduces the upwelling of cold water off the coast of the Americas. When this happens, fish either die or migrate into areas where they'll find more to eat. With the fish gone, sea birds that depend on them may die or go elsewhere. Kelp forests are often destroyed by storms and ocean swells.
Off California, fish populations may also be reduced. Marine mammals, such as seals and sea lions, that feed on fish may be affected. Californians may see an increase in dead and live strandings of seals and sea lions along the coast, and poor seal and sea lion pup survival at island breeding sites. However, despite the increased number of deaths of marine mammals during an El Niño, scientists report the long-term growth rate for California sea lions and harbor seals to be 6 to 10 percent annually. An occasional increase in strandings or deaths of these animals is not a threat to their overall population.
In California, warmer than normal ocean temperatures mean fishes normally found off Mexico, such as yellowfin tuna and dorado, are more common off southern California. However, bait, such as anchovy and squid, may travel to the north, and some game fish, such as white seabass and California halibut, may follow them.
Every El Niño is somewhat different in magnitude and duration.
In the tropical Pacific Ocean, the National Oceanic and Atmospheric Administration operates a network of buoys that measure temperature, currents and winds in the equatorial band. The collected data are evaluated by complex computer models designed to predict an El Niño. Even these complex models, however, cannot predict the exact intensity or duration of an El Niño, nor can they predict how areas will be affected.
Both refer to different phases of ENSO. El Niño refers to a pattern characterized by the tropical Pacific's warmest water spreading eastward to the coast of South America. The opposite condition is characterized by unusually cold ocean temperatures in the tropical Pacific; hence the strongly contrasted name, La Niña.