A slightly colder than average winter is indicated in the UK with plenty of high pressure. Prolonged spells of cold and snowy weather will be present at many times across central & eastern Europe but only occasionally might these advect west into NW Europe and parts of the UK – late January into February
Hello and thanks for reading. This forecast covers astronomical winter from December 21st 2021 to March 20th 2022
Long range (LR) forecasting is experimental. No LR forecast can tell you when it will snow, nor can it tell you weeks in advance when a two day northerly cold spell will strike. But assuming the key drivers are correctly identified and enough weighting is given to factors that may well skew prolonged cold chances in Europe, then this forecast will try to pick out periods of winter when prolonged cold is feasible. It won’t be until later down the road, during winter, that we will truly be able to ascertain the extent of any cold air advection to the UK, and of course any subsequent snow risk.
As I’ve done since the first forecast in December 2013, I will lay down the key drivers with a brief explanation. Each section is then scored in an attempt to clearly demonstrate the influence each driver has on cold winter weather prospects. The forecast then follows the conclusions at the end.
As usual, no seasonal model guidance has been factored into this forecast.
If you want to skip the long technical part, please scroll down to the conclusions and then the forecast below that.
1. The stratosphere (polar vortex), QBO, solar activity & ozone.
Before we get into this, I will briefly explain the significance of the relationship between the troposphere and stratosphere and the implications for winter weather.
Each winter the stratosphere cools significantly. The difference in temperatures between the Arctic and warmer latitudes further south results in the formation of a strong area of low pressure, called the stratospheric polar vortex. Below this is the tropospheric polar vortex (the area of low pressure located around the north pole that we see on weather charts). The colder the stratosphere is, the tighter/stronger the stratospheric vortex becomes and consequently, the tropospheric vortex too. The strength/position of the tropospheric vortex influences the AO (Arctic Oscillation), which is a measure of pressure between the north pole and the Azores, as shown in the image below, courtesy of NASA.
A stronger vortex can result in a positive Arctic Oscillation which, for Europe, equates to a less cold winter. The opposite is the case with a weaker vortex.
Sometimes, during winter, a strong vortex is put under pressure, warming it and displacing cold air from the pole down to the mid latitudes. Rossby/planetary waves circumnavigate the globe and during winter, when a powerful large wave encounters a mountain range (eg. Himalayas, Rockies, Andes), if the wave is large enough, some energy is deflected poleward (mountain torque). It needs to be a sizeable deflection to achieve this, but these waves can penetrate into the stratosphere, creating a warming disruption to the otherwise usually cold and stable wintertime stratospheric environment (think of the sea-shore where waves break all the time but only the strongest will push more inland resulting in coastal erosion).
There are two main types of disruption to the vortex via this process; a displaced vortex, where wave breaking and consequent warming moves the core of the vortex away from the pole; or a split vortex where the vortex is put under even more pressure and is split in two. These events are often referred to as sudden stratospheric warmings (SSW) where the zonal winds at 60N/10HPA are reversed from westerly to easterly. In both cases, the warming and movement of the vortex, pushes cold air into the middle latitudes as higher pressure builds over more northern latitudes. Generally speaking the number of the wave (1,2,3) refers to the number of waves at that time. Wave 1 usually displaces, and strong wave 2 can cause splits. Following a displacement, if wave activity subsides, it is common for the vortex to fairly rapidly regroup and cool. Following a split, if wave activity wanes, it can take much longer for the vortex to recover and regroup. So a split vortex is the ideal scenario we are looking for and historically the UK has benefited more in terms of prolonged cold weather from splits rather than displacements. Split vortexes can also lead to faster response at the surface.
1.1 The QBO (Quasi-biennial oscillation)
The QBO is a measure of wind flow across the equator high up in the stratosphere (measured at 30mb). There are two phases of the QBO, east and west – referring to the direction of those winds. These cycles or phases last roughly 18 months or so. The QBO has a significant effect on the state of the polar stratosphere during wintertime, and is therefore of much interest for the winter forecast.
An east phase QBO started ascending through the stratosphere earlier this year and has continued to upwell and strengthen since. By November, we have easterly flow (E QBO) at all but the very top and very bottom of the stratosphere. This east phase will continue through the winter.
The two images depict nicely how E QBO years usually feature warmer temperatures over the polar stratosphere and colder than usual temperatures across the tropical stratosphere, indicative of enhanced BDC (see ozone section 1.3 below for more on this).
Possible implications of E QBO for UK winter 2021/22?
In isolation, this makes a winter featuring a weakened polar vortex and a negative AO more likely during the upcoming winter and therefore scores a goal for winter
(Winter 1 Anti winter 0)
1.2 Solar Activity
After the recent solar minimum in 2019, we’re into a new solar cycle and gradually seeing solar activity increase, ahead of the next solar maximum in 2025.
It’s a slow process and we are still seeing fairly low solar activity which will continue through the winter, albeit with some inevitable spikes.
Labitzke et al have published several papers alluding to 10.7cm solar flux relationship with the stratosphere and northern hemisphere winters, specifically drawing attention to an increased incidence of SSW occurrence in east QBO winters when the 10.7cm flux is around or below 110 units.
The image below (Labitzke & Kunze) shows this relationship between solar flux, the qbo and SSW’s clearly, with nearly double the SSW occurrence during an east phase QBO with solar flux below 110 units vs above.
Implications on winter 21/22?
Whilst spikes (& dips) are inevitable, the expectation for the winter is for flux levels to bump around the 70-90 mark. This, allied with the EQBO phase suggests a slightly elevated chance of seeing a major mid winter warming in the stratosphere (SSW). We should, however, note that it is the spikes that will likely have the effect of polar vortex intensification, which could well result in milder spells at times. (Winter 2 Anti Winter 1)
1.3 Ozone/BDC (Brewer-Dobson Circulation)
Levels of ozone concentrations are also known to have an influence on stratospheric temperatures. The BDC refers to the transport of ozone from tropics to pole during Autumn into the winter. As mentioned earlier, the QBO plays a role here too. During east phases, the tropical stratosphere is cooler than average and the polar stratosphere warmer, because higher levels of ozone release heat into the surrounding air, sharpening the thermal gradient, reducing polar westerlies which in turn can lead to a weakened polar vortex.
Monitoring tropical stratospheric temperatures during the Autumn can give us an indication as to how strong the BDC is, as shown in the image below
We note that through Autumn, temperatures were largely below average, indicative of a fairly robust BDC.
Implications of BDC on winter 21/22?
An active BDC from Autumn into the winter bodes well for northern blocking in the latter half of winter.
(Winter 3 Anti Winter 1)
2. ENSO (El Nino Southern Oscillation)
Enso refers to the warm and cold phases of the waters along the equatorial Pacific. Warm being El Nino, cold phases are La Nina.
Climate influences of the warm and cold phases of ENSO have more pronounced implications on weather in the tropical regions, but do still influence weather patterns globally. Strong ENSO events can see a more direct tropospheric influence on European weather patterns (like the very strong EL Nino a couple of years back), but the effects of weaker or neutral ENSO events in Europe are smaller and can often be overridden by stronger signals elsewhere.
However, this years La Nina, albeit quite weak, has seen strong atmosphere/ocean coupling and a very classic La Nina atmospheric pattern playing out over summer, Autumn and into early winter. Through December into January , this becomes ever more crucial to cold weather prospects with a modest chance of continued westerly inertia (El Nino-esque) being added to global budget, in turn limiting the traditional European winter time course of a La Nina event – namely colder start, milder and wetter end. Uptick in wave 2 activity is expected as we move through January, which should help keep pressure on the polar vortex.
The 2015/16 forecast talked in detail about tropospheric vs stratospheric pathways influencing European/Eurasian weather patterns, and if you like, we can say a more typical tropospheric pathway is currently playing out, but into January and February, this is likely to weaken, whilst the stratospheric pathway becomes a key driver for European weather.
Reanalysis of similar east based La Nina events (albeit small sample), yield some interesting results and give some credence to the idea of sustaining cold pattern for Europe into January & February.
It’s interesting to note that comparison of La Nina analogues with cooler waters in the easternmost part of ENSO region (regions 1+2 & 3) vs further west in regions 3.4 & 4 yield notable signal for high latitude blocking in the Norwegian Sea across to Iceland & Greenland with notable European trough (-NAO). This is in contrast to analogues with coolest waters in regions 3.4 & 4 vs regions 1+2 & 3 which demonstrate a less convincing -NAO signature
Implications on winter 21/22?
Based on the assumptions on the atmosphere/ocean evolution over the next 4-5 weeks, we can give this another goal to winter. However, as described, there is a small risk that easterly inertia rules the roost, and the scoring system is meant to identify the key factors that pose the biggest threat to cold weather chances and therefore this section has to be marked a score draw.
(Winter 4 Anti winter 2)
2.1 MJO (Madden-Julian Oscillation), global winds & momentum
Through Autumn we have seen waves through the Maritime Continent into western Pacific. During December, we are currently seeing a robust wave propagating through central and western Pacific. As mentioned in the ENSO section, this is destructively interfering with La Nina, adding westerly inertia to the system, effectively keeping La Nina responses more muted & aiding the more amplified Atlantic profile (and giving models a real headache regarding Christmas to New Year weather in nw Europe). A key assumption in this forecast is for continued eastward propagating tropical waves emerging from East Indian Ocean and across the Pacific late December and through January with subsequent torque feedback.
(Winter 5 Anti winter 2)
2.2 PDO (Pacific Decadal Oscillation)
The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. The phases of the PDO are known to influence global sea level pressure patterns, amongst others. Historically, the UK’s coldest winters have coincided with low solar activity and cool phase of the PDO. There are exceptions, but the general theme is consistent.
A cool PDO and low solar output certainly do not guarantee cold winters, but when combined with other variables (eg. east QBO, low sea ice, rapid Oct snow cover/advance etc) they can increase the chances of a negative Arctic Oscillation/NAO and a cold winter for Europe.
The phases last roughly 20-30 years. Despite the main phases of the PDO, there are still
cool PDO years within warm phases and vice versa. We are currently in a cool phase which we entered in the latter half of 2007, albeit 2014 – 2020 saw a +PDO dominate the northern Pacific. However, since summer 2021, we are seeing one of the most emphatic -PDO’s seen for many years. A -PDO often aids wave 2 activity which combined with the ENSO phase, EQBO and solar activity, increases the chances of significant stress on the polar vortex in mid winter (SSW).
(Winter 6 Anti Winter 2 Final score)
Conclusions and summary
- A weak tropospheric and stratospheric polar vortex is likely to be a theme throughout the majority of winter
- Some PV intensification is likely at times, which may well result in a milder, zonal period of weather for western/nw Europe, especially if allied with expected spike in solar activity. But this/any milder phase is currently seen as transitional.
- In fact this PV intensification may well come just prior to a SSW later in January, with chances of SSW looking moderate to high late Jan into Feb.
- The score of 6-2 suggests a very good chance of prolonged cold in parts of Europe. But it won’t be until nearer the time that we see if and when the UK can tap into what is expected to be some very cold air sitting over Europe through January and much of February.
- SSW or not, continued stratospheric disruption is likely from mid January through to the end of winter/early Spring.
- Without the SSW, it will be cold shots rather than sustained cold for w/nw Europe
- A cold winter with repeated cold spells/sustained cold spells for Northern, Central and Eastern Europe is very likely, high confidence.
- At times Western/NW Europe (UK/France) may tap into this cold air
- There will be plenty of high pressure across west/nw europe
21st December – 20th January
Frequent cold spells for central, northern and eastern Europe, away from milder SW Europe. Further west into NW Europe more of a mixed bag with Atlantic lows moving in at times, bringing a snow risk along the boundary between milder and colder air. Quite often, this pattern wont be cold enough for snow to low levels, instead favouring higher ground. There will also be spells of high pressure bringing fine, settled, frosty and cold weather into the UK, especially southern uk, via an easterly or se’ly feed
January 21st – February 19th
NOTE: Charts & forecast are based on significant stratospheric disruption through January, culminating in mid winter SSW. Blocking expected to be most prevalent in the Norwegian Sea, Iceland & Greenland areas initially, perhaps drifting east closer to Scandinavia later.
If we do not see the anticipated stratospheric disruption and SSW, then milder weather is more likely, especially across NW and western Europe with stormy weather possible too. This will be something to monitor nearer the time
February 20th – March 20th
Being furthest away, always the trickiest part of a long range forecast. Whilst the blocking does seem to persist, the concern is that a cold period for eastern usa and lack of Atlantic block, could allow for deep low pressure systems to track towards the uk bringing stormy weather to west/NW Europe. Not a particularly mild pattern, but not a cold and snowy pattern either. On the flip side of this, should stratospheric disruption anticipated in late January/early February end up being delayed, then this period could be colder than the chart below suggests. But that’s something to monitor further down the line
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