Special recap of winter 2020/21

September 17, 2024

Below I try to weave a narrative of winter 2020/21.  The predominant thinking in the field is that sequential weather or synoptic events are random and unrelated known as chaos theory.  However I have tried to show how different events throughout the winter are related and one follows the next more akin to a domino effect and events at the beginning of the winter are responsible for evets at the end of the winter.  I feel that this is a unique perspective on the winter season that can only be accomplished through troposphere-stratosphere coupling and involvement of the polar vortex.  Both the hemispheric weather and the polar vortex showed extreme variability this winter and an in-depth understanding of the variability of each is impossible without the other.  Such an explanation of the extreme variability is much more difficult using ENSO, which was weak most of the winter and not surprisingly ENSO based forecasts performed poorly. 

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Summary

• Winter 2020/21 was characterized by some of the most extensive cold weather in recent years across the mid-latitudes continents including the United States (US), Western Canada and Asia but especially Siberia.  In contrast it was relatively warm across Eastern Canada, New England and the Arctic including Greenland and the Barents-Kara Seas. 
• The phase of the El Niño/Southern Oscillation (ENSO) was cold or negative and hence La Niña.  ENSO may have been factor that winter.
• October Siberian snow cover advance was relatively anemic all month relative to recent years, but October Siberian snow cover extent (SCE) still managed to finish above normal.  As I have argued much of my career above normal SCE in the fall across Siberia is favorable for disrupting the polar vortex. October SCE across North America was a record high mostly due to a rare strong early season stretched polar vortex (PV) event that was a foreshadowing of the upcoming winter.
• Arctic sea ice was below normal during the fall especially in the Barents-Kara seas. This pattern of sea ice anomalies is thought to be conducive to disrupting PV.
• The PV weakened in early January and remained in this weakened state through early February.  This was an unusually extended period of PV weakness.  This was followed by a rapidly strengthening PV in late February and the PV remained exceptionally strong up until the Final Warming at the end of April.
• Overall, during the winter, the PV was weak except at the very beginning and at the end of the winter season.  The weak PV was coupled with a negative Arctic Oscillation (AO) in January but less so in December and February.  The pattern of Northern Hemisphere (NH) surface temperature anomalies was consistent with a weak PV and negative AO especially across Siberia and contributed to one of overall coldest winters across the NH continents in recent years.
• The AER winter forecast which incorporates Arctic predictors performed exceptionally well in winter 2020/21.

Winter Forecasts

The climate community focuses on El Niño/Southern Oscillation or ENSO in making seasonal forecasts and a La Niña event was predicted by the models for winter 2020/21 and therefore ENSO was a strong factor in the winter dynamical forecasts.  In Figure 1 I show the forecasts from dynamical models that show general warmth across the NH with the notable exception of the iconic northwest North America/Southern US dipole associated with ENSO variability. And because it is a La Niña winter, relatively cold temperatures are predicted for Alaska and Northwest Canada.

At AER we use ENSO in producing seasonal forecasts, but in addition we have pioneered the use of Arctic boundary forcings in winter seasonal forecasting including Arctic sea ice but especially Eurasian snow cover in October.  We have demonstrated using observational analysis and model perturbation experiments that extensive Eurasian October snow cover is related to/can force a strengthened Siberian high, increased poleward heat flux and a weak stratospheric polar vortex (PV). For most of my research career I have argued that the weak PV is consistent with sudden stratospheric warmings (SSWs where the zonal mean zonal wind reverses from westerly to easterly at 60°N and 10 hPa) which culminates in an extended period of a negative Arctic Oscillation (AO) and relatively cold temperatures across northern Eurasia and eastern North America but mostly focused in Siberia (Kretschmer et al. 2018).  Scientists including those at AER, have shown a similar atmospheric response to low Arctic sea ice. 

In Figure 1 I also include the AER winter forecast, which based on Arctic predictors was considerably colder than the dynamical model forecasts.

Boundary Forcings

In our recent paper Cohen et al. (2021) we also argued that above normal Eurasian snow cover extent (SCE) can favor reflective or stretched PV events that result not in a negative AO but rather a negative Nort Pacific Oscillation (NPO) pattern that favors widespread cold across North America east of the Rockies and Central/East Asia but not Europe.  Winter 2020/21 was a strong motivator for me in writing this paper.

There are different ideas how variability in Arctic sea ice might influence winter hemispheric weather but the trend has been a convergence to a similar set of mechanisms first proposed for Eurasian snow cover.  Also, there is growing consensus that it is Barents-Kara sea ice in the late fall and early winter that has the greatest impact across Eurasia.  Similarly, low Arctic sea ice in the Chukchi-Bering Seas might favor colder temperatures downstream across North America. 

October Eurasian SCE was above normal but less than when compared to recent Octobers (Figure 2).  Snow cover advance was fairly close to normal for much of the month.  I also compute the snow advance index (SAI) which is a measure of the pace or speed of the snow cover advance across Eurasia (see Cohen and Jones 2011 for more detail).  The value was slightly negative, something that I have interpreted in recent years as favoring a PV disruption more so in the second half of winter than in the first half, however the signal was fairly weak.

Fall 2020 Arctic sea ice was below normal (Figure 3) especially in the Barents-Kara region in October and in early November.  Therefore, Arctic sea ice was favorable for forcing a PV disruption (either an SSW or stretched PV) often followed by a negative AO and cold temperatures across the NH mid-latitudes compared to other recent years. 

In Figure 4 I show the correlation between sea ice extent anomalies in the Barents-Kara Seas for October and the NH winter surface temperature anomalies.  The strongest relationship is in Central Asia and in winter 2020/21 I do think that the low sea ice contributed to the cold temperatures across the NH but especially in Asia.

The quasi-biennial oscillation (QBO) was in its westerly phase that winter.  The QBO is a periodic oscillation of the zonal winds in the equatorial stratosphere and in the westerly phase the zonal winds are stronger.  The westerly phase is thought to favor a stronger PV, which favors milder temperatures across the NH continents. 

And of course, the near record warm global atmosphere and ocean provided an overall warm backdrop heading into the winter of 2020/21.  Siberia was exceptionally warm heading into the late fall.  From January through October 2020 Siberia was record warm by a lot!

Mid-fall

Typically, I don’t discuss the weather before November in the winter recaps but there was an interesting weather event in late October 2020 that seemed to foreshadow one of the most impactful weather events of winter 2020/21.  From Figure 5 you can see the polar vortex and mid-troposphere circulation (10 and 500 hPa geopotential heights respectively) at the end of October.  The PV is not circular but rather stretched along an axis from Siberia to Western Canada and the Western US with warming emanating from Siberia and directed towards the northern North Pacific (Figure 5a).  This is consistent with a stretched PV.  In the mid-troposphere there is strong ridging of high pressure in the Gulf of Alaska and Alaska forcing deep troughing of low pressure in Canada and the Western US, exhibiting that the PV and tropospheric circulation are coupled (Figure 5b).

  The impact on North American weather was highly anomalous more typical of mid-winter.  A deep and expansive pool of cold air covered much of Canada and the US centered on the Plains (Figure 6).   This cold air brought record cold, with one notable temperature of -29°F in Potomac, Montana on 25 October 2020 that is the earliest such a cold reading in the observational record for the contiguous 48 US states (https://www.ncei.noaa.gov/access/monitoring/monthly-report/national/202010).  

In addition, the resulting snow cover was highly anomalous as can be seen in Figure 7a.  The tail end of this event even resulted in up to half a foot of snow in Southern New England including the suburbs of Boston.  Had an event of similar magnitude occurred in the winter months, the impact on society and the environment would have been more severe. 

Late fall/very early winter

November 2020 was not that eventful from a winter weather viewpoint.  The AO was fairly positive with low pressure dominating the Arctic and high pressure dominating the mid-latitudes (Figure 8).  

This pattern both inhibits troposphere-stratosphere coupling and supports widespread mild temperatures across the NH continents (Figure 9). However, despite the mild temperatures across the NH, in contrast to October, North American snow cover advanced more slowly while snow cover advance across Eurasia notably accelerated. 

  As mentioned above, October 2020 Eurasian snow cover extent was above normal but only marginally so.  Still above normal snow cover across Siberia in October favors a strengthened Siberian high in the late fall and early winter with the largest positive sea level pressure (SLP) anomalies northwest of the climatological center (see Figure 10a taken from Cohen et al 2014).  The advance of snow cover in October and likely even more so in November favored the deepening of low pressure in East Asia that extends eastward into the northern North Pacific coupled with a northwestward expansion of the Siberian high which dominated the monthly mean for December (Figure 10b).  

Below normal sea ice in the Barents-Kara seas is also associated with the northwestward expansion of the Siberian high and the persistent below normal sea ice in the Barents-Kara Seas both in October and November in combination with the rapid advance in snow cover in November contributed to an episode of blocking centered over the Urals for the month of December.  From Figure 10b, the northwestward expansion of the Siberian high is clearly evident as well as the “classic” tripole SLP anomaly pattern with relatively high pressure near Scandinavia/Urals and low pressure in both the North Atlantic and North Pacific Ocean basins that is the hemispheric circulation that is most favorable for disrupting the stratospheric PV. 

This tripole pattern is optimal for forcing increased vertical transfer of Rossby wave energy (vertical wave activity flux or WAFz) and poleward heat flux.  The WAFz plot in Figure 11 shows active WAFz from the troposphere and then in the stratosphere in two major pulses in the month of December.  The first pulse of active WAFz, occurred in early December and the second larger pulse occurred in the second half of December that peaked in early January. 

As I have discussed in the blog and in past winter summaries one of my focuses of my research has been to demonstrate that behavior of the stratospheric PV is not just binary, i.e., a strong PV and a weak or disrupted PV that is only really considered when a major mid-winter warming (MMW) or SSW is observed, which is identified when the zonal mean zonal wind reverses from westerly to easterly at 60°N and 10 hPa. 

It turns out that the tropospheric response to a polar vortex disruption where WAFz is “reflected” is quite different from when WAFz is “absorbed.”  The tropospheric response to a PV disruption where the WAFz is absorbed is the “classic” response to stratospheric PV disruptions.  The tropospheric response is characterized by Greenland blocking, negative North Atlantic Oscillation (NAO), relatively cold temperatures across northern Eurasia and milder across North Africa, the Middle East and the North American Arctic.  Also, the tropospheric response is usually delayed relative to the WAFz pulses, and the response can be of long duration, lasting of up to two months. In contrast the tropospheric response to a PV disruption where the WAFz is “reflected” is characterized by blocking near Alaska, relatively cold across much of Canada, the Eastern US and Central Asia and mild across Alaska and Europe.  The response is not associated with a negative NAO but rather a negative North Pacific Oscillation (NPO). Also, the tropospheric response is usually rapid relative to the WAFz pulses, and the response is of relatively short duration lasting on the order of days and up to two weeks. Reflected or stretched PV events were of significant consequence in winter 2020/21.

The first positive WAFz of December 2020 was quickly followed by a negative WAFz pulse – in mid-December (Figure 11).  This is strongly suggestive of a reflective event that involves upward WAFz pulses over Asia that reflect off the polar vortex and then are directed downward over North America.  As I just mentioned, reflective WAFz results in blocking/high pressure as well as warming near Alaska with upstream and downstream troughing and cold temperatures across Central and Eastern Asia and North America east of the Rockies first in the stratosphere and quickly followed in the troposphere.  In Figure 12, I present the geopotential heights for both 10 hPa from 15-17 December 2020 and 500 hPa from 16-18 December 2020.  Figure 12a closely matches the “reflective” cluster 4 for the stratospheric PV (see Figure 1 from Kretschmer et al. 2018a) with a stretched PV from Asia to North America and positive geopotential height anomalies centered near the Dateline, stretching from Eastern Siberia to Alaska with downstream negative geopotential heights across Asia but especially North America.  Admittedly at 500 hPa there is ridging in the North Pacific but broken into two pieces in the mid-latitudes and the Arctic (Figure 12b) and therefore there is lacking a full longitudinal ridge often present during these events.  The main troughs are in East Asia, Canada and the Central US.  There is another trough in Western Europe. 

  This pattern results in below normal temperatures widespread in Asia, Canada and the Eastern US (see Figure 13).  On the flip side there are relatively warm temperatures under the mid-tropospheric ridging along the West Coast of North America, Eastern Siberia, Alaska and Europe.  

One notable weather event was a record snowstorm in the interior Northeast with snowfall totals exceeding 40 inches (a meter) in four different states and ten plus inches in New York City and Boston (see Figure 14).  For the interior Northeastern US this was a highly anomalous snowfall event.

  The tropospheric response to stretched/reflective PV events is relatively short on the order of typically a week or less for the average stretched PV. However as seen in Figure 12b the pattern across Eurasia remains ridging in the west and troughing in the east.  Ridging near the Urals helps to maintain troughing in East Asia and relatively cold temperatures dominate Central and East Asia while milder temperatures overspread eastern North America in late December (as evidenced in the monthly means in Figure 15).  

The western ridge and eastern trough pattern in late December across Eurasia is conducive to triggering more WAFz.  Looking at the polar cap geopotential height anomalies (PCH) plot for winter 2020/21, the warm/positive in the troposphere in mid-December is a classical tropospheric precursor to an SSW (see Figure 16).  This is step-two of the six-step model or pathway that begins with Arctic boundary forcings, forces a troposphere-stratosphere-troposphere coupling event that culminates in an extended period of a negative AO.  I included the six step process in the 14 Dec 2020 blog in anticipation of such an event.  Following the tropospheric precursor of m-December, the strongest pulse of the winter commences in late December and continues into early January (see Figure 10).  The PV started off record strong in early December but evolved into a stretched PV in mid-December and transitioned into a mature SSW late December and early January.

Mid-winter

Following the tropospheric precursor of mid-December and the strong WAFz pulse of late December and into early January, the PCHs in the stratosphere turn warm/positive in late December and lasts incredibly until mid-February!  This is highly unusual and maybe unique in the observational record.  Typically, the disruption in the PV is dramatic, quick to climax and quick to relax back to normal and most of the time even stronger than normal.  This clearly didn’t happen in January 2020 and in real time was confusing to me.

On January 5^th, an SSW was achieved when the zonal mean zonal wind reverses from westerly to easterly at 60°N and 10 hPa.   And looking at the circulation at 10hPa in the stratosphere, the PV is highly disturbed and displaced with two centers one over Scandinavia and the other over Baffin Bay coupled with strong warming and high pressure spreading across the Central Arctic (see Figure 17a).  Though two PV centers can be seen they don’t seem to be independent from each other and I think at this point the disruption can be described as a PV displacement rather than a PV split.

But one strong impression that was left on me from working on the analysis for our paper Kretschmer et al. (2018b) is that for classical SSWs the most robust negative temperature departures are across Northern Asia and Northern Europe (see in Figure 4 from the paper).  There are also negative temperature departures in the Western US but the Eastern US following an SSW/MMW is typically warm, though our analysis doesn’t show a strong signal. However, the odds for cold weather in the Eastern US do increase with time. 

Looking at the mid-tropospheric circulation for the month of January, it is fairly classical at the time and following the SSW with Greenland blocking and a negative NAO clearly evident (see Figure 18).  This is coupled with troughing across Europe and Northern and Eastern Asia.  

Looking at the surface temperature anomalies (see Figure 19), I would also argue that the temperature pattern is consistent with the temperature pattern heading into at the time and following an SSW.  It is relatively cold over Northern and Western Europe, Northern and Eastern Asia but especially Siberia and the Western US but quite mild across much of the remainder North America and Southeastern Europe.  In December and January the most impressive cold and snow including record cold and snow was in Asia as discussed in this article from the Philadelphia Inquirer from 22 January 2021. Records were also broken in Europe in January (e.g., Euronews 7Jan2021).

However, looking at the mid-tropospheric circulation in January what was not classic post an SSW was persistent high pressure ridging in the Barents-Kara Seas and Urals (see Figure 18).    That is why upward WAFz didn’t shut down after the SSW as expected with a post SSW pulse in mid-January (see Figure 11).  This results in a further weakening of the PV and probably the climax of the long duration PV disruption where it did achieve a daily weak record.  The high pressure and warm temperatures spread across even more of the Arctic and the PV is displaced even further south and split into two centers with one centered over Southern Greenland and the other center over Western Russia (see Figure 17b). This appears to me more of a PV split.

But the active WAFz still wasn’t completely done, there was one more pulse of WAFz from the troposphere through the stratosphere at the very end of January (see Figure 11).  This results in one more disruption of the PV in early February (see Figure 17c).  And looking at the PCH plot you can observe three peaks in warm/positive PCHs in the mid-stratosphere (10hPa) in early January, mid-January and early February (see Figure 16).  However, the character of the PV disruption in early February was different from the two PV disruptions in January.  Where the PV was distorted throughout January, in early February the PV is not distorted but rather stretched or elongated from Western Asia into eastern North America.  Also, the high pressure and warming is not spread throughout the Arctic as in January but rather focused in the North Pacific sector of the Arctic.  As we argued in our paper Cohen et al. (2021), the early February PV disruption was fundamentally different than those in January. The January PV disruptions were classical SSWs while the early February PV disruption was a reflective or stretched PV event. 

Figure 20 is from in the Supplementary Figures from Cohen et al. (2021).  And it shows how the WAFxyz differs between early January and early February 2021, where in early January the WAF was absorbed in the stratosphere and in early February it was reflected off the stratospheric PV.   You might respond that the winds went easterly or negative at 60°N and 10 hPa in early February so maybe it is an SSW and not a stretched PV that is a more minor PV disruption.  The first researcher that I know of who discussed stretched PV events was Kunihiko Kodera and he only discussed reflective events following SSWs (e.g., Kodera et al. 2013).  With our research we tried to show stretched PVs can occur independent of SSWs.

Looking at the mid-tropospheric circulation in early February (see Figure 21a), impressive high latitude blocking is immediately apparent, not only is there highly amplified ridging south of the Aleutians and all of the North Pacific sector of the Arctic, consistent with a stretched PV, but also across Greenland, possibly a residual of the SSW from January.  The high latitude blocking supports deep troughing across much of Canada, the US and Northern Eurasia.  The pattern mostly persists through late February (see Figure 21b) though less amplified, and the Greenland blocking is mostly gone allowing a ridge to spread across Europe.  The duration of the pattern associated with the PV stretching is unusually long.

  Figure 5 from Kretschmer et al. 2018a compares the temperature anomaly pattern between a stretched PV and an SSW.  And while the temperature anomalies in January 2020 more closely resemble that following an SSW, the temperature pattern in early February more closely resembling that associated with s stretched PV event (see Figure 22a).  In contrast to the negative temperature anomalies that were focused in Eurasia in December and January, in the first half of February they are of greater magnitude in North America and more widespread.  An area of well below normal temperatures stretches from Alaska to the Central US.  Cold temperatures are spread out across Northern Europe and Northern Asia as well. The areas of below normal temperatures mostly persist in the second half of February though the core of the cold in the US settles south, the cold deepens in Siberia while Europe turns milder (see Figure 22b).     

In the second half of February, it does appear that North America achieved record snow cover extent for any month (see Figure 7b). The tri-state region around NYC received a large amount of snow during the entire month.  The biggest snowstorm was the first couple of days of February, when widespread 2-3 feet were reported in NJ, PAN, NY, MA and CT. and close to two feet was reported in NYC (not Central Park).  It does seem to me there were many similarities between the October 2020 record cold and snow and the February 2021 record cold and snow.  I often talk about how the weather likes to foreshadow (but at times it can also be a head fake so easier in hindsight).  But I would argue the highly anomalous cold and snow of October 2020 was an iconic example of foreshadowing.

This was a truly iconic stretched PV event in my opinion, which resulted in one of the most extreme winter weather events in US history.  I believe that it was the costliest winter weather event in US history and the costliest in Texas’ history including hurricanes.  I believe that it contributed to the coldest February in the US since the 1980’s and it was snowy in large regions of the US, including the Northwestern US, the Southern Plains, the Tennessee Valley, the lower Great Lakes and the Mid-Atlantic.  Some highlights are included in the Wikipedia write-up and news references provided.

Late winter

In late February the PV quickly strengthened and coupled to the surface with a strong positive AO.  This is more consistent with a stretched PV or reflective event than an SSW.  For the month of March, the Arctic was dominated by low pressure with high pressure ridging circumnavigating the NH mid-latitudes (see Figure 23).  In contrast to the winter months, mild temperatures dominated the NH continents (not shown).  

Observed winter circulation

 In Figure 24 I show the winter mean (December-February) circulation in the mid-stratosphere (10 hPa geopotential heights) and mid-troposphere (500 hPa geopotential heights).  The weak stratospheric PV from mid-December through mid-February is evident on the winter mean anomalies. Positive height anomalies cover the Arctic with negative height anomalies across the mid-latitudes of the North Atlantic sector readily recognizable as a weak PV.  The coupling of this pattern clearly translates into a related pattern in the troposphere. The Central Arctic is characterized by positive height anomalies with mostly negative height anomalies and/or troughing across the mid-latitudes.  I would just add that the PV is not perfectly circular in shape and suggests elongation or stretching towards North America.  So, despite the long duration SSW in January, wave reflection/stretched PV in December and especially in February has a visible imprint on the winter stratospheric circulation.

During the winter the weak PV from mid-December through February coupled to the troposphere.  And evidence of the coupling seems to present itself most clearly with high latitude blocking stretching from Northeastern Canada, across Greenland and into the Barents-Kara Seas.  This is coupled with troughing east of the Rockies across Canada and the US and especially Northern and Eastern Asia.  Troughing is also present across Western Europe.  Ridging is present in the North Pacific, consistent with the stretched PV configuration for the winter but is weak along the West Coast of Canada. Given the negative AO pattern both in the troposphere and the stratosphere it is not surprising that the surface temperature anomaly pattern for the winter projects on to the surface temperature pattern associated with a negative AO.  Above normal temperatures are nearly universal across the Arctic except near Alaska and Northwestern Canada (Figure 1). 

As they say in English I "saved the best for last." My last graphic or display item is an animation of the stratospheric PV for the entire winter.  It shows many iconic states of the PV from record strong start in December, to the long duration SSW January that started as a displacement and evolves into a split and then transitions one last time into a stretched PV while maintaining the SSW in early February before rapidly strengthening and ending where it began, much stronger than normal by the end of February (see Figure 25).  

Winter Forecast Verifications

The main predictors in the AER winter forecast are October Eurasian SCE, the Arctic sea ice anomaly and La Niña.  The dynamical models rely strongly on ENSO that was negative and therefore the one exception to the global warming are the forecasts of cold temperatures centered on Alaska and Northwestern Canada (my opinion).  The AER forecast and those from key dynamical models for NH winter surface temperature anomalies are shown in Figure 1 and the observed temperature anomalies are included in Figure 1.  Dynamical models include the national multi-model ensemble (NMME- an ensemble of North American models) forecast for NH temperatures and the European model ensemble (C3S) in Figure 1. 

As is the case every winter now, the dynamical models predict almost universal above normal temperatures across the NH continents.  The AER forecast was colder especially across Asia but also across Alaska, Western Canada and the Northern and Western US.  The difference in the forecasts is the emphasis of Arctic predictors, which is lacking in the dynamical models and emphasized in the AER model.  For a seasonal forecast the AER performed exceptionally well and much better than the dynamical model forecasts.

Concluding remarks

I believe that winter 2020/21 is a winter to remember.  I have approached winter prediction with the beleive that Arctic boundary forcings are the best available predictors of the possible behavior of the polar vortex.  I do think that the extensive snow cover across Eurasia in the late fall favored a more disrupted PV relative to a strong PV Arctic sea ice was low, especially in the Barents Kara Seas which also favored a more disrupted PV.  These boundary forcings would favor both stretching/reflection PV events and the large disruptions associated with SSWs.  The winter featured both impactful stretched PV events and SSWs.  The combination of both focused the cold temperatures in magnitude and persistence across Siberia, which was especially impressive given the unprecedented warmth across Siberia much of the previous year. The cold was also at times impactful in Europe, Canada, the US and East Asia.

It is understood that to form a stretched PV a reflective layer is required in the stratosphere which can shield the PV from upwelling standing or Rossby wave energy from the troposphere allowing the PV to strengthen or accelerate.  I think that February provided a textbook example of this.  The SSW in January preconditioned the polar stratosphere for reflection, as the weak westerly stratospheric winds would inhibit upwelling energy from the troposphere and reflect it back down into the troposphere.  Persistent Ural blocking even post the SSW maintained a steady stream of upwelling energy from the troposphere.

Arctic amplification is well established in theory and in the dynamical model climate projections.   In my mind, the winter of 2020/21 can serve as a paradigm for climate change winters simply because Arctic amplification was large that year yet impressive cold roamed the continents throughout much of the winter.  Though obviously not every winter features such a stark example of Warm Arctic/Cold Continents (WACC) pattern.

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