Although much of the early winter season had been mild in many places, January brought some exceptionally cold weather due to the polar vortex, which is a fast-flowing jet stream of air that circles the North Pole in the upper parts of the stratosphere about 20 miles above Earth’s surface.
When the polar vortex is strong, it keeps most of the frigid Arctic air in the polar region, resulting in mild winter temperatures in the middle latitudes of the eastern United States and in northern Europe and Asia. But when the polar vortex weakens, the once-trapped cold air can meander and push southward, bringing polar temperatures and extreme winter weather into the United States. (The changing back and forth of the polar vortex’s strength is referred to as the Arctic oscillation.)
Under normal winter conditions, the polar vortex is strong enough to keep the coldest air bottled up in the north, but in recent winters we have seen exceptionally mild temperatures at times in the far north. It is that unusual warming which causes the jet stream to wobble and eventually buckle, bringing the cold weather into the United States.
So, although it may sound counterintuitive, our cold January was caused by warmer weather in the far northern regions of Earth.
Figure 1. In the weak phase of the Arctic oscillation, the Arctic jet stream weakens and “wobbles,” allowing the polar vortex to descend southward.
The Wobbly Polar Vortex
The polar vortex first began to get wobbly in early January after showing signs that it might split apart. The final straw was rapid warming that occurred in the stratosphere, some 20 miles above Earth’s surface. It’s like a hacksaw of warm air cutting right through the cold air and releasing it to move away from its normal home in the far north.
This creates different vortices that are liable to wobble southward to different regions, like the U.S. and Europe at the same time, which happened in mid-January and brought very cold temperatures.
What exactly triggers these warming events high above the polar vortex remains unknown, but one theory that is gaining prominence blames the rapid diminishing of Arctic sea ice. With the Arctic warming more than twice as fast as the rest of the globe, the amount of Arctic sea ice cover is plummeting.
As a result, recent climate research suggests that without the ice cover to act as a blanket, more heat escapes from the oceans, warming the air above them. Ultimately, researchers found that this relatively warmer air interacts with and weakens the winds over the arctic, allowing frigid polar air to more easily escape to southerly places like Chicago and Boston.
What Does it All Mean?
It’s important to note, however, that these cold blasts do not mean that Earth is cooling. In fact, it is the warming of the planet that is causing these cold disruptions, displacing Arctic air that often stays put over the Arctic. Despite these cold periods, most of Earth still experiences warmer-than-average temperatures in any given month, and 9 of the 10 warmest years on record for Earth as a whole have come in the past 10 years.
Was your area hit by the polar vortex this year? Share in the comments below!