The extraordinary Joplin, Mo., twister — the single deadliest tornado since officials began keeping records in 1950 — was a rare destructive phenomenon known as a "multivortex," hiding two or more cyclones within the wider wind funnel.
Sunday's storm killed at least 116, with the death toll expected to rise. The storm injured another 1,500 and damaged or destroyed at least 2,000 buildings.
Added to the record 875 tornadoes that tore across the country in April, this latest disaster has experts asking why 2011 has spawned so many deadly storms. While researchers root out the causes for this year's record-breaking season, one thing is certain: Unusually big twisters are blasting through heavily populated areas.
"We have had more F4's and F5's than in past years," said Jack Hayes, director of the National Weather Service, referring to the two most destructive categories of tornadoes. And instead of touching down in farms and fields, storms have hit cities like Joplin and Tuscaloosa, Ala.
An emerging body of research points to a cyclical drop in temperatures in the Pacific Ocean as part of the answer. Called La Nina, the cycle lasts at least five months and repeats every three to five years. This year La Nina is pushing a strong North American jetstream east and south, altering prevailing winds. The jetstream's river of cool air high in the atmosphere pulls warmer, more humid air from the ground upward, forming thunderstorm "super-cells."
Such a pattern drove the outbreak of more than 300 tornadoes that swept from Mississippi to Tennessee in late April, killing at least 365, experts say. But it's too early for them to know whether La Nina alone accounts for what is shaping up to be a disastrously record-breaking tornado season, said tornado expert Grady Dixon of Mississippi State University.
"La Nina is probably part of it," he said. "But it's not the only reason."
Tornado experts predicted a devastating season this year, and many have begun studying whether global climate change is driving more frequent — and more intense — tornado-spawning thunderstorms. Such work is at an early stage, making it difficult to draw conclusions.
"This will be a rich topic of research in the coming years," said Russell Schneider, director of the Storm Prediction Center, part of the National Oceanic and Atmospheric Administration in Norman, Okla.
Warm air, moisture and specific wind patterns are the deadly ingredients that mix together to form tornadoes, and climate change impacts at least one of them by increasing the amount of moisture the air can hold.
"Climate change could be boosting one of those ingredients (for tornadoes), but it depends on how these ingredients come together," said Robert Henson, a meteorologist at the University Corporation for Atmospheric Research.
The Joplin twister touched down just west of town at 5:41 p.m. and blasted a path of destruction some three-quarters of a mile wide and six miles long. Tornado experts said the huge funnel cloud hid within it two or more swirling cyclones, a phenomenon known as a "multivortex" or "wedge vortex" tornado.
The centers of such intense wind funnels become unstable, wobble, and spin out two to six smaller twisters from within. The short-lived but intense sub-twisters dance around the edge of the cloud, spinning up to 80 mph faster than the wider mother funnel, said Ernest Agee, a tornado researcher at Purdue University.
Such tornadoes often blaze a peculiar destructive path that flattens buildings on one edge of the funnel while nearby structures survive relatively unscathed.