DEFINITION OF EL NIÑO AND THE SOUTHERN OSCILLATION (ENSO)

El-Niño–Southern Oscillation is the name given to one of the

better-known variations in global atmospheric 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-Niño–Southern Oscillation can also have profound influences

on the development of drought conditions and desertification

of stressed lands. Hadley Cells migrate north and south

with summer and winter, shifting the locations of the most

intense heating. There are several zonal oceanic-atmospheric

feedback systems that influence global climate, but the most

influential is that of the Austral-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 and 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 here 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

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 this 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

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°F–43°F (22°C–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 guano

normally produced by birds (which eat fish and anchovies) that

also die during El Niño events. The normally cold dry air is

replaced with warm moist 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 4–25

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 it is thought to account for about a

third of all 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.

See also CLIMATE; LA NIÑA.

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