Winter 2020/21

Hello & thanks for reading. The forecast is pretty much in the same format as it’s been since the first one in 2013, but this year there is a new forecast summary & conclusions at the beginning, below this paragraph. The usual forecast charts are at the end of the document

Forecast summary & conclusions UK (for rest of Europe, please see the forecast charts at the bottom)

For fans of prolonged, well below average temperatures and snow, things do not look great. But for those who have more realistic winter expectations, it’s not all bad news, far from it. Firstly, stormy winter conditions seem unlikely due to the relative dominance of high pressure. Secondly, if high pressure results in cold air sitting over parts of Europe, then we could certainly see some battleground snow events at times. There will also be spells of unsettled weather bringing air from the NW & north which may bring some snow – especially, but not exclusively, to higher ground

There is likely to be plenty of high pressure around, interspersed with spells of unsettled weather during periods of Atlantic amplification, as described in the ENSO section in the write up below. Precipitation is likely to be below average overall (DJF). Temperatures are hard to call and will often depend on orientation of high pressure cells. More likely than not ending up around average for the three winter months. There is likely to be a higher than average number of air frosts. Away from the unsettled spells which may bring marginal & extremely finely balanced snow risk to some areas as described above, the chances of more prolonged cold and snowy weather rest upon stratospheric influences (SSW). The chance of SSW seems rather low – but not entirely ruled out, as described in the Solar and ENSO sections below. Of course, should we see a SSW and the downwell into troposphere is favourable, then there will certainly be a prolonged cold spell in parts of Europe. But given the small chance of this occurring, a SSW is not factored into the forecast and instead will be something to monitor over the course of winter.

Technical

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 hit the UK. 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 that each driver has on cold winter weather prospects (the scores are more focused on UK). The forecast then follows the conclusions at the end.

As usual, no seasonal model guidance has been factored into this forecast.

1. The stratosphere (polar vortex), QBO, solar activity & ozone.

atmosphere

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.

ao infographic

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.

We started 2020 with a east QBO filtering through most levels of the stratosphere. The expectation was that the east QBO would be present throughout 2020, but as we saw in 2016, a west QBO has emerged through the year and will remain in place for the duration of winter.

wave example
wqboEG
eqboEG

Possible implications of West QBO for UK winter 2020/21?

A west QBO gives that bit more credence to the idea of vortex strengthening further into winter vs east phase. (Winter 0 Anti Winter 1)

1.2 Solar Activity

ises-solar-cycle-sunspot

After the recent solar minimum of cycle 24, we are now moving in to solar cycle 25 which is expected to peak in 2024 or 2025 (so we have a few stormy and mild winters coming up over the next few years, but probably not just yet).

ises-solar-cycle-sunspot (1)

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 west QBO winters when the 10.7cm flux is above 110 units.

The image below (Labitzke & Kunze) shows this relationship between solar flux, the qbo and SSW’s clearly. In the sample of 65 years (1942-2007), there was only ONE (two, see text below image) SSW during a west QBO winter when flux was below 110 units (February 2007).

qbo

Incidentally, there was another in early February 2009 which is not covered in this graphs dataset, with flux around 75 units. The key point is between 1942-2007 there were 11 SSW’s during west QBO winters with flux above 110 (and 10 of those, the flux was above 150 units).

For much of this year, solar flux has been steadily hovering around the 70 mark. But since the end of October, we have seen quite significant activity, with the most recent spike taking us close to 110 units – the highest for a few years now.  This is undoubtedly the first sign of the new solar cycle kicking off.  The expectation is for further solar activity (flares, solar storms etc) to continue over the course of winter (and, of course, beyond).  That could actually work in favour of a SSW in the second half of winter but it’s very hard to predict the exact levels we will see during the winter, apart from to say that solar activity will continue to be more active than it has been for the bulk of 2020.  

Implications on winter 20/21?

Solar flux & W QBO relationship suggests less chance (but not no chance) of mid winter SSW. Despite a likelihood that we will continue to see spikes in solar activity, they will more than likely average out at lower than 150 units until later in 2021 so we have to score this section to anti winter

(Winter 0 Anti Winter 2)

1.3 Ozone/BDC (Brewer-Dobson Circulation)

OMPS-NPP_NMTO3-L3-DAILY-Ozone-Image_v2.1_2020m1123_2020m1125t012034

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. The phase of the QBO is also influential here. 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 in January/February.
During W QBO phases, a generally weaker BDC is more typical, with ozone concentrations tending to linger in the mid latitudes. To ascertain how active the BDC is, we therefore monitor tropical stratospheric temperatures through the autumn. The image below shows the zonal mean temperature across the tropics at 30HPA. We see that since the end of September/early October, temperatures in this region have been around average which is fairly typical in a west QBO pattern.

This suggests that ozone transport via the BDC is average to weak this Autumn.

bdc

Implications of BDC on winter 20/21?

This average to weak BDC is not what we want to see at this stage with regards to blocking features in Jan/Feb so it’s another score to anti winter

(Winter 0 Anti Winter 3)

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. The effects of weaker or neutral ENSO events in Europe are smaller and can often be overridden by stronger signals elsewhere.

sst

We have a firm La Nina state this winter. When La Nina and atmosphere are coupled, we see increased easterly wind flow which has the effect of agitating mid latitude ridges. We will continue to see the waxing and waning (but increasingly waxing) of this coupled environment through winter. It is therefore likely that we will see high pressure dominating SW Europe and ridging further north and east into Europe at times during periods of low angular momentum (La Nina signature). It will only be via feedback processes involving stratospheric disruption/forcing in the troposphere that a more amplified Pacific and Atlantic profile may be attained. This having the effect of weakening the influence of Azores HP over SW (west) Europe, tilting on a more NW/SE axis.

Scoring this section, we have to focus on La Nina which is likely to result in plenty of high pressure with only occasional cold spells for some regions. Yes, things will look colder for Europe if we manage some stratospheric influences like SSW etc. resulting in weaker polar vortex but scoring ENSO section in isolation means another point for anti winter

Winter 0 Anti winter 4

3. Snow cover

Cohen et al’s work on snow cover/extent is well documented.
To summarise, the rate of Eurasian snow cover during October is linked to upper and surface weather patterns during winter, particularly January. During Octobers where snow cover rate is higher than average, distinct feedback mechanisms are observed over the course of the following 3 months.

High snow cover increases diabatic cooling aiding formation of a strong Siberian high pressure cell, which fluxes energy poleward into lower stratosphere. This energy flux (see stratosphere and waves section for more), disrupts the stable stratosphere, causing strong stratospheric warming which then feeds back to the troposphere in the form of high pressure at northern latitudes, pushing the jet stream south. The high pressure/warmer air at higher latitudes effectively squeezes the cold air locked over the pole south towards mid latitudes. Resulting in a typical -AO pattern. The following infographic (Cohen, 2014) explains the processes and feedback mechanisms involved.

snowcover_infographic

Reanalysis of high snow extent October years reveals high pressure at northern latitudes to be a distinct theme and January in particular showing a strong -AO, as shown below:

high snow

snowextent

October 2020 saw well above average snow extent in the second half of the month before dropping off in the last few days of the month into early November

3.1 Sea Ice

Reduced Arctic sea ice, primarily in August & September, influences tropospheric and stratospheric circulations and has been linked to a propensity for -AO conditions in the following January (Overland and Wang 2010).
Cohen, Jones, Furtado & Tziperman have linked this sea ice loss in late summer to increased snow cover in October.
This years sea ice is well below average.
For the purposes of this forecast, snow cover and sea ice extent are scored together.

Final score Winter 1 Anti winter 4

Forecast charts

December into January

Confidence is quite low throughout this very tricky seasonal forecast. These charts are quite self explanatory, along with the forecast summary at the top of the page.

decemberchart

feb chart

February

Should we see a SSW in January, this forecast will need to be changed and trend colder.

But as identified in the write up, SSW chances look low this season, and more than likely will come down to what extent the polar vortex has strengthened before the crux of stratospheric disruption occurs in January.

febchart

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