By Wally Kennedy

wkennedy@joplinglobe.com

Don’t blame it on global warming or climate change.

Blame this year’s extraordinarily violent tornado season on La Niña, a periodic cooling of waters in the Pacific Ocean that is the flip side of the better-known El Niño phenomenon.

La Niña shifts the polar jet stream into a position where storms from the West collide with moisture from the Gulf of Mexico. This year, those collisions have occurred with some frequency in southern Missouri and Arkansas.

La Niñas and El Niños happen every three to five years. The La Niña shaping our weather since January — the strongest on record since the winter of 1988-1989 — has nearly disappeared.

If new research is correct, the strength and frequency of the recent tornadoes that have ravaged the heartland should decline.

But, the experts say we are only half way through the peak of the spring tornado season, which continues through June. So, don’t let down your guard.

The pattern

Research to be released this summer will confirm that La Niña repositions the polar jet stream over a region of the country that stretches from Texas to the Great Lakes.

When that happens, the jet stream — a shifting river of air at high altitudes — brings an abundance of warm moist air from the Gulf of Mexico into the Midwest. That, coupled with high winds and a storm system with cooler air from the West, provides the primary ingredients for violent weather.

“This weather pattern is having a significant bearing on tornado development,” said Joseph Schaefer, director of the Storm Prediction Center at Norman, Okla. “What happened on May 10 in your area is indicative of this pattern.”

During La Niña, the favored area for tornadic activity stretches from Houston, Texas, to lower Michigan, he said.

“It goes right through Southwest Missouri,” he said. “During an El Niño, which is a warming of the Pacific Ocean, tornadoes are more likely to happen along the Gulf Coast, in places like Florida. When we are in a neutral season between La Niña and El Niño, the area of favored activity stretches from Oklahoma to the Carolinas.”

Schaefer and a colleague, Ashton Crook, have studied the pattern and their findings will be published in “Monthly Weather Review” this summer.

“We just looked at the cold (winter) season, but it’s probably true of the warm season as well,” he said. “But what’s important about this pattern is that the tornadoes are stronger and track greater distances during a La Niña.”

Not global warming

The study by Schaefer and Crook follows a similar study in 2005 by J.B. Knowles and Roger Pielke Sr., former atmospheric scientists at Colorado State University in Fort Collins.

The study resurfaced after the “Super Tuesday Outbreak,” on Feb. 5 and 6, when U.S. Sen. John Kerry, a former Democratic presidential nominee, blamed global warming. The authors of the study said “such statements are extremely misleading.”

The study found that tornadoes in La Niña-event years (between 1953 and 1989) were found to have longer-than-average track lengths, were likely to be more violent and there was a good probability of having an outbreak of 40 or more tornadoes. The opposite, they said, was true for El Niño years.

In response to Kerry’s remarks, they said, “The conclusion is that regional climate variability, the La Niña, is the likely explanation for the recent family outbreak of tornadoes, not global warming. Media who do not present this scientific perspective, but perpetuate only the claims of Sen. Kerry that the event was due to global warming, are clearly guilty of biased journalism.”

Too many to count

The ingredients for severe weather came together in early January when 75 tornadoes scoured southern Missouri and three nearby states, killing three people in Missouri.

It happened again on “Super Tuesday,” when 131 tornadoes ravaged southern Missouri, the Southeast and the Ohio Valley, killing 57 people.

And, it happened again May 5-12 when 88 tornadoes strafed Oklahoma, Missouri, Arkansas and Georgia, killing 25 people.

The May 10 deaths of six people at Picher, Okla., and 16 people in Newton, Jasper and Barry counties in Missouri were part of that outbreak. That tornado through Picher and Newton County, which tracked for more than 75 miles, reached wind speeds of 175 mph. It was EF4 tornado, the second strongest tornado on the Enhanced Fujita Scale.

Those were the main outbreaks. There were smaller ones in Virginia and North Carolina. Even Brooklyn, N.Y., had a tornado.

“The Super Tuesday Outbreak showed the classic La Niña pattern,” Schaefer said. “This La Niña appears to have ended. Hopefully, things will calm down now and return to normal.

“It could be just like last year when the major outbreaks just stopped after the May 4 outbreak at Greensburg, Kan.,” he said.

By the numbers

According to the National Oceanic and Atmospheric Administration (NOAA), the United States was pummeled by more tornadoes in the first two months of 2008 than in any previous comparable period.

NOAA tracked 369 twisters during January and February, which is about five times the average.

This year’s tornado season is on track to be one of the worst on record. The death toll stands at 98 people. In an average year, 44 people are killed by tornadoes between Jan. 1 and May 11.

So far, it’s the deadliest year for tornadoes since 1998 when 130 people were killed. The country averages 60 tornado deaths a year.

Before May 11, the Storm Prediction Center in Norman has counted 905 tornadoes. The final tally could be somewhat lower given that storms often are overcounted.

Weather makers

Bob Henson, with the National Center for Atmospheric Research in Boulder, Colo., said El Niño and La Niña are powerful weather makers when it comes to the absence or presence of tornadoes. But global warming could be having an impact, too.

“La Niñas and El Niños really make the weather because they are related to ocean temperatures. When you think about it, it sort of makes sense. It’s hard to warm water and it’s hard to cool water,” he said.

“La Niña tends to concentrate frontal systems that make them sharper and more vigorous. The warm-moist air and cold air are heightened. That is not as true with an El Niño.”

Henson said climate change might be stretching the season.

“Most tornadoes occur from April to June. What we are seeing is an increase in very early tornado outbreaks. There has been no definitive study on this, but we think the timing is changing. The season could be getting longer due to general warming.”



Worst year

The highest number of tornado-related deaths came in 1953, when 519 people died.



Hurricanes

A La Niña year also is likely to generate more frequent and more powerful Atlantic hurricanes. The neutral phase between La Niña and El Niño episodes can also be severe. The record-breaking hurricane season of 2005 occurred during a neutral phase, as did the most destructive hurricanes ever to hit the mainland, Katrina in 2005 and Andrew in 1992, according to NOAA.