El Nio and the Southern Oscillation ENSO

El Niño-Southern Oscillation is the name given to one of the better-known variations in global atmo spheric circulation patterns. Global oceanic and atmospheric circulation patterns undergo frequent shifts that affect large parts of the globe, particularly those arid and semiarid parts affected by Hadley Cell circulation. It is now understood that fluctuations in global circulation can account for natural disasters including the dust bowl days of the 1930s in the midwestern United States. Similar global climate fluctuations may explain the drought, famine, and desertification of parts of the Sahel, and the great famines of Ethiopia and Sudan in the 1970s and 1980s.

The secondary air circulation phenomenon known as the El Nino-Southern Oscillation can also profoundly influence the development of drought conditions and desertification of stressed lands. Had-ley cells migrate north and south with summer and winter, shifting the locations of the most intense heating. Several zonal oceanic-atmospheric feedback systems influence global climate, but the most influential is the Austro-Asian system. In normal Northern Hemisphere summers the location of the most intense heating in Austral-Asia shifts from equatorial regions to the Indian subcontinent along with the start of the Indian monsoon. Air is drawn onto the subcontinent, where it rises and moves outward to Africa and the central Pacific. In Northern Hemisphere winters the location of this intense heating shifts to Indonesia and Australia, where an intense low-pressure system develops over this mainly maritime region. Air is sucked in, moves upward, and flows back out at tropospheric levels to the east Pacific. High pressure develops off the coast of Peru in both situations, because cold, upwelling water off the coast there causes the air to cool, inducing atmospheric downwelling. The pressure gradient set up causes easterly trade winds to blow from the coast of Peru across the Pacific to the region of heating, causing warm water to pile up in the Coral Sea off the northeast coast of Australia. This also causes the sea level to be slightly depressed off the coast of Peru, and more cold water upwells from below to replace the lost water. This positive-feedback mechanism is rather stable—it enhances the global circulation, as more cold water upwelling off Peru induces more atmospheric downwelling, and more warm water piling up in Indonesia and off the coast of Australia causes atmospheric upwelling in that region.

This stable linked atmospheric and oceanic circulation breaks down and becomes unstable every two to seven years, probably from some inherent chaotic behavior in the system. At these times, the Indonesian-Australian heating center migrates eastward, and the buildup of warm water in the western Pacific is no longer held back by winds blowing westward across the Pacific. This causes the elevated warm

Normal conditions

El Niño water mass to collapse and move eastward across the Pacific, where it typically appears off the coast of Peru by the end of December. The El Niño-Southern Oscillation (ENSO) events occur when this warming is particularly strong, with temperatures increasing by 40-43°F (22-24°C) and remaining high for several months. This phenomenon is also associated with a reversal of the atmospheric circulation around the Pacific such that the dry downwelling air is located over Australia and Indonesia, and the warm upwelling air is located over the eastern Pacific and western South America.

The arrival of El Niño is not good news in Peru, since it causes the normally cold upwelling and nutrient-rich water to sink to great depths, and the fish either must migrate to better feeding locations or die. The fishing industry collapses at these times, as does the fertilizer industry that relies on the bird guano normally produced by birds (that eat fish and anchovies) that also die during El Niño events. Warm moist air replaces the normally cold dry air, and the normally dry or desert regions of coastal Peru receive torrential rains with associated floods, landslides, death, and destruction. Shoreline erosion is accelerated in El Niño events, because the warm water mass that moved in from across the Pacific raises sea levels by 425 inches (10-60 cm), enough to cause significant damage.

The end of ENSO events also leads to abnormal conditions, in that they seem to turn on the "normal" type of circulation in a much stronger way than is normal. The cold upwelling water returns off Peru with such a ferocity that it may move northward, flooding a 1-2° band around the equator in the central Pacific ocean with water that is as cold as 68°F (20°C). This phenomenon is known as La Niña ("the girl" in Spanish).

The alternation between ENSO, La Niña, and normal ocean-atmospheric circulation has profound effects on global climate and the migration of different climate belts on yearly to decadal timescales, and is thought to account for about a third of all

Normal conditions

Cold water

Upwelling

Cold water

Upwelling

Reduced upwelling

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Schematic diagrams of the different patterns of ocean and air circulation over the Pacific associated with El Niño and normal conditions the variability in global rainfall. ENSO events may cause flooding in the western Andes and southern California, and a lack of rainfall in other parts of South America, including Venezuela, northeastern Brazil, and southern Peru. It may change the climate, causing droughts in Africa, Indonesia, India, and Australia, and is thought to have caused the failure of the Indian monsoon in 1899 that resulted in regional famine with the deaths of millions of people. Recently, the seven-year cycle of floods on the Nile has been linked to ENSO events, and famine and desertification in the Sahel, Ethiopia, and Sudan can be attributed to these changes in global circulation as well.

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