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DIRECTIONS: While studying readings from barometric stations in Australia and Tahiti, Sir Gilbert Walker found that when atmospheric pressure was low in Australia, it was high in Tahiti, and vice versa. Walker called that see-sawing of atmospheric pressure between east and west the Southern Oscillation and concluded that the parallel oscillations indicated an as yet unexplained relationship among weather patterns in distant areas of the world. Then, in the late 1960s, Jacob Bjerknes, a meteorologist at the University of California, discovered a connection between warm ocean temperatures and the low atmospheric pressure that accompanies heavy rainfall. That discovery eventually led scientists to recognize the connection between El Niño and the Southern Oscillations. In fact, what most people refer to as El Niño is known to scientists as El Niño-Southern Oscillation, or ENSO.


El Niño, or ENSO, actually refers to a complicated chain of events that begins in the Pacific Ocean and spreads to affect the entire world. In the most general terms, this is what happens:

A huge pool of warm ocean water, heated by the tropical sun, lies in an area of the Pacific Ocean near the Galapagos Islands. Most years, strong trade winds blowing over the Pacific push this warm water westward toward Australia and Indonesia. At irregular intervals, however, for as yet unknown reasons, the trade winds weaken and the warm water sloshes back toward South America, accumulating instead along the Peruvian coast. The water heats the air above it, spawning massive thunderstorms. Those storms pump huge amounts of moisture and humidity high into the atmosphere, interfering with and redirecting the flow of jet stream winds. And those changes -- the relocation of Earth's heaviest rainfall from the western Pacific to the eastern Pacific and the disruption of the jet stream winds -- cause unexpected weather patterns around the world.


The redistribution of the warm waters of the Pacific Ocean can have worldwide ecological ramifications. The rise in sea level in the eastern Pacific resulting from the weakened trade winds can cause erosion of coastal areas and the destruction of animal habitats and food supplies. The corresponding drop in sea level in the western Pacific exposes and destroys fragile coral reefs, an integral part of many marine food chains.

Alterations in water temperature also take their toll. As water in a particular area becomes warmer or cooler than normal, some ocean animals are forced to migrate to locations with more ecologically favorable conditions. Other animals -- both sea and land animals -- die, either directly because of the change in temperature or because of the disruption of their habitat and food supply resulting from the temperature change.

El Niño affects people too. Warmer temperatures in normally cold places result in reduced snowfall amounts and lower fuel usage, which can have severe economic repercussions for people living in those areas. Unusually high or low rainfall amounts damage crops and affect both local and worldwide food supplies. Some El Niño weather changes, on the other hand, may be both welcome and beneficial. For example, some scientists attribute this year's quiet hurricane season in the southeastern United States to El Niño.

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