If you’re a follower of meteorologists (weather and climate scientists) on Twitter, there’s a good chance you’ve heard of something called a ‘sudden stratospheric warming’ (SSW). You’ve probably seen excited discussions on Twitter regarding these SSWs and winter weather in the USA, the UK, or Europe.
So, what’s the big deal with them? Well, they have the capacity to seriously shake up the weather patterns that affect such mid-latitude regions.
Major SSWs in Recent History
In recent years, two of the most dramatic SSW events occurred February 2018 and January 2009. Both led to major cold weather outbreaks across western Asia and then a large part of Europe.
The 2018 event was the foundation for the ‘Beast from the East’ that delivered freezing cold winds and heavy snow to northern Europe in late February. In some places, the snow depth was the highest recorded since the early 1990s! The very cold air than traversed the entire width of the North Atlantic from east to west. That’s an extraordinary feat, completely reversing the usual progression of airmasses. It’s extremely rare for eastern North America to receive air that was over Europe or the UK less than a week ago!
1st March 2018: Deep snow drifts (by Southern England standards) engulfed the composter and bins behind my garden shed. The peak depth was more than my 30 cm ruler could measure. Meanwhile, the deepest level snow in the garden was about 18 or 19 cm (allowing for the ruler probably not being perfectly vertical).
In 2009, the UK weather turned generally cold and snowy for about a week. Some parts snow lying snow for at least 5 days straight. In parts of mainland Europe, freezing conditions lasted even longer. The event became known to many as ‘The Big Freeze’ – testament to how rarely cold weather has persisted for more than a few days in recent decades.
In records going back to the mid-20th Century, more than 50 SSWs have been logged. Yet we don’t see as many severe cold weather outbreaks and sometimes it appears that the SSW had little appreciable impact on the weather. To understand this, we need to establish an important fact: Not all SSWs are equal.
Displacements and Splits
Broadly, these events can be placed into two main categories: Displacement and Split. This is based on what effect they have on a phenomenon known as the polar vortex. In the Northern Hemisphere, that is a vast anticlockwise circulation in the stratosphere that typically centres above the Arctic Ocean during Dec-Feb, which is also when it’s usually at its strongest.
These diagrams the locations of the polar vortex (blue outline), stratospheric warming (orange outline) and the boundary zone between relatively warm and cold stratosphere (grey dashed outline) on the central date of a historical displacement SSW on the left and split SSW on the right.
Broadly speaking, weather systems are driven from west to east beneath the periphery of the polar vortex. In the typical setup, that brings frequent spells of mild, wet weather to the northern half of the USA, northern Europe, and to some extent northern Asia.
SSWs cause that periphery to move southward in one or two regions, depending on the type. Meanwhile, it moves northward in other regions. This results in abnormal weather conditions in parts of the Northern Hemisphere.
Typically, the polar vortex takes longer to recover from being split than being displaced. So, split SSWs tend to cause the longest periods of unusual weather; 3-6 weeks, as opposed to 1-4 weeks for displacements.
The Impact of SSWs on the Likelihood of
‘Severely Cold’ Weather
For this blog, I’m going to focus attention on how SSW-driven changes to weather patterns affect the chance of temperatures falling far enough below average that severe winter weather (e.g. snow, ice, dangerously low temperatures) is significantly more likely.
I’ve made an assumption that temperature observations follow a ‘normal distribution’ (a good summary of that here). In data science, this is widely practiced for temperature observations.
In this distribution, a temperature that’s one sigma (σ) below the mean (average) is in the lowest 15.9% of historical observations. This is my benchmark for ‘severely cold’ conditions.
For this study, I’ve used weather station observations for months meeting three different criteria. The first is little or no influence from a major sudden stratospheric warming event (SSW). The second and third are at least half the month influenced by a displacement or split SSW event, respectively.
The influence of a SSW event is defined as being the 4 week period after the onset of a displacement event, or 6-week period after the onset of a split event.
The plots and analyses are divided between three highly populated mid-latitude regions: Europe, the contiguous US and southern Canada, and Northeast Asia.
When it comes to SSWs, Europe has perhaps the strongest ‘relationship’ with them in the Northern Hemisphere.
This is because weather patterns over the North Atlantic tend to respond strongly to SSW events. Just what that response is depends on the nature of the SSW. Especially whether they are of the displacement or split variety. This is plain to see when comparing the three plots below (please click on the thumbnails to view).
With no SSW, other driving forces are in control, which we’ve not organised the data by. So, it’s no surprise to see no coherent pattern to the count of days reaching -1 σ or lower.
When there’s a displacement SSW in play, a pattern is just about evident. The north and northwest see more -1 σ or lower days than usual, while the east sees fewer. The weakness of the pattern suggests a lot of variability between individual displacement SSWs. So, the chance of such conditions is higher than normal but perhaps not what you’d call ‘likely’.
Split SSW are another matter. For those, we see an impressively clear pattern – colder in the north, milder in the south. Many locations in the north typically experience at least a week of -1 σ or lower temperatures, with a high risk of snow and ice. This is a strong signal for increased severe winter weather risk.
Below the maps, I’ve included some box-and-whisker plots to further hammer the results home. These provide a quick overview of how the observations within different groups are distributed. As a bonus, in case -1 σ events aren’t extreme enough for you, I’ve included -2 σ anomaly observations too. Those are temperatures in the lowest 2.8% of historical cases!
Contiguous United States & Southern Canada
Here, we find that SSWs are much less associated with an increased probability of severe winter weather. There’s just a suggestion of that in the western US for a split SSW and even then, the pattern isn’t strong.
For the displacement SSW event, the bigger story – by far! – is a reduced likelihood of days being at least 1 σ below average. That’s the only strong signal for SSW impacts in this region of the planet.
From this, we can deduce that unusually cold spells in the USA typically occur because of other key driving forces. For example, it’s well established in literature that Pacific sea surface temperature patterns have a strong effect.
In Northeast Asia, there are some interesting parallels with both other regions.
In response to displacement SSW events, the situation resembles that observed on the other side of the Pacific. The frequency of -1 σ or lower temperatures is below normal for nearly all the region. Toward the coasts, this means appreciably lower heating bills and reduced risk of ice or snow at low elevations.
Looking at split SSW events, we instead see a high resemblance to the impact on Europe. Most of the north of the region sees more days much colder than average, while the southwest sees fewer. The pattern isn’t as strong as it is for Europe, but it’s still impressive. Most noteworthy is the increased risk of freezing temperatures with snow and ice to low elevations in northeast China, South Korea, North Korea, and Japan.
Keep Your Eyes on the Stratosphere
Clearly, SSWs are a big deal for many places – just not always in the same way, depending on the type of event and where you are.
Now, I don’t want to cause undue excitement or concern, but as I write this in mid-Dec 2020, forecast models are suggesting that a SSW may occur in Jan 2021. This is the possible culmination of some strong ‘assaults’ on the polar vortex that have already got underway in recent days.
It’s far from guaranteed, I must stress, but things are moving in the right direction. Soon we may stop asking ‘will it happen’ and instead ask ‘how will it shape up?’.
James Peacock MSc
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY