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The cycle begins when warm water in the western tropical Pacific Ocean shifts eastward along the equator toward the coast of South America.
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Normally, this warm water pools near Indonesia and the Philippines.
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During an El Niño, the Pacific's warmest surface waters sit offshore of northwestern South America.
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Forecasters declare an official El Niño when they see both ocean temperatures and rainfall from storms veer to the east.
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Experts also look for prevailing trade winds to weaken and even reverse direction during the El Niño climate phenomenon. These changes set up a feedback loop between the atmosphere and the ocean that boosts El Niño conditions.
