Scientists show climate patterns may influence extreme U.S. tornado seasons

Contact: Erica Rule, 301-361-4541

The record-breaking U.S. tornado outbreaks in the spring of 2011 caused unprecedented destruction, leaving citizens and scientists alike wondering what caused the season to produce so many strong tornados. NOAA and university researchers believe they have found a climate signal related to a specific phase of the El Niño-Southern Oscillation (ENSO) that could be linked to, and possibly serve as a predictor of, such massive tornado outbreaks.

Climate scientists at NOAA's Atlantic Oceanographic and Meteorological Laboratory and the Cooperative Institute for Marine and Atmospheric Studies (CIMAS) at the University of Miami recently published their findings online in the Journal of Climate. CIMAS is a partnership between NOAA and the University of Miami.

In the study, they describe a link between a phenomenon scientists describe as "Trans-Niño" and the large-scale atmospheric patterns over the central United States which contribute to tornado outbreaks. Trans-Niño represents the transition of tropical Pacific sea surface temperatures during the beginning or end of the ENSO in spring.

The study looked at the worst tornado outbreaks from 1950-2010 and found that seven out of ten years were linked with a strongly positive phase of Trans-Niño. The number of intense tornadoes during these worst outbreak years, those rated as F-3 to F-5 on the Fujita scale, was nearly double the number in other years.

A positive phase Trans-Niño is characterized by colder-than-normal sea surface temperatures in the central tropical Pacific and warmer-than-normal sea surface temperatures in the eastern tropical Pacific.

Robert Detrick, Ph.D.

Detrick is the new assistant administrator of the NOAA Office of Oceanic and Atmospheric Research. Credit: NOAA

"This is exactly what we saw in 2011," said Sang-Ki Lee, Ph.D., a CIMAS scientist and lead author on the study. "It appears that another positive Trans-Niño influenced both the number of storms and the occurrence of the intense tornados."

"In fact, in looking at other significant tornado outbreaks in U.S. history, such as the tri-state tornado outbreak on March 18, 1925, and the super tornado outbreak on April 3, 1974, both occurred during this ENSO phase," he said.

Scientists are hopeful that this correlation will not only help explain what happened in 2011, but may also help in the future prediction of such destructive outbreaks before the season begins.

"A seasonal outlook for extreme U.S. tornado outbreaks may be possible if a seasonal forecasting system has significant skill in predicating the Trans-Niño and associated teleconnections to the U.S.," said Scott Weaver, Ph.D., of the NOAA Climate Prediction Center. "While NOAA’s primary goal is to save lives by improving the lead time for individual tornado warnings, this understanding may help communities better prepare resources in the region if extreme impacts can be anticipated."

While this research may eventually enable scientists to predict the severity of future tornado seasons, it will not allow them to predict when or where an individual tornado will form, the scientists say. This research was supported by grants from the NOAA Climate Program Office and the National Science Foundation.