On the Polar Vortex: What is hype, what isn’t, and how it may affect us

The Polar Vortex is coming, and when it arrives it’s going to freeze you, your house, and everything around you – taking you back to the Arctic from where it came. Kidding. Ah, it has been a while since we’ve had to really dive into this. As a meteorologist, these are exciting times. The atmosphere is getting ready to put on a bit of a show, and the community is starting to pick up on it. You may recall the last time the term Polar Vortex went “viral”, back in 2014. It was a media frenzy – the polar vortex was coming to invade the United States.

You may also recall the reality of it – it was cold, and in some places it was snowy – and anomalously so. The truth is that the polar vortex is not unheard of, unthinkable, or unfathomed. It is not new, and it is not appearing for the first time since 2014. It is, however, a very neat atmospheric feature, and it deserves our full attention. It affects the weather greatly, and its positioning and intensity throughout the Winter months is a critical component of many forecasts.

We’re approaching this article a bit differently. This afternoon, we opened up discussions on social media regarding the polar vortex – looking for as many questions as we possibly could get. Our goal is to open up the lines of communication between meteorologists, hobbyists, and readers, in an attempt to help everyone understand what is happening – and even garner an appreciation for how amazing all of these atmospheric processes really are.

With that in mind, we’ll be ditching our typical article format and diving in to questions regarding the polar vortex itself that were submitted to us today. Please note that the questions get increasingly technical as they go along. If you need a more basic explanation, feel free to contact us directly on social media or through the contact form. We tried our best to keep these answers detailed while also not diving too much into the technical meteorology. Here we go!

What is the polar vortex?

The Polar Vortex, in our hemisphere, is a persistent cyclone that is typically located near the North Pole. It features a counter-clockwise rotating pool of cold, dense air — some of the coldest on the globe. An increased temperature gradient between the mid and polar latitudes strengthens the vortex during the winter season. The vortex moves around to a certain extent, but generally remains in the vicinity of the poles for most of the year. In the troposphere (where we experience most of our weather) the polar vortex can often fracture, and pieces that separate and move away from the pole can bring anomalous weather (often very cold air) to areas that don’t normally experience it.

The strength of the polar vortex can fluctuate in both the troposphere and the stratosphere. In a very general sense, a stronger polar vortex tends to stay closer to its home near the pole. It can often become a behemoth of sorts, difficult to move around and with a strong upper level jet stream around it. When it weakens, however, the vortex can become much more “wavy” and move around quite a bit. Warming near the vortex or disruptions near the vortex in the stratosphere and troposphere can cause pieces of it to move far away from its typical location.

Polar Vortex Comparison, Darryn W. Waugh, Adam H. Sobel, and Lorenzo M. Polvani, 2016.

What is the stratospheric aspect of the polar vortex, and why is it important?

The stratosphere, while not part of the direct/sensible weather we observe every day, has an important role in meteorology. The term polar vortex is most often used to describe the planetary circumpolar vortex. The polar vortex discussed in news media is more often referencing the tropospheric polar vortex, which exists in the troposphere where most of our sensible weather occurs. The stratospheric polar vortex exists at its most notable strength from Autumn to the end of Winter, and is much smaller in size than the polar vortex in the troposphere.

The stratospheric polar vortex is quite important. While a separate entity from the polar vortex that impacts our weather on a direct basis, its strength and positioning can have major implications on our weather. The stratosphere and troposphere often work together, especially when they are directly coupled, and changes in the strength and positioning of the stratospheric polar vortex can lead to major changes in the troposphere as well.

What is a Sudden Stratospheric Warming Event, or SSW?

One of the more significant events that we monitor are Sudden Stratospheric Warming events, or SSW’s. These occur when the stratospheric polar vortex is suddenly disrupted by major warmth – and splits or fractures into multiple pieces. The definition has been debated, SSW’s are most often declared when zonal winds “reverse” to easterly (an atypical direction) at 10hPa near 60N. There are also often displacements and disruptions of the stratospheric polar vortex, led by warming at the arctic regions and the North Pole. Both of these types of events can lead to tropospheric impacts and direct impacts on our weather down the road.

In general, Sudden Stratospheric Warming events, or “SSW’s” have been known to lead to arctic outbreaks and a propensity for winter storms to develop in the Eastern United States. From a simple standpoint, the rising heights in the stratosphere make it easier for high latitude blocking to develop in the troposphere. With high latitude blocking developing in areas like Canada and the Arctic, cold air is displaced southward into Canada and the United States, and the weather pattern becomes amplified and anomalous. Chances for a SSW appear to be increasing from late December and January, as heat flux increases in the stratosphere and tropospheric forcing from cyclonic and anti-cyclonic wave-breaking events occurs.

GFS Model showing a Sudden Stratospheric Warming event in late December.

These SSW events feature a very large increase in temperatures in the stratosphere around 10hpa (that’s way up there!) with a reversal of zonal winds from westerly to easterly in the high-latitudes between 60N and 90N. However, while they can have long-lasting impacts for the remainder of the winter, a Sudden Stratosphere Warming event — as officially defined– is not really needed in order to cause major changes in our weather pattern.[/av_toggle]

What causes and impacts a Sudden Stratospheric Warming event?

A variety of atmospheric events can cause or attribute to a Sudden Stratospheric Warming event. Warming events in the stratosphere are not atypical – one or two typically occur per winter. However, sudden stratospheric warming events are more rare. Favorable conditions for stratospheric warming events have been tied back to several things (there is a correlation with the QBO phase, for instance). Most notably, though, the events are typically “set off” by waves traveling upward from the troposphere to the stratosphere, which gradually decelerate stratospheric winds. Repetitive blocking patterns in the troposphere (upward propagating) are often predecessors of stratospheric warming events.

This year, for instance, a repetitive blocking pattern has been observed near Scandinavia. Blocking ridges have formed in that region, from Scandinavia towards the Barents/Kara Sea, a few times already this Autumn. The blocking pattern in the troposphere can often cause Rossby waves to grow (wavenumber 1 or 2) – propagating into the stratosphere. Thereafter, downward propagating warming events can occur – where the warming and zonal wind reversal actually begins at the higher stratospheric levels with decelerating or reversing zonal winds at the critical levels (10hPa, for example).

When the stratospheric zonal winds in critical levels reverse from westerly to easterly, significant polar warming can occur and the stratospheric polar vortex can fracture, split, and move into unusual positions. When the stratosphere and troposphere are properly coupled, the reversing zonal winds can often lead to the development of high latitude blocking ridges in the troposphere over time – which dislodge cold air and cause unusually cold/snowy weather patterns across the United States and Europe.[/av_toggle]

Are impacts from a SSW guaranteed?

Some form of impact to tropospheric weather is extremely likely when a SSW occurs. The exact impacts, however, are not guaranteed and remain highly uncertain. The exact intensity, positioning and propagation of the stratospheric polar vortex has a large impact on what type of weather is observed, and where. The positioning of the strongest warming and stratospheric polar vortex itself can heavily influence where/if high latitude blocking develops.

Average impacts from all SSW events (Dr. Amy Butler, 2017).

The average impact from a SSW event, however, tends to favor high latitude blocking in the polar regions and Canada, with colder than normal air across the Eastern and Northeastern United States, as well as much of Europe. Mean sea level pressure anomalies are also lower near the United States East Coast (signifying heightened storm potential) and much higher in Canada from Hudson Bay toward the Davis Strait and Greenland (again, signaling blocking). Individual events remain intricate and some have not produced typical or average impacts at all. [/av_toggle]

What can we expect from the upcoming potential Sudden Stratospheric Warming event?

The progression throughout the globe is favorable for a Sudden Stratospheric Warming event from late December into early January. For a few weeks now, forecasters have discussed the potential for a notable warming event, as multiple tropospheric waves have impacted the stratosphere, and multiple Wave 1 events have progressed through the stratosphere as well. The stratospheric polar vortex remains weaker than normal – and has been most of the season so far. Forecast models are beginning to suggest that a major event may transpire late this month.

With the vortex notably weakened, and a major Wave 1 warming event forecast – we can dive a bit more in to the details. Forecast model and ensemble guidance are suggesting two key things: Heat Flux into the stratosphere and a reversal of uwind/zonal winds at 10hPa. This suggests that the major warming event being depicted in late December is suggested to significantly impact the stratospheric vortex.

Exactly how the major warming event transpires is still yet to be determined. While ensemble guidance has been consistent, the potential event is still 7-10 days away. It is important to note also that if/when the stratospheric polar vortex splits will have an impact on sensible weather impacts down the road. Nevertheless, with a large stratospheric relative high located over British Columbia and a major warming event forecast with the potential for a zonal wind reversal as warming moves through the North Pole and then towards Northern Canada, the potential exists for a stratospheric polar vortex split – with one piece of the vortex moving towards Europe and another towards Eastern Canada.

Such an event would (in addition to many other meteorological factors which we are not discussing here) support the development of high latitude blocking in the Atlantic and Canadian domains (-NAO, – NAM) and the potential for colder than normal and stormier than normal weather in both the Eastern USA and Europe.

Wrapping it all up, and what to watch for as we move forward 

The stratospheric polar vortex, in its own right, is extremely important. As detailed, it has a huge impact on our sensible weather, even if often times indirectly. The tropospheric polar vortex, which you’re much more accustomed to hearing about – is not hype. It is real, and it does exist almost all of the time. What is hype, however, is the notion that it is some sort of unprecedented feature in the atmosphere.

Over the next few weeks, forecast models suggest that a sudden stratospheric warming event might occur, which could eventually disrupt the tropospheric polar vortex as well. Overall, confidence is rising that higher latitude blocking will develop in Canada and parts of the Atlantic during January, which could lead to a colder and stormier pattern across the Northeast US and parts of Europe during that time and perhaps continuing into February.

Research on the stratospheric polar vortex, its behavior, and what impacts its intensity and breadth is still ongoing. There are a multitude of talented scientists working on this constantly, and the meteorological field hopes to continue to advance understanding and prediction of the stratospheric polar vortex’s behavior in the future.