What is a polar vortex, the weather event causing winter storms in the US? 

The United States has been experiencing severe winter storms, which is likely to affect more than 60 million people in the country’s east. The ongoing cold spell, which may bring down the mercury to as low as -50C (-60F), is caused by the expansion of the polar vortex – an area of extremely cold, rotating air around the Arctic – southwards.

Sub-freezing temperatures can extend as far south as the Gulf Coast and Florida Peninsula in the US.

The polar vortex primarily affects countries situated in the Northern Hemisphere’s mid to high latitudes. These regions are particularly susceptible to extreme cold weather events.

What is the polar vortex?

The polar vortex spins anticlockwise around the North Pole with wind speeds of about 155mph (250km/h).

There are two types of polar vortexes – tropospheric and stratospheric.

The tropospheric polar vortex occurs at the lowest layer of the atmosphere where most weather phenomena take place. The vortex creates milder weather across northern latitudes.

The polar vortex we are currently experiencing is a stratospheric polar vortex which typically occurs about 10-30 miles (16-48km) above the Earth’s surface and forms in autumn but disappears in the spring of each year.

When the stratospheric polar vortex is in a stable state, frigid arctic air is confined to the polar regions.

But when it is in a weakened state, the chilly winds expand beyond its circular North Pole region and extend as far south as Florida.

The rising warm air from the lower atmosphere weakens the polar vortex, allowing it to spread more widely. This upward movement of warm air can be triggered by significant weather patterns or phenomena, such as powerful winds in mountainous regions or changes in tropical climate systems.

Both types of polar vortexes are important for global atmospheric circulation and climate regulation.

The extreme cold temperatures from a weakened polar vortex, although seasonal and cyclical in nature, are not an annual phenomenon. Indeed, the term polar vortex did not become a popular public term until 2014.

The 2013-14 polar vortex resulted in record-shattering cold and snowfall which swept across Canada and the eastern US, plunging the regions into one of their coldest winters.

Major metropolitan areas, including New York, Philadelphia, and Chicago, were blanketed with snow, with accumulations ranking among the top 10 highest in their recorded histories.

According to an article published by NASA in 2015, the 2013-14 polar vortex resulted in 92.5 percent of the Great Lakes being covered in ice, the second most extensive ice cover observed via satellite. Lake Michigan’s surface froze to an unprecedented 93.3 percent on March 8, 2014, marking the highest ice coverage ever recorded for this Great Lake.

In 2014, many news outlets and weather reports extensively discussed this atmospheric pattern, making it a household name. The extreme cold not only captured public attention but also provided a widely accessible explanation for the severe weather conditions, cementing the polar vortex within the cultural zeitgeist as a key factor in understanding winter weather patterns in the Northern Hemisphere.

Does climate change affect the polar vortex?

Researchers have been trying to understand the effect of climate change on the intensity or frequency of colder temperatures due to the polar vortex. Some data suggests that climate change may be affecting the polar vortex.

“It makes sense that the polar vortex tends not to be as strong due to global warming because the planet isn’t warming uniformly. It’s warming more at the pole, overall decreasing the strength of the polar vortex and the jet stream and making it more susceptible to being dislodged and sent our way,” Steven Decker, the director of the Meteorology Undergraduate Program at Rutgers University, said last February.

Global warming makes the polar vortex weaker because the Earth is not heating up the same everywhere. The North Pole is warming faster than other places, which weakens the polar vortex and the jet stream – a fast-flowing air current in the atmosphere. This makes it easier for the vortex to get knocked out of place and bring colder air down to many regions, including Europe and North Asia.

“The change is warming higher latitudes and reducing the temperature difference between the warmer mid-latitude and polar regions. This weakens and destabilises the polar jet stream, causing it to dip into lower latitudes, bringing polar air farther south,” Paul Ullrich, an associate professor of regional climate modelling at the University of California, Davis, wrote in a 2021 article.