La Nina Cycle Puts New Twist On Tornado Season

Purdue University Professor Ernest Agee says those areas may expect twice as many tornadoes as last year, based on the findings of his graduate student, Suzanne Zurn-Birkhimer, who compared the geographical variation in tornadoes for El Nino years vs. La Nina years during an 81-year-period.

"Though the study provides little reason to expect more or fewer tornadoes overall, the findings show clear evidence of geographical shifts in tornado activity within the United States when comparing strong El Nino years to La Nina years," says Agee, professor of atmospheric sciences at Purdue who has studied tornadoes for more than 30 years.

Zurn-Birkhimer compared tornado activity during El Nino and La Nina events by calculating a ratio of tornadoes on a state-by-state basis. Her findings show more tornadoes in the central and southern plains and the Gulf Coast during strong El Nino years, with a shift to more tornadoes in the lower Midwest, the Ohio and Tennessee valleys, and the mid-Atlantic region during La Nina years.

"That means that this year's La Nina event is likely to increase tornado activity in Mississippi, Arkansas, western Tennessee, western Kentucky and Indiana," Agee says. "In fact, these areas might expect twice as many tornadoes as last year, when they were at a lower risk of tornado activity due to the strong El Nino."

La Nina, which means "the little girl," is characterized by unusually cold ocean temperatures in the eastern tropical Pacific, as compared to El Nino, which is characterized by unusually warm temperatures in the same waters.

The changes in ocean temperatures also cause a shift in the jet streams patterns, Zurn-Birkhimer says. In her study, she calculated the positions and strengths of the polar and subtropical jet streams during El Nino and La Nina events from 1916 to 1996 to study the effect on the distribution and strength of tornadoes in the tornado alley region of the United States.

"During an El Nino event, the polar jet stream -- which carries cold, dry air from the north -- shifts south, bringing cooler air to the Midwest and Southeastern regions of the country," she says. "This cooling effect might also serve to suppress tornado activity in those areas."

By contrast, during a La Nina event, the subtropical jet -- the jet stream that brings warm moist air from the south -- shifts to the far north, bringing an influx of warmth and moisture to these regions, and increasing the odds for tornadoes, Zurn-Birkhimer says.

"There has to be a threshold of heat and moisture to build severe thunderstorms," she says. "If the atmosphere's too dry or too cold, you just can't get large dynamical cloud systems like the super-cell storms. When the jet stream is farther north, as it is in a La Nina event, you have a better chance of achieving these kinds of temperatures and dew-points."

Despite the popularity of blaming El Nino for all of last year's weather woes, Zurn-Birkhimer says her study shows there is little evidence that El Ninos are associated with more or less tornado activity.

"La Nina events, however, seem to favor an above-average annual number of tornadoes in select geographical regions," she says.

Interestingly, she adds, the years with neither an El Nino or La Nina event tend to favor a below-average number of tornadoes by more than 2-to-1.

Agee notes that the 1999 tornado season already is off to a record-breaking start, with two major tornado outbreaks in January.

"Preliminary reports from the National Weather Service show that there were 169 tornadoes in January, and 19 tornado-related deaths," he says.

Zurn-Birkhimer presented her study last fall at the 19th annual meeting of the National Severe Storms Conference in Minneapolis.