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.