It mightn’t be quite a case of the flapping of a Brazilian butterfly’s wings causing a tornado in Texas, but a team of climate researchers has found a correlation between melting Arctic sea ice and the formation of tornadoes in the United States, with fewer tornadoes being reported when northern sea ice is unseasonably low.

"A relationship between Arctic sea ice and tornadoes in the US may seem unlikely," says study co-author Jeff Trapp, an atmospheric sciences researcher with the University of Illinois at Urbana. "But it is hard to ignore the mounting evidence in support of the connection."
Building upon earlier research illustrating that sea ice loss in the Arctic can lead to extreme weather events in certain regions around the world, Trapp, along with Purdue University’s Kimberly A. Hoogewind, wanted to see if these extreme events also extend to tornado activity in the US, due to the waning frequency of twister activity there.

Employing three decade’s worth of weather and climate data, the researchers found a definite correlation between the extent of Arctic sea ice and tornado activity in the US, particularly in the month of July: if the ice extent is abnormally low, tornado activity also winds up being low.

The researchers caution that this is simply a correlation, and as the old saying goes "correlation doesn’t necessarily mean causation": they point out that they don’t know if there is an actual connection between these two events, and if there is, they are unaware of what the underlying mechanism connecting the two is.

If there is a connection, the study’s authors speculate that the effect Arctic ice has on the flow of the jet stream may be affecting tornado formation in regions of the US that are prone to these cyclones, particularly in the Great Plains. The transition from winter to summer typically sees a retreat of Arctic air northward, drawing both the jet stream and warmer air from the south with it.

"When the jet stream migrates north, it takes the wind shear along for the ride, but not always the moisture, Trapp explains. "So, even though thunderstorms may still develop, they tend not to generate tornadoes because one of the essential ingredients for tornado formation is now missing."

Trapp and Hoogewind plan to continue their research to try to understand how the loss of Arctic sea ice to global warming is affecting regional weather in parts of the world farther south, a piece of the puzzle that will be required if we are to build climate models that can make reliable predictions as the climate continues to change.