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7th Jan 2020, temperatures in my home country of England widely reached 14 to 15°C. That’s what we consider a mild day in the middle of March!
This is one of many unusually high temperatures observed during a period of exceptional winter warmth across much of Eurasia. This has been especially pronounced since mid-Dec, with some national records falling. Provisionally, the UK saw it’s warmest recorded December temperature; a spring-like 18.7°C at Achfary in Scotland on the 28th.
Cumulative Heating Anomalies
These balmy conditions are evident when considering cumulative heating degree days (CHDDs; Figure 1). This is a measure used to estimate the demand for artificial heating of buildings. The higher it is, the greater supply heating companies will need to plan for… and the more profit they can potentially make.
Figure 1: An approximation of cumulative heating degree days in Nov-Dec 2019 (Figure 1), made using a threshold for building heating requirement of 18°C and the monthly mean temperatures.
As you can see, CHDDS for Nov-Dec are typically 1000+ from around Switzerland and southern Germany eastward. Yet in 2019, you have to look some 2,000 km east from there to find such values.
Away from Scandinavia, there’s essentially been an eastward retraction of the cold winter conditions that usually extend well westward from Asia.
‘Away from Scandinavia, there’s essentially been an eastward retraction of the cold winter conditions that usually extend well westward from Asia.’
The final month of 2019 predominantly featured a weather pattern that drives mild air from the North Atlantic well across Europe from the west (Figure 2).
Figure 2: Map showing the Dec 2019 mean 500 mb geopotential height (a measure relating mostly to air pressure) anomalies (versus 1981-2020 long-term average) across Europe and surrounds. Arrows indicate the resulting anomalous movement of mild air from over the Atlantic Ocean. ‘L’ indicates more low pressure systems than usual and ‘H’ more high pressure areas than usual.
This air also contains a lot of moisture, making for a lot of cloud cover and tending to prevent freezing temperatures.
In fact, it can be hard to find even a touch of frost during such a weather regime!
So, onto the big question – will the patterns tend to remain this way for the rest of the winter?
Tropical Cycles Suggest: As Before in West but Changes in East
7th-21st Jan 2020, an organised area of strong thunderstorms will progress eastward across the equatorial Maritime Continent, then on into the West Pacific. Which is to say, the Madden-Julian Oscillation (MJO) will be active there. An active MJO has a pronounced impact on weather patterns worldwide.
‘An active MJO has a pronounced impact on weather patterns worldwide.’
The MJO location is categorised into 8 phases. A crossing of the Maritime Continent entails the MJO moving between phases 5 and 7. By looking at what patterns occurred when it did this in previous Januarys 1950-2019, we can see what this MJO activity indicates for European weather patterns.
Figure 3: Composite geopotential height (left) and temperature (right) anomaly maps for January MJO activity in phases 4-5 (top) and 5-6 (bottom), for Europe and surrounds. Arrows indicate resulting anomalous movements of air; orange represents warm air, green cool and blue cold.
This suggests most western and central parts of Europe will continue unusually mild (overall) for the rest of January, suppressing heating demands. Alongside this, it becomes increasingly unsettled, potentially leading to a return of flooding risk to the northwest.
For Scandinavia and easternmost Europe (and western Asia), however, the picture generally looks colder than what was seen last Nov-Dec, albeit with a notable exception around Ukraine.
Numerical Weather Prediction Models Suggest: Staying Mild for Most
Computer model forecasts for weather patterns in the coming weeks have been very consistent lately. Day after day, they send a strong ‘mostly mild’ message for Europe.
In fact, temperatures look to be exceptionally far above the long-term average (1981-2010) for many north-eastern parts (and north-western Asia).
‘…temperatures look to be exceptionally far above the long-term average (1981-2010) for many north-eastern parts [of Europe]’
Figure 4: The global ensemble forecast system (GEFS) means of geopotential height anomaly (left) and temperature anomaly (right), for the 5 days 14th to 19th Jan 2020, for the run initiated at midnight GMT on 10th Jan 2020. Sourced from tropicaltidbits.com. Arrows again indicate anomalous movement of mild Atlantic air.
The overall weather pattern being forecast has many similarities to the MJO phase 4-5 composite, but with one major discrepancy; how far east the westerly regime reaches.
They predict that the mild, moist Atlantic air will frequently be driven all the way to north-western Asia.
So, what could be behind these particularly far-reaching maritime airflows?
The Powerful Polar Vortex
Weather patterns during the months of Oct-Mar are strongly influenced by a large-scale circulation called the polar vortex*.
Looking at what went on during the historical years since 1950 with the ten strongest polar vortexes (Figure 5), the consequences for Europe are clear.
Figure 5: Composite geopotential height (left) and temperature (right) anomaly maps for the ten years with the strongest Jan-Feb mean zonal wind speed at 30 hPa; a good measure of the polar vortex strength (faster wind, stronger vortex).
North Atlantic westerlies reach well eastward across Europe.
Low pressure systems travel west-east on a path that passes near or over Scotland and then continues into Scandinavia. High pressure is prevalent across much of mainland Europe, bringing an abundance of dry weather with anomalously mild temperatures France-Belarus northward.
‘…an abundance of dry weather with anomalously mild temperatures France-Belarus northward.’
Based on this, it’s hard to see the Nov-Jan CHDDs being anything other than below-normal for all but Southeast Europe and Iberia.
Further Ahead: February
By early Feb 2020, there are signs that the MJO could be active in phases 6-7 (composite Figure 6), while the polar vortex will likely be winding down from a ferocious mid-Jan peak.
Figure 6: Composite geopotential height (left) and temperature (right) anomaly maps for February MJO activity in phases 6-7, for Europe and surrounds. Arrows indicate resulting anomalous (relative to where it’s going) movements of air; orange represents warm air, green cool and blue cold.
This may allow weather patterns to become less ‘flat’, with weaker westerlies from the North Atlantic.
The MJO activity promotes a southward shift in the path of low pressure systems, but with a tendency for them to become slow-moving to the west of Iberia, sometimes overlapping the region.
As a result, the temperature signal is anomalously high for most of the continent.
Iberia makes for a notable exception – this and the far-southeast of Europe are the areas in which the Nov-Feb CHDDs look most capable of being near or above normal.
‘…the far-southeast of Europe are the areas in which the Nov-Feb CHDDs look most capable of being near or above normal.’
Elsewhere, 2019-20 could prove to be a very mild winter overall. Much may depend on the second half of February, by which time the MJO looks inactive but the polar vortex may be weak enough to allow for some colder weather – even if it’s just some frosty nights!
James Peacock MSc
Head Meteorologist at MetSwift
* Broadly speaking, the polar vortex is an anticlockwise rotation of air that extends through a vast depth of atmosphere. It’s usually strongest in the stratosphere – the layer of the atmosphere above the one in which we reside; the troposphere. It spends most of its time over the Arctic region.
That strength varies a great deal depending on how much other weather patterns interfere with it. The stronger it is, the more weather patterns across regions peripheral to the Arctic, such as Europe and the United States, tend to be driven by it, accelerating their eastward movement.
The resulting increased eastward inertia reduces the extent to which airmasses move north or south. Meteorologists refer to this is a ‘flat’ weather pattern.