El Nino: The Heartbeat of Climates

By Bohn, Louise; Hulme, Mike | The World Today, December 1997 | Go to article overview

El Nino: The Heartbeat of Climates


Bohn, Louise, Hulme, Mike, The World Today


The world is experiencing what some call the climate event of the century. This is an El Nino, or more accurately an El Nino Southern Oscillation event. It is one of the most important known causes of large-scale climate variability and is associated with disrupted weather patterns worldwide. Newspaper headlines talk of famines and floods and the current phenomenon has been blamed for Indonesian forest fires and resulting smog, for drought in Papua New Guinea and for hurricane devastation in Mexico. The months ahead could see further extremes of weather with both famines and plenty predicted for different regions.

THE EL NINO SOUTHERN OSCILLATION (ENSO) is a large-scale natural fluctuation of the global climate system, a result of interactions between the ocean and the atmosphere. El Nino is the oceanic component which produces a marked warming of the waters of the central and eastern Pacific. Southern Oscillation refers to alternations in atmospheric pressure between the eastern and western Pacific, often described as a see-saw effect.

There are two phases of ENSO. The best known is El Nino, meaning the boy or Christ child, so named by Peruvian fisherman in the eighteenth century since it normally peaks around Christmas. The less well known phase is called La Nina - the girl - which displays opposite characteristics.

La Nina has received less attention both in the scientific community and in the media because it is merely an extreme phase of normal conditions and tends to have fewer adverse impacts worldwide.

ENSO is quasi-cyclical, normally occurring every three to six years and lasting on average up to about 18 months. An exception to this pattern was the El Nino which began in 1991 and persisted until early 1995.

COLD OR WARM

The strength of ENSO is generally measured by two main indicators: sea surface temperatures and the Southern Oscillation Index (SOI). Sea temperatures are monitored where ENSO shows itself most strongly in an area known as Nino3.

The pressure fluctuations that accompany these changes in sea surface temperature are measured by shifts in the Southern Oscillation Index. This is the difference in atmospheric pressure between Tahiti and Darwin, which represents the changes in pressure over the whole Pacific. An El Nino (or warm) event has positive sea surface temperature anomalies in the eastern Pacific and a negative SOI. La Nina (or cold) events have the opposite characteristics.

There are a number of physical changes during the evolution of a typical El Nino. Sea-level pressure drops in the eastern Pacific and rises in the west over Indonesia. This is accompanied by variations in the strength of the trade winds over the equatorial Pacific Ocean, and occasionally in their direction. Surface currents of the Pacific and its thermal structure also alter.

Although these physical changes can be monitored, how an ENSO event starts is uncertain. There is a close interaction between the ocean and the atmosphere, but which system triggers a response in the other is debated. Once an event is underway, however, there is no doubt that changes in the ocean and atmosphere reinforce each other.

RAIN... RAIN...

ENSO alters climate almost worldwide with the strongest influence in the tropics and subtropics. Rainfall patterns are widely affected. Areas that can experience low rainfall or droughts include southern Africa, Ethiopia, the Indian sub-continent, Australasia and northern South America. This can lead to low river discharges in large catchments such as the Nile and the Amazon. The timing of these rainfall deficiencies can vary, but they normally coincide with the region's wet season.

Other areas of the world have heavier that average rainfall during El Nino, often resulting in flooding. These include parts of eastern Africa, southern India, Peru and south eastern Argentina. As with the majority of El Nino's impacts on climate, a La Nina event has the opposite effect

El Nino also influences other aspects of climate. …

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