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by David Herring
Fishermen who ply the waters of the Pacific off the coast of Peru and Ecuador have known for centuries about the El Niño. Every three to seven years during the months of December and January, fish in the coastal waters off of these countries virtually vanish, causing the fishing business to come to a standstill. South American fishermen have given this phenomenon the name El Niño, which is Spanish for "the Boy Child," because it comes about the time of the celebration of the birth of the Christ Child. During an El Niño, the physical relationships between wind, ocean currents, oceanic and atmospheric temperature, and biosphere break down into destructive patterns that are second only to the march of the seasons in their impacts to weather conditions around the world. |
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| El Niño: A temperature anomaly
The vast tropical Pacific Ocean receives more sunlight than any other region on Earth. Much of this sunlight is stored in the ocean in the form of heat. Typically, the Pacific trade winds blow from east to west, dragging the warm surface waters westward, where they accumulate into a large, deep pool just east of Indonesia, and northeast of Australia. Meanwhile, the deeper, colder waters in the eastern Pacific are allowed to rise to the surface, creating an east-west temperature gradient along the equator known as the thermocline tilt. | What is El Niño?
El Niño: a temperature anomaly El Niño's impacts on the biosphere
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The trade winds tend to lose strength with the onset of springtime in the
northern hemisphere. Less water is pushed westward and, consequently,
waters in the central and eastern Pacific begin to heat up (usually several
degrees Fahrenheit) and the thermocline tilt diminishes. But the
trade winds are usually replenished by the Asian summer monsoon, and
the delicate balance of the thermocline tilt is again maintained.
Sometimes, and for reasons not fully understood, the trade winds do not replenish, or even reverse direction to blow from west to east. When this happens, the ocean responds in a several ways. Warm surface waters from the large, warm pool east of Indonesia begin to move eastward. Moreover, the natural spring warming in the central Pacific is allowed to continue and also spread eastward through the summer and fall. Beneath the surface, the thermocline along the equator flattens as the warm waters at the surface effectively act as a 300-foot-deep cap preventing the colder, deeper waters from upwelling. As a result, the large central and eastern Pacific regions warm up (over a period of about 6 months) into an El Niño. On average, these waters warm by 3° to 5°F, but in some places the waters can peak at more than 10°F higher than normal (up from temperatures in the low 70s Fahrenheit, to the high 80s). |
This animation (left) shows the relationship between the direction
and intensity of the Pacific trade winds, and the formation of El
Niño. The arrows show surface wind dynamics, while the colors
represent sea surface temperature. Notice how the warmer water expands,
while cooler water contracts. This animation was made using ADEOS
scatterometry data to show winds and Topex-Poseidon data to show water
temperature and topography. (Courtesy of Greg Shirah, Goddard SVS) (3.4MB)
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In the east, as temperatures increase, the water expands, causing sea levels to rise anywhere from inches to as much as a foot. But in the western Pacific, sea level drops as much of the warm surface water flows eastward. During the 1982-83 El Niño, this drop in sea level exposed and destroyed upper layers of coral reefs surrounding many western Pacific islands. |
This series of images shows how the thermocline tilt changes during the onset of an El Niño. Notice how in January 1997 (left) there is a steep temperature gradient between the western and eastern Pacific Ocean. In April 1997 (center), much of the deep pool of warm water in the west has migrated eastward, lessening the temperature gradient. By July 1997 (right), there is a full-blown El Niño and the thermocline is flat. (Courtesy of Greg Shirah, Goddard SVS) | ||
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