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El Nino is the term given to a disturbance of the ocean atmosphere system in the tropical pacific. This disturbance causes widespread weather changes over a vast area of the earth.
In normal conditions, the Eastern Pacific, off the coast of North America and Peru, is cold, whereas the Western Pacific, off the coast of, for example Indonesia, is warm. This is due to the direction that the wind blows, towards the West. This wind pushes the warm surface water along with it, and so the water in the Western Pacific remains warm. As the wind pushes the warm water along, cold water rises in the Eastern Pacific to replace it. The water in the Western Pacific is at a higher level than that in the East. The warm air in the West rises causing rainfall, whereas off the coast of North America there is little rain.
When the El Nino phenomenon occurs, the trade wind from the East decreases dramatically. Instead, a trade wind develops from the West, and so the opposite occurs. The warm water is now pushed from the West to the coast of North America and Peru, in turn preventing the rising of cold water on the Eastern coast. The sea temperature off the Eastern coast rises, and this warm air rises, producing rain.
The consequences of El Nino mean weather changes in many areas of the worlds, but are typified by increased rainfall in southern areas of North America and Peru, often causing flooding. Conversely, the islands of the West Pacific are affected by drought.