ENSO and US tornadoes - a new study

[Jeff Duda]

A new study published recently offers more data and results as to the relationship between El Nino Southern Oscillation (ENSO) status and tornado and hail frequency in the US. The study covered activity during two three-month periods: meteorological winter (Dec-Jan-Feb) and spring (March-April-May).

The results were perhaps not surprising. Springtime hail and tornado activity was found to correlate negatively with ONI (Oceanic Nino Index - the official index used to measure ENSO status...basically the SST anomaly over a longitudinal band of the central tropical Pacific), which means tornadoes and hail occur more frequently during La Nina years and less frequently during El Nino years. It's important to note that the activity frequency changes were focused farther to the south and east, centered more over the Oklahoma-Arkansas-Louisiana-Texas region as opposed to areas of the plains farther north and west. Activity farther north and west is much less correlated with ENSO status.

See related summary article at http://www.climate.gov/news-feature...a-niña-affect-spring-tornadoes-and-hailstorms.

The actual paper can be found at http://www.nature.com/ngeo/journal/v8/n4/full/ngeo2385.html, although you may have to pay for access.

 

[NealRasmussen]

I didn't see in the summary as to what degree of influence the El/La Nino has? Would it be like a 5% better chance or a 25%, or? Is this in the pay-for-knowledge one? Thanks for posting it.

 

[Mike Benefield]

A new study published recently offers more data and results as to the relationship between El Nino Southern Oscillation (ENSO) status and tornado and hail frequency in the US. The study covered activity during two three-month periods: meteorological winter (Dec-Jan-Feb) and spring (March-April-May).

The results were perhaps not surprising. Springtime hail and tornado activity was found to correlate negatively with ONI (Oceanic Nino Index - the official index used to measure ENSO status...basically the SST anomaly over a longitudinal band of the central tropical Pacific), which means tornadoes and hail occur more frequently during La Nina years and less frequently during El Nino years. It's important to note that the activity frequency changes were focused farther to the south and east, centered more over the Oklahoma-Arkansas-Louisiana-Texas region as opposed to areas of the plains farther north and west. Activity farther north and west is much less correlated with ENSO status.

See related summary article at http://www.climate.gov/news-features/featured-images/el-niño-and-la-niña-affect-spring-tornadoes-and-hailstorms.

The actual paper can be found at http://www.nature.com/ngeo/journal/v8/n4/full/ngeo2385.html, although you may have to pay for access.

It is becoming increasingly obvious that during La Nina years tornadoes and other severe weather activity increase. And you are correct about the areas affected by La Nina activity. The states you mentioned as well as states in the southeastern U.S. (especially Alabama and Mississippi) also experience an increase in the intensity and frequency of tornadoes. The most obvious example of the La Nina effect contributing to an increase in tornadic activity is the 2011 Super Outbreak. There are are several other years in which La Nina contributed to tornado outbreaks. If I'm not mistaken, 1974 was a La Nina year. If that is true, the 1974 Super Outbreak is additional proof of the La Nina/tornado outbreak correlation.

This year, El Nino is slightly elevated, and could be one of the reasons that tornadic activity was reduced in areas that usually have more tornadoes. In April 2015, fewer than 10 tornadoes were confirmed in Alabama. In April 2011, at least 109 tornadoes were confirmed in Alabama, an 11-1 ratio.

You know a lot more about this subject than I do, but I definitely see a correlation between ENSO weather patterns and severe weather.