"This is a great study," Cook said. "It's the next step in assessing the role of ENSO [El Niño] on severe weather, not just tornadoes."
Warm ocean, few tornadoes The El Niño-La Niña cycle, or ENSO, is a natural climate pattern in the Pacific Ocean. During an El Niño, warm sea surface temperatures spread across the tropics. In a La Niña year, the opposite happens: Cool sea surface temperatures dominate in the eastern tropical Pacific. These temperature shifts have a ripple effect on wind patterns around the world, which, in turn, affects where storms form. [Fishy Rain to Fire Whirlwinds: The World's Weirdest Weather]
NOAA declared El Niño's arrival last week, after Pacific Ocean sea surface temperatures crossed a warm threshold and wind patterns shifted in response.
So far, the 2015 tornado season is off to a slow start, with 28 tornados reported, according to the Storm Prediction Center. However, Allen said the cold weather across the eastern United States likely had a stronger effect than El Niño conditions on suppressing tornadoes so far this winter.
In an El Niño year, the jet stream is more southerly, which tamps down the wind patterns that generate severe storms. (For instance, the southerly flow brings cool, dry air from the plains and Canada.) The weather patterns that form twisters and hail decreased by 25 to 50 percent during an El Niño, the study reported.
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.
"There is a geographical dependence, which explains why it might be hard to untangle the impact if you were to just look at the total number of tornadoes [each year] in the U.S.," said study co-author Michael Tippett, a climate scientist at Columbia University.
Direct observations from earlier studies agree with the findings. For example, there were spikes in tornado activity during strong La Niña years, such as in 1999 and 2011. Strong El Niño years brought a drop in tornados, in 1969 and 1988.
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