Of all the media attention the polar vortex has received in recent years, the majority of people just know it as “a system that brings cold air to my city every winter.” Although this is a true statement, let’s discover together what exactly the polar vortex is. After all, the term “polar vortex” has been in use even before the Civil War.
The polar vortex is a low-pressure system that resides over the poles with its strongest extent during the winter months. It is strongest in the winter because of the vast temperature contrast between the Polar regions and the mid-latitude regions. People often associate the polar vortex with cold fronts that produce snow. Although this vortex does bring a punch of cold air, the association of it with snow is simply not the case. The polar vortex primarily occurs in the stratosphere, a layer of the atmosphere about 6-30 miles above the troposphere where most of the weather occurs. Although it is 30 miles above the ground, it still has a profound impact on day-to-day weather.
Changes in the stratospheric polar vortex can effect jet-stream patterns by forcing them southward which in turn can influence the location and persistence of cold air masses across North America and Europe. Here’s the kicker: when the polar vortex is strongest, we are less likely to see a deep cold air mass plunge into North America or Europe. It’s actually when the vortex is at its weakest, when we typically will see the cold air punch. An easy analogy to remember this would be when it is strongest (like in the left picture above) with no waves, it keeps the cold air at bay, kind of like a barrier. When it is at its weakest (like in the picture on the right) with multiple waves, the cold air rushes through the barrier and into North America and Europe.
Like North America and Europe is currently seeing, the polar vortex occasionally weakens. This is due to energy being brought upward from the lower atmosphere. This happens when the stratosphere suddenly warms, an event referred to as sudden stratospheric warming (SSW). This event raises temperatures to about 50 degrees Fahrenheit, or more, in just a few days in the stratosphere. Although this event sounds rare, it occurs at least once every other winter season. This sudden warming disrupts and weakens the polar vortex which shifts the location of it southward and can physically split the vortex up. When this happens, the weakened vortex forces the jet-stream to become more prone to “blocking.” This blocking usually happens in colder months and can easily break down the “cold air barrier” as stated earlier. This leads to a more persistent cold weather pattern for weeks.
A lot goes into fully understanding the polar vortex. Even more goes into forecasting it and knowing exactly when or if it will impact weather patterns across North American and Europe. As we travel through colder weather months, the ever-impending polar vortex may soon make an appearance but before that happens, much more forecasting is needed.
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©2018 Weather Forecaster Alec Kownacki