Above the Arctic hovers a counter-clockwise spinning mass of cold air that grows and shrinks with the changing seasons.
Occasionally, some of that intensely cold air escapes from its spinning rotation and heads south. In January 2019, a large mass of that air escaped from the Arctic and descended upon the middle and eastern portions of North America. Many cities recorded historically low temperatures. In Chicago, temperatures reached negative 45 degrees Fahrenheit, making it colder than Antarctica on that same day.
At least 21 people died as a result of the 2019 polar vortex. In extreme cold, frostbite can attack exposed skin in minutes, and prolonged exposure leads to hypothermia.
The polar vortex is a well-documented, long-existing pattern, but some scientists say climate change could be making it unstable, with potentially serious consequences.
How does the polar vortex work?
The vortex is constantly spinning in a counter-clockwise direction around the North Pole. During the summer months in the Northern Hemisphere, the edge of the vortex sits at a higher latitude, and in winter months, it edges south.
At the bottom edge of the vortex is the polar front, or polar jet stream. The jet stream moves from west to east, which is why the northeastern portions of North America are often hit the hardest by polar vortex weather. (Read about the difference between weather and climate.)
According to the National Oceanic and Atmospheric Administration, a strong jet stream moving roughly along the same latitude as the vortex makes the vortex more stable and traps cold Arctic air within the Arctic Circle. But occasionally the jet stream weakens, moving around the globe in a wavy pattern called arctic oscillations with peaks and troughs that allow warmer southern air to move north, and cold Arctic air to rush south.
Climate change and the polar front
One single weather event isn't enough to prove climate change, but taken over time, scientists are looking at how warming temperatures will likely affect weather patterns like low-lying polar vortex winds. In fact, scientists say that cold snaps are getting warmer: The polar vortex that plunged the U.S. Midwest temperatures into polar territory in January 2019 was pretty tame compared with what used to happen during the winter.
Jet streams are propelled forward by temperature differences and the Earth's rotation. Warm air is less dense than cold air and rushes to fill in low-pressure regions. Wider temperature differences create faster-moving winds. The polar front is a jet stream that generally moves across Canada's northern territories. Farther south, the subtropical jet stream tends to move through northern Mexico, though the streams fluctuate throughout the year.
Those jet streams tend to sit at the same altitudes that planes fly, 30,000 feet off the ground, and pilots will often catch them to travel faster and save fuel. Jet streams can also usher in weather changes like cold fronts and heat waves.
Scientists are now finding that the Earth is warming more quickly at the poles than at the mid-latitude regions, meaning the temperature contrast that drives jet streams has decreased. In a study published in Science Advances last October, climatologist Michael Mann of Penn State linked extreme, deadly weather to slower jet streams that allow the conditions to stay in place.
What's more, early research has connected a more unstable polar jet stream to climate change. In other words, as the Arctic continues to warm, cold snaps to the south from the polar vortex could increase for some time. But more research is needed.