The extraordinary Joplin twister — the single deadliest tornado since officials began keeping records in 1950 — was a rare destructive phenomenon known as a “multi-vortex,” hiding two or more cyclones within the wider wind funnel.

Sunday’s storm smashed the southwest Missouri city’s hospital, left nothing but splintered trees where neighborhoods once stood, and killed at least 116, with the death toll expected to rise. The storm injured another 500 and 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 try to figure 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 such as 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 jet stream east and south, altering prevailing winds. The jet stream’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.

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 intense twister that whipped through Joplin on Sunday spun with windspeeds approaching 200 mph, ranking it as an F4 , just below the top of the tornado scale. The death toll Monday stood at 116, according to the Associated Press, increasing to 481 the number killed in tornadoes this spring with five weeks until the traditional end of the season.

“We are now on pace for a record year for tornado fatalities” since national record-keeping began in 1950, said Russell Schneider, director of the Storm Prediction Center, part of the National Oceanic and Atmospheric Administration in Norman, Okla.

The April total of 875 U.S. tornadoes shattered the previous record of 267 set in April 1974. The first two weeks of May were relatively quiet until this weekend’s outbreak of tornadoes.

The extraordinary 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 “multi-vortex” 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.

In an online video filmed by a survivor of the Joplin tornado, the blasting roar of the storm quiets for a few seconds before a second roar arrives — a telltale sign of a multi-vortex tornado, Agee said.

Mississippi State’s Dixon was following the violent “supercell” thunderstorm with eight students in a van just outside Joplin when they broke off the chase.

“We let it go,” said Dixon, an atmospheric scientist. “It was just getting too unsafe.”

The windows of their van open, Dixon and the students felt blasts of warm air as they followed the backside of the supercell — a sign of an unusually violent storm, Dixon said. “Normally it’s cold air on the backside. So we knew it was going to be a big storm. But when we left it . . . we didn’t think it was going to be catastrophic.”

Over the past decade, deeper understanding of how tornadoes form and move — coupled with advanced radar that can detect telltale swirls at the center of a storm — have lengthened tornado warning lead times broadcast by the weather service. On Sunday, the service announced a tornado warning for Joplin at 5:17 p.m., with the twister touching down 24 minutes later — a “phenomenal” lead time, Dixon said. The nationwide average is 14 minutes, according to the weather service.

Despite this warning, the huge tornado is likely to set a record for the deadliest single tornado in U.S. history. The previous most deadly tornado on record killed 116 people in Flint, Mich., in 1953, an extraordinary year that also saw 114 die in a tornado in Waco, Texas, while 90 perished in a Worcester, Mass., twister that June.

Sunday’s deadly storms come during a busy stretch for the Federal Emergency Management Agency, which is already responding to 11 other tornado-related federal disasters this year.

As of Monday, FEMA said it had paid out $79 million this year to more than 20,000 tornado survivors and another $3.3 million to cities and towns to begin rebuilding schools, libraries, firehouses and other public buildings destroyed by twisters. More requests to rebuild public infrastructure are expected in the coming weeks, a spokeswoman said.

FEMA said survivors of Sunday’s storms are already eligible to apply for aid, after two affected counties were added to a previous disaster declaration for Missouri.