Right Now: Sudden Stratospheric Warming
At the time of publication, a powerful warming of temperatures is occurring high up in the Earth’s atmosphere. This is called a sudden stratospheric warming (SSW).
This is occurring on the flank of a vast anticlockwise circulation of very cold air, known as the polar vortex. The warming doesn’t play nice with this; it attacks, distorting the shape of the polar vortex. This causes its circulation to become less efficient, slowing it down and reducing its ability to hold onto the very cold air within.
The warming also pushes the polar vortex away from its usual position above the Arctic Ocean. This is known as a polar vortex displacement and is what’s taking place today in the mid-upper stratosphere (6th January 2021; see charts below).
These charts are forecast model predictions for midday 6th January 2021, issued 24 hours prior (so they’re very reliable). The left-hand one is for around halfway up the stratosphere, while the right-hand one is for right down at the bottom. That’s just above the troposphere, which is the layer of the atmosphere we experience the weather within. Notice that at this time, the impacts on the polar vortex are strong at the higher height than the lower one; the impact hasn’t yet propagated down.
During a SSW, the strongest impacts typically manifest in the mid to upper stratosphere (hence we’re looking so high up), then propagate downward to the lower stratosphere. As of today, this hasn’t happened much, but as the next two charts show, it’s expected to become more evident by about ten days from now.
In the circled region, there’s a split of the polar vortex in the mid-stratosphere. This is distinguished by there being two separate circled areas with the very cold air (darker blue colours) in the left-hand map. Such a split can happen if the SSW is strong enough or has two areas of warming on different flanks of the polar vortex.
What we see predicted is only a very small split with little distance between the circulations (some past events moved them nearly ten times further apart!). Even so, the model suggests that some impact is possible lower down – and it’s not the only model to do so.
That impact is to create an anomalous ‘ridge’ in the lower stratosphere. Such a feature promotes the development of high pressure areas beneath it, which in this case is just west and northwest of the UK. In that position, it tends to bring cold air down from the Arctic to affect the UK and Northwest Europe.
If the SSW was the only strong player on the field, this would make for an easy forecast for mid-month; we’d almost certainly see that weather pattern manifest sometime in the period 14th-18th January. As it is, there’s another heavyweight battling for dominance.
The Competition: La Niña
Last autumn, sea temperatures in the eastern half of the tropical Pacific became much cooler than usual. This marked the onset of a La Niña event, which has since persisted through into early 2021.
Though the ocean was fully into this event, the atmosphere was not. This can be seen by looking at a measure known as normalised Global Atmospheric Angular Momentum (GLAAM). Broadly speaking, persistent values of this below -0.5 indicate that the atmosphere has a La Niña setup to it. Yet in Dec 2020, values were largely within a neutral zone; above -0.5 but below +0.5 (above that, there’s an El Niño setup).
This effectively meant that the La Niña event had little say in weather patterns away from the tropics that month.
Recently, though, GLAAM has dropped. You can see that starting at the end of Dec in the left-hand plot below. This trend has continued into early Jan 2021.
So, the atmosphere is responding to the Pacific tropical waters and entering a La Niña setup.
Initially, this transition supports a move of cold air across Europe from the east (see above-right), but typically not on the scale of 2018’s ‘Beast from the East’. This occurs via high pressure from Greenland to Scandinavia, so it would cooperate with the effects of the SSW in mid-January.
I say would, because forecast models predict that GLAAM will have levelled off at weakly negative values (at or below -0.5 but above -1.0) by this time.
This drives a very different weather pattern for Europe and surrounds, as is shown below.
Now, we see high pressure over western Europe with low pressure over Scandinavia. In this setup, mild Atlantic air tends to move along the northern flank of the high pressure, spreading across much of Europe from the west. Near-normal temperatures can lead to a low of snowfall in parts of Scandinavia but elsewhere anything that falls tends to be rain. Prolonged dry weather can occur in western Europe, including southern UK.
This pattern conflicts with what the SSW is expected to try and bring about for mid-January. There’s not much sign that GLAAM will change much for the rest of the month, either (see right-hand chart; an example model forecast out to 9th Feb). So, we could be looking at a lengthy ‘war of the forcing’ on our weather patterns!
When two strongly conflicting forces drive the weather patterns, the outcome tends not to be a blend of the two. Instead, one forcing dominates how things unfold, while the other has a minor influence.
Below, I show an excellent way to visualise the various scenarios that forecast models are currently coming up with for mid-January. In this case, it’s for a run of the Ensemble Prediction System, hosted by the European Centre for Medium Range Weather Forecasting (ECMWF), initialised at midnight 6th Jan 2021.
The red shading indicates areas where high pressure is more likely to occur than usual. The blue shading is the same but for low pressure. The panels each show one ‘cluster’, which is a group of ensemble members – among 50 in total – that have many similar features.
Of the outcomes shown, the two on the left would bring slightly milder and somewhat drier than average conditions to northwest Europe. Cooler and very dry over Iberia. The top-middle one may be similar, but it’s less clear-cut as there is more extension of high pressure northward from Europe – it could be cold and dry for the most part. In the top-left and top-middle scenarios, Scandinavia is cold and snowy, but bottom-left is much milder, also drier in the west.
The remaining two (ignoring the blank one), especially bottom-middle, show cold outcomes for northern Europe in general. There would be a risk of snow widely, perhaps as far northwest as the UK. Given the low pressure to the east, that cold weather could arrive from the east or northeast. This would feel more like 2018’s ‘Beast from the East’ but it wouldn’t really be the same; that legendary event saw very cold air drawn across from way over in Siberia!
So… Which Will It Be?
A sensible answer to that question doesn’t exist at the time of writing. The trouble is who wins this atmospheric argument is likely to come down to very fine margins.
I’m observing forecast model runs in which just a few hundred miles of variation in placement of the lesser polar vortex circulation over the North Atlantic in a week’s time makes the difference between mild and cold weather for Western Europe in mid-January.
When it comes to modelling that covers the whole globe, a few hundred miles is a gnat’s whisker. Typical model uncertainty suggests that we won’t get a reliable prediction until as little as a few days in advance of the period!
What I can say, though, is that something akin to 2018’s ‘Beast from the East’ looks a long shot. In early 2021, it’s more likely that any cold air will be sourced from closer to the east. That doesn’t preclude some very cold conditions, though. A few recent forecast model runs have predicted that it will develop in-situ over western Asia or Northeast Europe.
If at First You Don’t Succeed…
Even if we see the mild outcome for mid-January, it won’t be good cause to let our guard down. SSWs tend to have residual, periodic impacts that can manifest 2-8 weeks after the event. So, it would not be surprising to see further attempts to shift the weather patterns in a cold direction (for the region) in late January and early to mid-February. Be sure to keep your eyes on my Twitter (@peacockreports) for regular updates!
James Peacock MSc
Head Meteorologist at MetSwift