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.
During a La Niña year, the jet stream across North America shifts to the North, which favors more tornadoes in the Southeast. This brings warm, moist air into Tornado Alley, the twister-prone regions of the United States. Tornado and hail activity doubled across Oklahoma, Arkansas and northern Texas during strong La Niña years, the researchers reported. The opposite pattern is seen in the Gulf Coast and Florida panhandle, with an increase in tornado activity during El Niño and a drop during La Niña years, the researchers also noted.