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European Nov-Dec 2021: Are Comparisons to 2010 Warranted?

18th October 2021

Late 2010, northern Europe was plunged into a deep freeze of an intensity rarely seen in the first month of winter. In the UK, it was the coldest December since 1890.

There was widespread snowfall, heavy in places. Parts of Southeast England saw over a foot of level snow, extraordinary by recent standards. More than half of Germany saw record high snowfall for December.

Travel disruption occurred widely, but especially in lowland UK, where persistent winter snowfall is so uncommon that snowploughs are scarce, and few people possess winter tyres or snow chains. There were also lengthy school closures, and some businesses were unable to operate normally due to staff shortages.

It was a stark reminder that a warming climate doesn’t mean an end to extreme cold weather; it only makes it less frequent.

Looking Back to Look Forward

In the past few weeks, there has been increasing discussion of late 2010 on social media. It’s been proposed as a good source of guidance for how weather patterns could evolve in late 2021. This stems from the recent and expected behaviour of several major driving forces of weather patterns (teleconnections), which as shown below, matches well with 2010.

Table showing recent and expected behaviour of several major driving forces of weather patterns.

Niño 3.4 is a measure of sea surface temperature (SST) anomalies in the central-eastern tropical Pacific Ocean. The Pacific Decadal Oscillation (PDO) is based on SST patterns in the northern Pacific Ocean. The Indian Ocean Dipole (IOD) is based on the relative warmth of the western versus eastern halves of the Indian Ocean. The sunspot count is a measure of the 10-11 year solar cycle.

Both years feature a La Niña event (Niño 3.4 of -0.5 or lower), negative PDO (-0.5 or lower), neutral or negative IOD, and low monthly sunspot counts. On this basis, you might expect northern and western Europe to be in for a major cold spell sometime this Nov-Dec… but that would overlook one key player: The North Atlantic Ocean.

Oceanic Role: Sea Surface Temperature ‘Tripole’

Research by Buchan et al. (Feb 2014) identified the North Atlantic sea surface temperature (SST) pattern to be a crucial driver of the extreme Nov-Dec 2010 weather.

Specifically, a ‘tripole’ pattern: Unusually warm SSTs in the north and south sandwiching anomalously cool SSTs between. By changing the temperature gradient of the North Atlantic, this caused typical path of Atlantic storm systems to shift well south of usual, facilitating the observed cold weather patterns. They were associated with a very negative state to something called the North Atlantic Oscillation (NAO).

As can be seen below-left, the tripole pattern was already in place in October 2010.

When we look at October 2021 to date, the pattern is almost the reverse. This is a big deal for the probability of a very negative NAO this Nov-Dec.

Maps showing the average sea surface temperature anomaly plots for October 2010 (left) and 1st-15th October 2021 (right).

Average sea surface temperature anomaly plots for October 2010 (left) and 1st-15th October 2021 (right).

Re-emergence

You may ask, could the SST pattern become more similar by Dec? To gauge how likely that is, we can look back to last winter. You see, strong SST anomalies in winter can extend well below the surface, giving them great staying power. Even if the SSTs change a lot during spring through autumn (which they often do), the pattern of the preceding winter can persist beneath the surface.

This pattern can then be mixed back up to the surface by Atlantic storm systems during the following Oct-Dec. This is known as ‘re-emergence’ (see Alexander and Deser, 1995 for a deeper dive).

Comparing winter 2009-10 (left) with 2020-21 (right) reveals less of a disparity than seen earlier:

Maps showing the average sea surface temperature anomaly plots for Nov-Feb in each of 2010 (left) and 2021 (right).

Average sea surface temperature anomaly plots for Nov-Feb in each of 2010 (left) and 2021 (right).

Both saw very positive SST anomalies in Baffin Bay (west of Greenland), with some anomalous warmth north of Iceland too. However, there are significant differences elsewhere.

2009-2010 was unusually cool west of the Azores, while 2020-21 was warmer than average. It also saw unusually low SSTs to the west of the UK, south of Iceland, which weren’t present in 2009-2010.

So, while the earlier winter had a tripole pattern, the later one did not.

Stratospheric Role

What brought about the tripole pattern in the first place? To answer that, we need to look way up above the ocean surface, into the stratosphere. Specifically, at a type of event known as a ‘sudden stratospheric warming’ (SSW).

For reasons beyond the scope of this blog, sudden stratospheric warming events lead to a period of 2-8 weeks with an increased likelihood that the Arctic region will see strong, persistent areas of high pressure. Sometimes, a very negative NAO is also supported.

SSWs are defined by strict criteria, which usually need to be met for long-lasting impacts on weather patterns to occur. Dec 2009 was an exception: Events fell a little short of SSW standards, yet impacts lasted several weeks, from late Dec to 2nd week of Jan 2010.

A Negative NAO Setting Up a Negative NAO

As shown below, high pressure (orange-red shading) was unusually dominant across the Arctic latitudes and around Iceland. Meanwhile, low pressure systems (blue-purple shading) were travelling well south of normal. The NAO was very negative.

Map showing the average sea-level pressure anomaly for 28th Dec 2009 to 14th January 2010.

Average sea-level pressure anomaly for 28th Dec 2009 to 14th January 2010.

The altered path of low pressure systems (lows) has a big impact on the SSTs. Lows churn up the ocean, driving relatively warm surface water downward, and drawing colder subsurface water upward.

So, a southward shift in storm track reduces this mixing in the northern North Atlantic and increases it in the central slice.  In Jan-Feb 2009, this gave rise to the warm north, cool central North Atlantic SST pattern.

So it was that a strongly negative NAO in January set the stage for a strongly negative NAO 11-12 months later!

Easy Forecasting?

I expect many readers are wondering whether this means we can easily predict a very negative NAO about a year in advance.

Sadly, my response must be “if only it was that simple!”. The process of re-emergence is not dependable. It relies on abundant storminess across the northern half of the North Atlantic during Oct-Nov. When it’s unusually quiet, surface mixing is too weak to bring sub-surface temperature anomalies back to the surface strongly enough for a significant impact.

This Year: Probably Up to the Stratosphere

In conclusion, it’s unlikely that North Atlantic SSTs will force a period of strongly negative NAO sometime this Nov-Dec. If anything, the setup may be more conducive to the opposite – a positive NAO, bringing predominantly mild weather to northern Europe.

However, as detailed in my late September blog, guidance tools have been dropping hints that a SSW or near-SSW might occur. The La Niña and negative PDO combination may also be conducive to a negative NAO. Comparisons with 2010 do have some merit.

So, there is some scope for some significantly cold spells in northern Europe in late 2021, provided the North Atlantic doesn’t rule the roost.

 

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo, taken in Oxford, England on Dec 20th, 2010  by Juan Pablo Garnham, is licensed under CC BY-NC-ND

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