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El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific, is one part of what’s called the Southern Oscillation. The Southern Oscillation is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific; when the surface pressure is high in the eastern tropical Pacific it is low in the western tropical Pacific, and vice-versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon the El Nino/Southern Oscillation or ENSO for short. South American fishermen have given this phenomenon the name El Nino, which is Spanish for “The Christ Child,” because it comes about the time of the celebration of the birth of the Christ Child-Christmas.

Scientists do not really understand how El Nino forms. It is believed that El Nino may have contributed to the 1993 Mississippi and 1995 California floods, drought conditions in South America, Africa and Australia. It is also believed that El Nino contributed to the lack of serious storms such as hurricanes in the North Atlantic which spared states like Florida from serious storm related damage.

El Nino is thought to occur due to changes in the normal patterns of trade wind circulation. Normally, these winds move westward, carrying warm surface water to Indonesia and Australia and allowing cooler water to upwell along the South American coast. For reasons not yet fully understood, these trade winds can sometimes be reduced, or even reversed. This moves warmer waters toward the coast of South America and raises water temperatures. Warmer water causes heat and moisture to rise from the ocean off Ecuador and Peru, resulting in more frequent storms and torrential rainfall over these normally arid countries.

Unfortunately not all El Nino’s are the same nor does the atmosphere always react in the same way from one El Nino to another. This is why NASA’s Earth scientists continue to take part in international efforts to understand El Nino events. Hopefully one day scientists will be able to provide sufficient warning so that we can be better prepared to deal with the damages and changes that El Nino causes in the weather.

The most severe El Niño of the century occurred in the winter of 1982 and 1983. Disastrous effects and meteorological changes occurred around the world. Total damages were estimated at over $ 8 billion.
Scientists are questioning whether climate change may be affecting the observed increase in strength and frequency of El Niño events in recent decades, or whether the El Niños themselves are contributing to global warming. There is no consensus yet on what the link may be, although evidence is mounting for at least an indirect relationship between El Niño and climate change. Further research is needed before scientists can provide confident answers to these questions. The numerical models which are the main tools for projecting climate are constantly improving in their representation of El Niño.
The antithesis – La Niña

About every four to five years, a pool of cooler-than-normal water develops off South America. The effects of this cooler water are called La Niña. This usually brings colder winters to the Canadian west and Alaska and drier, warmer weather to the American southeast.

More information on El Nino

http://water.ezinemark.com/el-nino-and-the-southern-oscillation-4d0a5399196.html