The cycle begins when warm water in the western tropical Pacific Ocean shifts eastward along the equator toward the coast of South America. Normally, this warm water pools near Indonesia and the Philippines. During an El Niño, the Pacific's warmest surface waters sit offshore of northwestern South America.
Forecasters declare an official El Niño when they see both ocean temperatures and rainfall from storms veer to the east. 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.