Studying El Niño, La Niña helps predict frequency of tornadoes, hail storms

the insurer Munich RE. Powerful, isolated events such as the 2011 Joplin, Missouri, tornado can smash that average. That storm alone caused several billion dollars in damage and killed 158 people.

The idea that ENSO can affect the frequency and locations of tornadoes and other severe storm systems isn’t new. It is already known to exert a strong influence on temperatures and rainfall in the United States, and affect the position of the jet stream. Yet scientists have had difficulty quantifying ENSO’s role in tornadoes, for two reasons.

First, a variety of other factors can make them seemingly random: one year can see hundreds of twisters, while another sees few. Also, historical weather records are not reliable for long enough to make strong statistical connections. This is true especially for tornadoes, which often flare up and die quickly.

“Trying to tease out an ENSO signal from both the natural noise and the human noise becomes quite complicated,” said coauthor Michael Tippett, from Columbia University’s Fu Foundation School of Engineering and Applied Science. “You can’t get a robust correlation using the observational record alone.”

Past studies that have relied on eyewitness records alone have had limited success, said Allen. “For example, previous work has shown a clear linkage between ENSO and winter activity, but spring — the season when most of tornadoes occur in the southern U.S.— remained an enigma until now,” Allen said.

To get around these challenges, the Columbia University team created indices derived from environmental conditions such as wind shear, temperature and moisture. Each is a key ingredient in severe storm formation, and each is influenced by ENSO. The scientists then verified the indices using available observational records.

Adding a forecasting component was relatively straightforward. “We’re already set up to monitor and forecast ENSO,” said Tippett. “We know that ENSO affects the large-scale environment, and the large-scale environment affects the tornado occurrence.” During La Niña, both vertical wind shear and surface warmth and moisture increase significantly in the southern states, making conditions favorable to severe storm occurrence.

The release notes that agencies such as NOAA and its counterparts all over the world constantly monitor conditions in the Pacific to spot a developing El Niño or La Niña, so the authors say it would not be too difficult to issue a warning for tornadoes or hail based on the ENSO state.

They note caveats, however. First, ENSO is not the only driver of severe storms. “Any kind of extreme weather is at most only loosely controlled by coherent, predictable climate phenomena like ENSO, and tornadoes are no exception,” said coauthor Adam Sobel, who also is at Columbia’s engineering school, as well as its Lamont-Doherty Earth Observatory. Second, the current study shows robust correlation only in the southern states, where the ENSO signal is especially clear. “A lot of the year-to-year variability is for all practical purposes random and unpredictable,” said Sobel, who also directs a new Columbia University Initiative on Extreme Weather and Climate.

— Read more in John T. Allen et al., “Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States,” Nature Geoscience (16 March 2015) (DOI: 10.1038/ngeo2385)