A one-of-a-kind combination of weather factors make the United States the twister capital of the world, with the ominous funnels 10 times more common in the states than anywhere else on the planet, scientists say.
The four main ingredients all are geographical, all unique to America's borders: a massive mountain wall to the west, a warm ocean to the southeast, a cold-air “shield” to the north – and above these particular latitudes, a narrow river of wind, the jet stream, that surges eastward at hundreds of miles per hour.
“I call the area from the Rockies to the Appalachians and from the Gulf Coast to the Canadian border the ‘tornado super bowl’ of the world,” said Ernest Agee, a professor at Purdue University, affiliated with the school’s Earth, Atmospheric and Planetary Sciences Department.
“Other places have tornadoes, that’s for sure, but not as many,” Agee said.
“The Super Bowl is where you get the best together to create an event, so the 'tornado super bowl' is where you put the best ingredients together to make tornadoes."
Indeed, the sorts of heat-transferring “convective” storms that spawn tornadoes have been known to strike on every continent, except Antarctica, experts say.
But America has a permanent hold at No. 1 among tornado counters, averaging about 1,000 twisters per year, while Canada ranks second with about 100 annually, experts say.
According to the National Climatic Data Center – part of the National Oceanic and Atmospheric Administration (NOAA) – tornadoes have formed in northern Europe, western Asia, Bangladesh, Japan, Australia, New Zealand, China, South Africa and Argentina.
“The Super Bowl is where you get the best together to create an event, so the 'tornado super bowl' is where you put the best ingredients together to make tornadoes,” Agee said.
“And we’re on a part of the planet where these ingredients can come together sometimes very quickly as they did Sunday,” Agee added.
An outbreak of tornadoes in three states killed at least 16 people Sunday, carving a particularly savage route through Arkansas.
In the most simple terms, super-cell storms are born from warm, moist air at low levels of the atmosphere – usually near the ground – that rise and mix with dry, cold air way above, said Harold Brooks, a senior researcher NOAA’s National Severe Storms Laboratory in Norman, Okla.
But the fearsome foursome factors of American tornadoes begin, Agee said, with the fact that a “strong” jet stream blows above the nation, typically gaining speed during spring months as cold air from the northwest careens toward warm, wet air in the southeast.
“Not to sound sarcastic, but if you declared the land from the Yucatan (Peninsula in Mexico) to the Florida Keys to be a landfill, and there was no Gulf anymore, there would be no tornadoes."
That hot air wafts up and out over the salty currents and blue waves of the Gulf of Mexico.
“Not to sound sarcastic, but if you declared the land from the Yucatan (Peninsula in Mexico) to the Florida Keys to be a landfill, and there was no Gulf anymore, there would be no tornadoes,” Agee said. "The Gulf of Mexico is basically the fuel tank for these developing storms.
“But, of course, we have to have the jet stream, too. You get big storms in the tropics but they don’t have tornadoes because the jet stream is not there to help produce the tornadoes,” he added.
The northern regions of the country, meanwhile, offer the third geographical ingredient – a vast “continental shield” that extends from Western Canada to Greenland, behind which winter continues well into America’s spring, Agee said. That shield allows cold air to pool and remain to eventually fuel those mega-thunderstorms storms far south, carried there by the jet stream.
Of course, all of those cold, dry pockets up high, and that humid, juicy air down near the ground need an energy source to create what scientists like Agee call “vorticity,” or spinning. That’s where America’s greatest mountain chain enters the recipe.
“That we have a north-to-south mountain range like the Rockies help create the vorticity that’s needed for the storms, caused when the jet stream winds comes over those mountains," Agee said.
The Rockies, in fact, are considered “protypical” to Earth’s tornado-churning machine, Brooks said.
The 14,000-foot-tall range helps trap winds in the middle of the country.
“They extend a long way, north to south, and they’re wide, east to west, and there’s basically no way for the air in the central U.S. to get over there without having to go over the Rockies,” Brooks said.
"That we have a north-to-south mountain range like the Rockies help create the vorticity that’s needed for the storms, caused when the jet stream winds comes over those mountains."
In contrast, consider South America, which has the Andes, another north-to-south range yet very narrow when compared to the Rockies.
“In South America, the source of moist air for their severe thunderstorm region is the Amazon (rain forest),” Brooks said “It’s not quite as good as the Gulf because it's not a body of water.”
And on other continents, other huge mountain ranges run east-to-west, including the Himalayas in Asia and the Alps in Europe, allowing air to blow around the giant rock outcroppings instead of being forced to somehow push over the peaks, Brooks said.
“So that means, (in the U.S.) when we get the right wind profiles in the atmosphere, we are bringing in the temperature and moisture profiles that are most conducive to tornadoes,” Brooks said.
“Anywhere else on the planet,” he added, “the atmosphere has to produce the conditions and it has to work much work harder to bring these ingredients together.”
First published April 29 2014, 1:08 AM