La Nina, MJO May Have Delayed But Not Denied Sudden Strat Warming & Cold Response Across Northern Hemisphere

Winter 2020-21 will likely go down as a cold winter overall for the Northern UK, Northern Europe, Scandinavia, Russia, Mongolia and northern China. Warm across Southern Europe and perhaps Canada and even the USA despite the incoming cold now.

For the UK, despite a warmer than normal December, January was coldest since 2010 (but by no means extreme) and February’s first half could go down as a top 5 coldest of the last 50 years.

January 2021 was cold but unsettled thanks to a ‘weak’ stratospheric and tropospheric polar vortex producing a -AO but +NAO. While leaning cold with the -AO, there was still a pronounced westerly flow across Pacific and Atlantic but a more southerly displaced jet stream often kept the UK on the cold side of the jet. The UK found itself in the battleground between Atlantic (+NAO) and Arctic (-AO) with cold mainly winning over Scotland. For England, Wales and Northern Ireland there was more back and forth.

The weak vortex of December finally gave way to a major sudden stratospheric warming early January at 10hpa, this would prove significant in February.


Typically, the downward propagation of energy from top of the stratosphere down into the troposphere occurs 10-15-days after the SSWE. I had a cold mid to late January based on this rule. However, the response was delayed and seemed like it may be denied. A failed SSWE seemed likely to me by January 15-20 when Europe ‘should have turned very cold’ it in fact warmed.

However, by late January, the response finally came. This delay but not denial may have been caused by the masking influence of the strong La Nina and or MJO but forced on perhaps by the influence of the solar minimum (see winter forecast).

In the below cross section of the polar vortex, note the strong warming early January caused by the SSWE but note the strong responsive blocking didn’t truly reach the troposphere until the beginning of February. Perhaps a tug of war between La Nina and the solar minimum?

Sure, it’s been a strongly negative AO winter but without winds reversing within the lower atmosphere, the Atlantic never truly shuts off. Then came the block we had all been waiting for with positive heights extending south from Greenland into the mid North Atlantic.


January 2021 temp anomaly

Credit: Michael Ventrice

First 13 days of February 2021

Credit: Michael Ventrice

As forecast, the snow pack grew and the polar air ran the tracks all the way to the UK.

Cold, dry east winds blowing over the relatively warm North Sea produced significant ‘sea effect’ snowfall up and down the eastern side of the UK.

Several places in the east and within land ‘gaps’ such as through the Dornoch and Cromarty Firths and Central Lowlands measured 6-14 inches of snow even to sea level.

Similar to scenes back in 2010, snow covered most of Scotland and large swathes of central and eastern England. The stage was set for cold unseen in 25 years, comfortably surpassing any ‘air temps’ observed during the beast in 2018 which was extreme for low daytime temps and wind chill values. It even beat the cold of 2000-01 and of course 2009-10 and December 2010.

Credit: NASA

The bitter easterlies shut down and skies cleared out thanks to high pressure descending south.


With the deep snow cover and cold, dry continental air in place Britain witnessed it’s coldest night since 1995 and coldest night for February since 1955.

Persistent snow blowing in on stinging east winds drove the snow depth to a potentially record breaking 70cm in Braemar, Aberdeenshire.

Credit: @Alonso2012F

No wonder, it was Braemar which also took the prize for UK’s lowest temperature of -23C.

Several communities, particularly in the north shivered through their coldest February night on record.

From Sean Batty

Aboyne, Braemar, Gogarbank, Bishopton, Lossiemouth, Strathallan Airfield & Tain all look like they’ve had their coldest Feb night on record. Most have been recording 20-25 years. Most significant is Lossiemouth where this is coldest Feb night for over 67 years with low of -15.2C!

The effects of the weak PV and SSWE impacted the entire Northern Hemisphere. After a seemingly unprecedented summer of heat and fire, Siberia has endured an unusually long and cold winter even by their standards, much of China and Mongolia has suffered long periods with temperatures below -40C. Even the Chinese capital, Beijing observed it’s coldest night since 1967. Record snow was followed by record cold in Japan and the Koreas.

While it’s been mild across Canada and the parts of the US, record cold has now arrived here.

The city’s of Saskatoon, Regina and Edmonton have all exceeding lows of -40C with some communities between -45 and -51C. Coronach, SK fell to -46.5C (-52F) setting a new all-time record while Uranium City, SK tied their’s with -48.9C (-56F).

The core of North America’s cold is now descending southwards over the US.

Credit: Tropical Tidbits

The Atlantic winds return to Western Europe forcing the cold east with Greece and Turkey’s turn to shiver.

Credit: Tropical Tidbits

After shivering much of northern Asia and Europe, the finale of the SSWE focuses on North America. However, the upper stratospheric polar vortex is regrouping, strengthening. The question is could this declare winter over?

Not according to the CFSv2 for Europe for March.

Credit: Tropical Tidbits

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