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Instruction 5-6


Circulation Patterns in the Earth's Atmosphere and Oceans | The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers | The Origin and Effects of Temperature Inversion | Properties of Ocean Water | Location of Deserts and Rain Forests | Features of ENSO

Features of ENSO
CCSTD Earth Science 5.g.

ENSO stands for El Nino Southern Oscillation – but it is usually just called El Nino.

El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific Ocean.

It has important consequences all around the globe

These consequences include increased rainfall across the southern United States. And also in Peru, which has experienced disastrous El Nino flooding with great loss of life.

During an El Nino, some normally dry tropical habitats become virtual gardens, while normally abundant rain in other tropical areas becomes sparse and intermittent.

In the United States, El Nino causes temperatures to be warmer than normal in the North Central States and cooler than normal in the South and Southwest.

In 1991, El Nino caused the failure of the northeasterly trade winds that usually bring moisture to Australia.

The resulting drought was the worst in that country in 200 years. Half of the country's Eucalyptus trees were lost and many native animals, like the platypus and koala, died.

How El Nino Got Its Name

El Nino was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific Ocean. Climatologists named it El Nino, which means "Little Boy" or "The Christ Child" in Spanish, because it usually appeared around Christmas.

An El Nino generally lasts for about 18 months, although the 1990 El Nino lasted for five years. There is a big difference between normal conditions and El Nino conditions.

Normal Conditions

Under normal conditions, trade winds blow toward the west across the tropical Pacific.

These winds pile up warm surface water in the western Pacific, making the ocean surface half a meter higher in Indonesia than in Ecuador.

Normally, the surface temperature of the ocean is warmer in the west by about 8 degrees Celsius. Cooler temperatures off South America are caused by an upwelling of water from the deeper parts of the ocean. This nutrient-rich cold water supports abundant marine life and a large commercial fishing industry.

There is also rainfall in the rising air over the warmest water, while the eastern Pacific is fairly dry.

During El Niño

During an El Niño, however, the trade winds relax in the central and western Pacific. This leads to a depression of the thermocline in the eastern Pacific and an elevation of the thermocline in the western Pacific. (Remember that the thermocline is the transitional temperature zone between the Surface Zone and the Deep Zone in the ocean.)

This reduces the ability of the upwelling to cool the water at the ocean's surface. The resulting rise in temperature causes the death of great numbers of fish and is a disaster for the fishing industry.

Then, as this warm water spreads from the western Pacific Ocean toward the east, rainfall follows – with associated flooding in Peru and drought in Indonesia and Australia.

For a series of graphs that help explain the El Nino phenomenon, click:

La Nina

La Nina is also a disruption of the ocean-atmosphere system in the tropical Pacific Ocean. La Nina means "The Little Girl," although La Nina is sometimes called El Viejo, anti-El Nino or simply a "cold event" or a "cold episode."

Unlike El Nino, which is characterized by unusually warm temperatures in the Equatorial Pacific, La Nina is known for unusually cold temperatures.

In other words, El Nino and La Nina are opposite phases of the ENSO cycle. La Nina is sometimes referred to as the cold phase of ENSO and El Nino as the warm phase of ENSO.

The global climate effects of La Nina tend to be opposite those of El Nino. In the United States, for example, La Nina causes warmer winter temperatures than normal in the Southeast and cooler temperatures than normal in the Northwest.

For a series of graphs that help explain the La Nina phenomenon, click:

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for Students, Parents and Teachers

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