. . . Scientists discover number of twisters in the US has dropped as sea ice retreats.

Scientists have discovered a peculiar connection between Arctic sea ice activity and weather in the United States.

Among the many weather-related changes the US has seen in recent years, researchers say fewer tornadoes touched down in the last decade than in the past.

And, according to a new study, melting sea ice could partly be to blame.

As Arctic sea ice is known to affect the path of the jet stream, researchers say its retreat could be causing changes in atmospheric circulation that diminish the conditions favorable for tornado formation.

Arctic sea ice plays an important role in the jet stream – or the fast-moving air currents circling the planet. When the sea ice retreats, the path of the jet stream shifts, according to the researchers. This may do away with the conditions favorable for tornadoes. File photo

The new analysis of nearly 30 years of weather and climate data revealed that reduced tornado activity often lines up with periods of greater ice loss.

According to the researchers, the correlations were especially significant in the month of July.

‘A relationship between Arctic sea ice and tornadoes in the US may seem unlikely,’ said Robert Jeff Trapp, a professor of atmospheric sciences at the University of Illinois.

‘But it is hard to ignore the mounting evidence in support of the connection.’

Arctic sea ice plays an important role in the jet stream – or the fast-moving air currents circling the planet.

When the sea ice retreats, the path of the jet stream shifts, according to the researchers.

Instead of following its usual summer path across the central US through states such as Montana and South Dakota, it migrates farther north.

And, with that goes the atmospheric conditions that give rise to tornadoes.

As Arctic sea ice is known to affect the path of the jet stream, researchers say its retreat could be causing changes in atmospheric circulation that change tornado patterns in the United States 

‘Tornadoes and their parent thunderstorms are fueled by wind shear and moisture,’ Trapp said.

‘When the jet stream migrates north, it takes the wind shear along for the ride, but not always the moisture.

‘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.’

The apparent correlation has raised a number of questions. The researchers aren’t quite sure, for example, why the link seems strongest in July.

HOW DOES A TORNADO FORM? 

Recent supercomputer simulations revealed a glimpse at the catastrophic conditions inside a tornado-producing supercell thunderstorm.

In the new simulation from the University of Wisconsin-Madison, scientists used real-world observational data from the deadly ‘El Reno’ tornado in 2011.

New simulations have revealed a glimpse at the conditions inside a tornado-producing supercell thunderstorm

The atmospheric sounding from the archived data revealed the vertical profile of temperature, air pressure, wind speed, and moisture for the tornado.

There are a number of factors said to be ‘non-negotiable’ in the formation of a tornado, the researchers explained.

This includes abundant moisture, instability and wind shear in the atmosphere, and a trigger that moves the air upwards, like temperature or moisture difference.

But, this doesn’t always mean a storm will happen. 

‘In nature, it’s not uncommon for storms to have what we understand to be all the right ingredients for tornadogenesis and then nothing happens,’ said Leigh Orf, a scientist with the Cooperative Institute for Meteorological Satellite Studies (CIMSS).

‘Storm chasers who track tornados are familiar with nature’s unpredictability, and our models have shown to behave similarly.’

But, the new understanding could ultimately pave the way for better methods of seasonal severe weather prediction.

‘One of the reasons that we focused on sea ice is because, like the ocean and land, it is relatively slow to evolve,’ Trapp said.

‘Because sea ice and the atmosphere are coupled, the response of the atmosphere is also relatively slow.

‘We can use this property to help make long-term predictions for tornadoes and hail, similar to the way predictions are made for hurricane seasons.’