- WEST LAFAYETTE, Ind. -- Last year's El Nino brought a variety of weather-related
woes, but this year's La Nina may stir up double trouble during the tornado
season for Indiana, Arkansas and Mississippi and the western parts of Kentucky
and Tennessee.
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- 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.
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- "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.
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- 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.
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- "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."
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- 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.
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- 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.
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- "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."
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- 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.
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- "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."
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- 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.
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- "La Nina events, however, seem to
favor an above-average annual number of tornadoes in select geographical
regions," she says.
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- 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.
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- Agee notes that the 1999 tornado season
already is off to a record-breaking start, with two major tornado outbreaks
in January.
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- "Preliminary reports from the National
Weather Service show that there were 169 tornadoes in January, and 19 tornado-related
deaths," he says.
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- Zurn-Birkhimer presented her study last
fall at the 19th annual meeting of the National Severe Storms Conference
in Minneapolis.
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