Changes in the polar vortex can load the dice
At the start of February 2021, a major snowstorm hit the northeast United States, with some areas receiving well over two feet of snow. Just a few weeks earlier, Spain experienced a historic and deadly snowstorm and dangerously low temperatures. Northern Siberia is no stranger to cold, but in mid-January 2021, some Siberian cities reported temperatures below -70 degrees.
Media headlines hint that the polar vortex has arrived, as if it were some sort of ice tornado that wreaks wintry havoc wherever it strikes.
As atmospheric scientists, we cringe when the term polar vortex is used to loosely refer to blasts of cold weather.
The actual polar vortex can’t put snow in your backyard, but changes in the polar vortex can load the dice for wintry weather — and this year, the dice rolled Yahtzee.
The winds of winter
The polar vortex is an enormous, threedimensional ring of winds that surrounds the North and South poles during each hemisphere’s winter. These winds are located about 10 to 30 miles above Earth’s surface, in the layer of the atmosphere known as the stratosphere. They blow from west to east with sustained speeds easily exceeding 100 mph. In the darkness of the winter polar night, temperatures within the polar vortex can easily get lower than -110 degrees.
Fortunately for everyone, the stratospheric polar vortex itself won’t appear outside your front door. The polar vortex does influence winter weather, but it is more like a domino — when it is knocked over, it can start a chain of events that later result in wild weather.
The strength of the polar vortex can vary widely during winter, and these variations can lead to shifts in the strength and position of the jet stream, the fast-flowing river of air in the troposphere beneath the polar vortex. When the jet stream changes, it affects the movement of weather systems, causing different parts of the world to see much warmer or colder, or much wetter or drier conditions.
The domino effect
The polar vortex is interconnected with the weather that moves around the Earth at lower altitudes. Normal variations in the jet stream and weather can disturb the structure of the vortex in the stratosphere. Like an elastic band, the vortex usually rebounds back to its normal shape and size, maintaining its strong winds and low temperatures.
But sometimes, these weather and jet stream variations can knock the polar vortex off balance, causing significant wobbles in its shape, location, temperatures and winds. When this happens, the structural integrity of the polar vortex begins to break down. If this happens often enough over a period of time, everything can go haywire with the polar vortex as the winds break down and the vortex warms up.
On Jan. 5, the polar vortex was completely thrown out of whack by an event called a sudden stratospheric warming, the technical name for these violent disturbances that severely distort and weaken the vortex, knocking it off of the pole or even ripping it apart. When this happens, temperatures in the normally cold polar stratosphere explosively rise by as much as 90 degrees over the span of a few days — hence the name of these events.
At this point, the domino has tipped over: Eventually the jet stream feels the effects of the weakened polar vortex above, and it can begin to undulate. When the jet stream gets wavy, it can dip farther south, bringing cold air and winter storms with it.
It can take weeks or months for the polar vortex to recover from something like this. While the vortex pieces itself back together, the undulating, curvy jet stream can bring frigid Arctic air and winter storms to the U.S. and Europe while allowing unusually warm weather to get into the far north.
Warmer weather, too
Calling any blast of cold air a polar vortex is wrong.
The behavior of the polar vortex doesn’t just portend colder weather — it can also foreshadow much warmer weather.
Most of the time the polar vortex has little influence on winter weather.
But forecasting and monitoring huge disturbances to the polar vortex allows us to anticipate the chain of events that may leave feet of snow and frigid weather at your doorstep.