Why does the U.S. have so many tornadoes?

Throughout the warmer months in the Midwestern U.S., many people have become keen observers of the sky, watching for signs of storms and tornadoes.

Dan Chavas, an associate professor at Purdue University’s College of Science, goes further by studying tornadoes and severe storms daily. His work focuses on understanding what drives the formation and location of these extreme weather events.

Why do some regions have more storms?

“I study both the climate and extreme weather,” Chavas said. “My research asks, ‘Why do we have severe thunderstorms or tornadoes at all?’ There are specific regions on Earth that have more storms, more tornadoes than other places. What creates these stormy regions?”

Chavas uses advanced computer models to explore the factors contributing to these storms. “We have had these decades-old assumptions about what causes storms. We’re validating those hypotheses and figuring out what makes North America such a hot spot.”

Unlike storm chasers who track storms in the field, Chavas utilizes historical data and computer models to simulate different scenarios, referring to this process as being a storm tester. 

“We use weather and climate models, as well as extensive databases of thunderstorms, lightning strikes, atmospheric data and more, to ask, ‘What if the world was different?’”

Geographic features and storm formation

One significant study conducted by Chavas and his team involved simulating what would happen if the Rocky Mountains were flattened or if the Gulf of Mexico were filled in. The goal of studying these hypothetical scenarios was to investigate the role of these geographic features in storm formation.

For more than five decades, the Gulf of Mexico was believed to play a crucial role in the formation of tornadoes in the U.S. by supplying warm, moist air. 

Chavas’s research has begun to challenge this assumption. “It was a very reasonable hypothesis,” he said. “But no one had been able to test these 50-year-old ideas because they came about when there weren’t climate models with the necessary computational power.”

Environments with specific ingredients 

The team’s findings showed that a dry Gulf of Mexico influenced the frequency and severity of storms less than expected. Instead, severe thunderstorms shifted from the central Great Plains to Illinois, reducing their occurrence in southern Texas.

“Severe thunderstorms and tornadoes form in environments with specific ingredients for how temperature, moisture, and especially wind speed and direction change with height in the atmosphere. The climate determines where and when those ingredients can be found together to produce these types of storms,” Chavas said.

Severe weather potential in North and South America 

In a recent study published in the Proceedings of the National Academy of Sciences, Chavas and graduate student Funing Li compared severe weather potential in North America with South America. 

Despite having similar geography, South America experiences fewer tornadoes. The researchers found that the rough terrain east of the Andes, including the Amazon’s hills and tall trees, may prevent tornadoes.

Conversely, tornadoes in the U.S. often form east of the Rockies, where the wind flows from the smooth Gulf of Mexico surface.

“A rough surface upstream means that downstream the wind is no longer changing speed and direction with height very strongly near the surface, which we refer to as ‘wind shear,’” Chavas explained. “It doesn’t change ingredients for severe thunderstorms, but the wind shear in the one kilometer of air above the ground is a critical ingredient for tornadoes.”

Climate change and future weather patterns

Chavas’s fascination with real weather and its implications began after a storm-torn tree fell on his house in Wisconsin when he was four years old. His research aims to understand how climate change will impact future weather patterns. 

“If we want to understand how climate change will affect weather in the future, we need to understand how climate determines weather in the first place,” said Chavas.

“If we change the land surface and the trajectory of air flowing inland from the Gulf of Mexico, it may have a direct impact on these ingredients that give rise to tornadoes farther inland.”

“When we think about climate change, we think about it getting hotter and the land getting drier. But if the jet stream changes where and how quickly air flows inland, it can change where and how tornadoes form.”

“Places that didn’t see them before may see them more, and places that had more may see fewer. We need to understand the weather now to help us better predict the weather of the future.”

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