El Niño, La Niña patterns may be keys to predicting tornado outbreaks

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Weather forecasting at its most fundamental level is pattern recognition: knowing what usually happens before, during, and after certain patterns develop in the atmosphere and over the oceans.  Just the casual glance over years of data shows at least a little relationship between severe weather seasons in Alabama and Tennessee and ocean temperature patterns in the Pacific: El Niño and La Niña.

UAH researcher and PhD candidate Tony Lyza spends countless hours studying tornadoes and the patterns producing them in the Tennessee Valley.  To him, relationship goes far beyond the “eye test.”  Like many other meteorologists (our crew included), Tony believes there’s a connection between La Niña and tornado outbreaks in the South.  La Niñas following El Niño years are especially interesting to Tony; you may recognize a few of them: 1932, 1971, 1974, 1984, 1999, 2008, and 2011.

That seems to be where we are headed now.  El Niño is weakening, and long-term guidance points toward a moderate La Niña by the Fall.

Let’s be clear; this is not a slam-dunk forecast.  We don’t have all of the puzzle pieces to know if the secondary severe weather season this Fall or the 2017 Spring season will be active or quiet.  So don’t let this be a burden; right now, it’s just one piece of information that gives us clues to what our long-term future could look like.

Also, it’s important to remember that a single major event can occur outside of the bounds of what “should” happen in El Niño or La Niña.  Research like this just shows us when something like that is more likely.

The information below comes directly from the Atlantic Oceanographic and Meteorological Laboratory (a branch of NOAA):

In a recent paper published in Environmental Research Letters, scientists with NOAA and the University of Miami identified how patterns in the spring phases of the El Niño-Southern Oscillation (ENSO), coupled with variability in North Atlantic sea surface temperatures, could help predict U.S. regional tornado outbreaks.

“This is the first study to show that the most frequently occurring spring sea surface temperature patterns in the tropical Pacific and North Atlantic are linked to distinctive spatial patterns of the probability of U.S. regional tornado outbreaks,” said lead author Sang-Ki Lee, Ph.D., an oceanographer at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML).

Researchers at AOML and the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) at the University of Miami investigated the spatial patterns of springtime U.S. regional tornado outbreaks from 1950-2014 and their connection to springtime phases of ENSO. ENSO, or the El Niño-La Niña cycle, is a natural climate pattern in the Pacific Ocean. During an El Niño event, 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 large-scale atmospheric processes conducive to tornado outbreaks across the U.S.

ENSO impact on tornado outbreaksThe researchers focused on four variations of ENSO events: strong winter events that persist well into spring, and weak events that dissipate soon after their winter peak. They found that weak El Niños led to tornado outbreaks in May throughout the Upper Midwest, while strong El Niños led to outbreaks in February across Central Florida and the Gulf Coast.

In contrast, weak La Niñas led to April outbreaks throughout the South, particularly in Oklahoma and Kansas, while strong La Niñas led to April outbreaks along the Ohio Valley and in the Southeast and Upper Midwest.

The results suggest that each of the four dominant spring ENSO variations is linked to distinct and significant U.S. regional patterns of outbreak probability. The strongest tornado connection was with strong, persistent La Niñas, consistent with the Super Outbreak of 1974 and the record-shattering tornado outbreaks of 2011, both of which occurred during strong La Niñas.


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