In 2014, almost all of you became quite familiar with the term ‘polar vortex’ when it brought brutal cold to the United States during the 2013-14 winter. The polar vortex is nothing new among meteorologists and has been responsible for some of the United States’ worst Arctic outbreaks. Some winters, the polar vortex stays quite strong and well-placed over the North Pole, but other winters, it can become weaker and displaced, bringing bitter cold to different locations in the mid-latitudes. It’s important to note that all of the Arctic blasts that move across the United States are not caused by the polar vortex becoming displaced and/or splitting. Polar vortex has been rated one of the most overused terms in 2014, but nonetheless, it is still important.
What Is The Polar Vortex?
The polar vortex is an area of low pressure that is located over the North Pole in the upper troposphere and stratosphere which can become particularly strong during the winter months. Because of the sharp contrast of really cold air at the North Pole and the warmer air further south during the winter months, very strong winds rotate counter-clockwise around the polar vortex and bottle up all of that colder air. When the polar vortex is strong and well-placed, the cold air has a difficult time getting past those strong rotating winds. As I stated above, there are other ways to tap into that cold, Arctic air during the winter, but typically when the polar jet stream stays further north (north of the United States), then the nation doesn’t experience extended periods of cold. When it all comes down to it, it all depends on where that jet stream is.
What Causes The Polar Vortex To Weaken?
You can kind of think of the polar vortex as one of those spinning tops. When you spin it really fast on a table, it typically rotates very quickly in one place. Now if you were to tap the spinning top or if it were to just slow down on its own, it may continue to rotate in one spot, but its rotations would become more wavy. In some cases, the spinning top may completely move all over the table. You can think of the waviness of this slower spinning top as tentacles of colder air being displaced due to the weaker spin. Last winter, the polar vortex was all over the place, or to put it another way, it was spinning all over the table. Because of the weaker polar vortex, the polar jet stream was all wavy, and it just so happened that a piece of the vortex became displaced and moved into the United States on multiple occasions last winter.
There are various ways to weaken the polar vortex. To put it simply, warming over the Arctic weakens the polar vortex. This creates a weaker temperature contrast between the North Pole and the warmer regions further south; therefore causing the polar vortex to displace, elongate, or even split. The jet stream that was once nicely rotating around the vortex becomes wavy and all over the place. If you are under one of those wavy tentacles of cold, then you are in for some really cold, Arctic air.
That’s why you hear so much about the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), Eastern Pacific Oscillation (EPO), sudden stratospheric warming events, etc. Those can indicate that blocking (the building of high pressure) is setting up over the Arctic, Greenland, Alaska, etc., which would indicate warming over the Arctic regions. Sudden stratospheric warming events can warm the North Pole enough to really break down and weaken the polar vortex. That will likely occur this month also, and the effects of this warming could be felt in the form of very cold air over parts of the United States later in January and February.
Will The Polar Vortex Pay The United States Another Visit This Winter?
So far this winter, the polar vortex has remained very well-placed and strong over the North Pole. While many regions around the globe in the Northern Hemisphere have already experienced really cold air so far this winter, other regions have not. Much of the United States has missed out on some of the brutal cold so far, but that could drastically change later in January. If the polar vortex weakens later in January, that could bring brutal cold to the mid-latitudes. Determining where that cold will set up is the challenge.
Model guidance continues to hint that strong warming in the stratosphere is going to weaken the stratospheric polar vortex, which will eventually make its way down to the troposphere. Over the next two to three weeks, this could have major consequences at the surface. We’ll have to watch where exactly this warming occurs, which will ultimately determine where this Arctic cold gets displaced.
To help you visualize this, check out the two images below. This shows you the North Pole, and notice all of the warming that is currently taking place. You’ll notice on the next image how in a few days, all of that warming weakens and splits the polar vortex into two vortices. While it takes time, these effects will eventually be felt at the surface.
Even over the course of a few days, things will begin to drastically change over the North Pole.