What is El Nino? How does it affect the climate?

In June 1982, oceanographers noticed a temperature rise in the surface waters of the eastern and central Pacific. The change, though barely discernible, marked the start of the most extensive climatic disturbance on record, affecting places as far apart as Chile and Alaska, Indonesia and New York. According to climatologists, the freakish weather worldwide sprang from conditions that give rise on a smaller scale to El Nino (the Christ Child), a periodic weather pattern that appears off South America shortly after December. Scientists are now trying to determine how El Nino relates to the Southern Oscillation, a major seesawing in temperature and atmospheric pressure across the southern pacific.
According to theory, El Nino arises when the trade winds slacken. Normally, these winds blow toward Asia, pilling up water in the western Pacific. If they die down, warm water flows back towards South America, displacing the cold Humboldt Current, which travels up the coast carrying nutrients vital to the marine food chain. As a result, fish and bird life are decimated and the Peruvian anchovy and guano industries ruined. Sometimes the trade winds give out altogether, and associated changes in air pressure brings torrential rains to Peru and Ecuador and drought to Indonesia and Australia at the other end. A major reversal in the seesaw would even affect the weather in the Northern Hemisphere. The El Nino occurs due to abnormal transfer of heat between the ocean and atmosphere – but just what causes this imbalance is yet to be known completely. So far, researchers are nowhere near a complete analysis of the El Nino cycle.

More reading:
El Niño-Southern Oscillation (Wikipedia)

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