The El Niño-Southern Oscillation, or ENSO, is a naturally occurring climate cycle in which sea-surface temperatures in the equatorial Pacific Ocean fluctuate. When waters are warmer than normal, as they are currently, it is described as El Niño; when cooler, La Niña.
Curious as to whether or not El Niño or La Niña, conditions could predict the amounts of tornadoes likely to be seen in a given year, researchers from Columbia University 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 team then verified the indices using available observational records from agencies such as NOAA that constantly monitor conditions in the Pacific to spot developing El Niño or La Niña.
Because ENSO affects the large-scale environment, and the large-scale environment affects the tornado occurrence, the team expected to find some correlation between these conditions and severe storm occurrence.
So was it El Niño or La Niña? It was during La Niña years that both vertical wind shear and surface warmth and moisture increased significantly in the southern states, which then led to more severe storms.
As lead author John Allen, a postdoctoral research scientist at the International Research Institute for Climate and Society (IRI), explains, “We can forecast how active the spring tornado season will be based on the state of El Niño or La Niña in December or even earlier.”
This might also explain why last year, 47 people died in tornadoes. But in 2011 – a La Niña year – tornadoes killed more than 550 people, higher than in the previous 10 years combined.
However, this effect acts mostly upon the southern states, such as Oklahoma, Texas, Kansas and other parts of the southern United States. On the other hand, El Niño events act in the opposite manner, suppressing both types of storms in this area.
The research team also notes a few caveats: ENSO is not the only driver of severe storms, as any kind of extreme weather is at best only loosely controlled by coherent, predictable climate phenomena like ENSO, and the study showed robust correlation only in the southern states, where the ENSO signal is especially clear.
What we can learn, however, is that in Southern states that typically experience tornado activity, it is the La Niña years that predict more frequent, and more severe storms.
Allen, J. Tippett, M., Sobel, A. (2015). Influence of the El Niño/Southern Oscillation on tornado and hail frequency in the United States. Nature Geoscience, 2015
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