El Niño–Southern Oscillation
In a typical El Niño, the strong easterly winds of the equatorial Pacific weaken due to a reversal of the normal atmospheric pressure gradient above the S Pacific between Australia and Tahiti. This allows warm eastward-flowing subsurface waters to rise, increasing surface temperatures 1–2℃ (2–3.5℉), and sometimes as much as 4–6℃ (7–11℉), in the central and E central Pacific. Along the west coast of South America, El Niño's warm waters persist and deepen, and cold, upwelling, nutrient-rich waters fail to reach surface waters; the resulting warm, nutrient-poor waters devastate coastal fisheries. Heavy rain falls along the South American coast, and heavy rainfall also moves from the western to central Pacific, causing drier than normal conditions in Indonesia and nearby areas. An El Niño also affects the climate of the northern latitudes, particularly North America, which experiences warmer temperatures along the N Pacific coast, increased precipitation in the Southwest and Gulf states and N Mexico, and weaker Atlantic hurricanes.
Severe El Niño events can be economically disruptive worldwide; those of 1982–83, 1997–98, and 2015–16 are regarded as the three strongest on record. The most devastating events have caused droughts in such areas as Africa, India, Southeast Asia, Australia, and the W Pacific, flooding along the Pacific coast of South America, in California, and the SE United states, and devastating coastal storms in California. The 1982–83 El Niño was blamed for 1,300–2,000 deaths and more than $13 billion in damage to property and livelihoods. The 1997–98 event, generally regarded as the strongest El Niño, also was blammed for a severe ice storm in the NE United States and neighboring parts of Canada; estimates of the losses due to it worldwide range from $32 billion to three times that. El Niños can also have positive economic effects; warmer weather in the N United States, for example, can lead to lower energy expenses. It has been estimated that in the United States the 1997–98 event in fact had a positive economic impact overall.
The effects of an El Niño were documented in Peru as early as the Spanish conquest in 1525. By the end of the 19th cent. the phenomenon was being studied by Peruvian oceanographers, although the effects were thought to be limited to the coast of W South America. It was not until the systematic studies of the International Geophysical Year of 1957–58 that the extent of the meteorological impact of El Niño was recognized.
See M. H. Glantz,
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