El Niño-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years. The Southern Oscillation refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean and in air air surface pressure in the tropical western Pacific.
Up to 1957 we regarded El Nino as a regional phenomenon confined to the shores of Ecuador and Peru. We gained that impression from the data gathered over the centuries by vessels that sailed across the Pacific. From these oceanographic data we learned that, along the equator, the Pacific is generally warm in the west, near the international date line, and cold off South America except during El Nino's occasional visits, when there is a temporary warming of the waters in the east.
During El Nino, when warm waters cover the eastern tropical Pacific, the stratus clouds disappear from that region and are replaced by cumulus clouds that bring heavy rains. The coastal zones of Ecuador and Peru can now experience floods when rainfall is reduced in the far western equatorial Pacific; northern Australia, New Guinea, and the Philippines can have droughts at such times.
The 1982-83 El Niño produced equally dramatic effects on land. In Ecuador and northern Peru, up to 100 inches of rain fell during a six-month period, transforming the coastal desert into a grassland dotted with lakes. Lush vegetation attracted swarms of grasshoppers, which fueled explosions in the toad and bird populations. The new lakes also provided a temporary habitat for fish that had migrated upstream from the sea during the floods and become trapped.
During the past 40 years, nine El Niños have affected the South American coast. Most of them raised water temperatures not only along the coast, but also at the Galapagos Islands and in a belt stretching 5000 miles across the equatorial Pacific. The weaker events raised sea temperatures only one to two degrees Fahrenheit and had only minor impacts on South American fisheries.
In other locations, the impact of El Niño can have two or more different "flavors." For instance, California can experience very wet conditions (such as in 1940-41, 1982-83, and 1991-92) or drought (1986-87 and 1987-88), depending on how far east the ENSO-related rainfall extends in the tropical Pacific. Predicting which flavor will dominate for a given event is difficult, because very small changes in SSTs can become magnified to produce large differences in rainfall patterns outside the tropics.
El Niño is normally accompanied by a change in atmospheric circulation called the Southern Oscillation. Together, the ENSO (El Niño-Southern Oscillation) phenomenon is one of the main sources of interannual variability in weather and climate around the world.
Today El Niño is known to be the warm extreme of an interannual climate fluctuation called El Niño Southern Oscillation. The cold extreme, (La Niña, El Viejo, cold phase) has consequences of equal or greater importance than the effects of El Niño.
The phenomenon known as El Niño has been observed as early as the 1600's off the coast of Peru. At varying intervals, anomalously warm waters off the Peruvian coast appeared around Christmas and were dubbed El Niño, for the Christ child. The development of the El Niño phenomenon has its origins in the western tropical Pacific Ocean.
El Niño events bring unusually warm water to many coral reefs worldwide. Mass bleaching on coral reefs around the world occurred during ENSO events in 1983, 1992, 1995, 1998, 2000, and 2002 (a minor ENSO year, but the most extensive bleaching event on the Great Barrier Reef). Severe bleaching lowers the corals’ reproductive output for a year or more while they recover.
During El Niño conditions, the trade winds weaken, causing Pacific Ocean currents to change. The shift in ocean currents allows warm water from the western Pacific to flow toward the east.