Winter 2018 Recap

May 7, 2019

Special recap of winter 2017/18

Summary

Winter 2017/18 was characterized by a very warm Arctic, the Southwestern United States (US), Canadian Maritimes, North Africa and the Middle East but cold in much of Canada, the Northern US and Western Europe, Northern and East Asia while Eastern Europe was mixed.
La Niña was anticipated for the winter and weak La Niña conditions were observed during the winter months.
October Siberian snow cover extent (SCE) was well above the long-term average. The month began with well above normal SCE but then had a fairly unusual stall where the snow cover extent nearly didn’t advance for two whole weeks. This was an unprecedented event in recent Octobers. However, in the third week of October the snow cover extent picked up once again through the end of the month.
Sea ice was below normal across the Arctic but especially in the Barents Kara seas during the fall. Above normal snow cover and low Barents-Kara sea ice is favorable for high pressure in the Scandinavian and Ural regions. Ural blocking resulted and in January forced a stratospheric polar vortex (PV) split in early February with the minor daughter vortex over Europe and the major daughter vortex over North America.
The PV disruption was the dominant weather event of the Eurasian and North American winters resulting in a relatively cold mid-latitude winter and mild Arctic.
Earlier in the winter in late December and early January there was a more minor “reflective” PV disruption that resulted in historic cold in eastern North America.
This winter retrospective is consistent with my scientific publications related to the importance of Arctic influence, troposphere-stratosphere-troposphere coupling and the polar vortex on mid-latitude winter weather. This retrospective while based on the scientific literature is my opinion only.
I think winter 2017/18 is somewhat unique in that it included exemplary cases of two flavors of PV disruptions one being a reflective PV disruption at the end of December and early January and the other of an absorptive PV disruption in the middle of February, whose impacts lasted into March and possibly right through April.
A lengthy discussion on this winter was included in my paper: Cohen et al. 2018.

Boundary Forcings

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 2017/18. La Niña favors colder temperatures in the Northwestern US and milder temperatures in the Southeastern US. La Niña also favors relatively heavy precipitation in the Northwestern US and lighter precipitation in the Southwestern US including California. During the winter of 2017/18, weak La Niña conditions did materialize. Most winter forecasts, especially from the government forecast centers, relied heavily on the anticipated La Niña for their temperature and precipitation anomaly forecasts. In general, these forecasts performed well for temperature across North America. As an example, NOAA’s Climate Prediction Center (CPC) United States (US) temperature forecast is shown along with the observed temperature anomaly forecast in Figure 1. My graphic of observed temperature anomalies is based on the relatively coarse NCEP/NCAR reanalysis and smoothing is applied so the anomalies don’t necessarily match station data, especially along the transition zone from positive to negative anomalies. Still the CPC forecast captured the large-scale temperature pattern across the US with generally below normal temperatures across the Northern US and above normal temperatures across the Southern US. Unlike in winter 2018/19, I think a credible argument could be made that ENSO had an impact on North American surface temperatures. Still, I will only briefly discuss ENSO in the rest of the retrospective.

Figure 1. a) Forecasted surface temperature anomalies from NOAA's Climate Prediction Center and b) observed surface temperature anomalies (°F; shading) for December 1, 2017 through February 28, 2018

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, a weak stratospheric polar vortex (PV), which culminates in an extended period of a negative Arctic Oscillation (AO). A negative AO is associated with below normal temperatures in the Eastern US and Northern Eurasia including Northern Europe and East Asia. Scientists, including those at AER, have shown a similar atmospheric response to low Arctic sea ice. 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 the Barents-Kara sea ice in the late fall and early winter that has the greatest impact across Eurasia. Therefore low Barents-Kara sea ice in November for example, favors a strengthened Siberian high, increased poleward heat flux, a weak PV and finally a negative AO. An important point in regard to the Siberian high, it strengthens or expands northwest of the climatological center. For low snow cover and/or high sea ice the opposite occurs.

Snow cover advance was rapid at the beginning and end of the month but relatively slow mid-month. However overall, October 2017 Eurasian snow cover extent (SCE) was above normal (Figure 2a). Above normal SCE favored a sudden stratospheric warming (SSW) and a weak PV in mid-winter followed by a negative AO and cold temperatures across the Northern Hemisphere (NH) mid-latitudes. 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 below normal for October 2017. In this regard the October SCE and SAI were at odds with the SCE indicating a weak PV and the SAI indicating a strong PV. To make sense of using the SCE and SAI when they disagree, I have used the SAI mostly as an indicator of the timing of a possible stratospheric PV disruption and I interpreted the below normal SAI to favor a later PV disruption more so than an earlier PV disruption for winter 2017/18. But I readily admit this is highly speculative. It has worked well the past two winters but is no guarantee of future success.

Figure 2. a) Snow cover extent across Eurasia on October 31, 2017 shown in white, Arctic sea ice shown in light blue. Gray shading snows all points north of 60°N across Eurasia. b) Observed Arctic sea ice extent anomalies November 2017. Negative anomalies shown in blue shading.

Fall 2017 Arctic sea ice was below normal (Figure 2b) and I used the anomaly for the entire Arctic basin as a predictor in the AER model. Below normal Arctic sea ice similarly favored a weak PV in mid winter followed by a negative AO and cold temperatures across the NH mid-latitudes.

The quasi-biennial oscillation (QBO) was in its easterly phase. The QBO is a periodic oscillation of the zonal winds in the equatorial stratosphere and in the easterly phase the zonal winds are weaker. The easterly phase is thought to favor strong PV disruptions referred to as a major midwinter warming, which is identified when the zonal mean zonal wind reverses from westerly to easterly at 60°N and 10 hPa.

And of course, the near record warm global atmosphere and ocean provided an overall warm backdrop heading into the winter of 2017/18.

Late fall/early winter

As mentioned above, October 2017 Eurasian snow cover extent was above normal due to early and late month surges in snow cover advance. Above normal snow cover across Siberia in October favors a strengthened Siberian high in November and into December with the largest positive sea level pressure (SLP) anomalies northwest of the climatological center (see below Figure 8a taken from Cohen et al. 2014). Extensive snow cover during October favors the northwestward expansion of the Siberian high in November and into December. Below normal sea ice in the Barents-Kara seas is also associated with the northwestward expansion of the Siberian high. I would argue that the combination of the extensive snow cover during October and low Barents-Kara sea ice during the fall and early winter contributed to high pressure/blocking centered near the Urals and across the North Atlantic side of the Arctic including Greenland for much of November and into December (Figure 3). Blocking centered near the Urals favors northerly flow and relatively cold temperatures downstream across Central and East Asia (Figure 4). Ridging/high pressure centered on western North America also favored downstream troughing and relatively cold temperatures across the US East Coast. Otherwise elsewhere across the NH continents temperatures were relatively mild.

Figure 3. a) Observed average sea level pressure (hPa-1000; contours) and sea level pressure anomalies (hPa; shading) across the Northern Hemisphere for November 10, - December 15, 2017. b) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for November 10, - December 15, 2017.

From Figure 3, the northwestward expansion of the Siberian high is evident but missing are below normal pressures in the two ocean basins to from the “classic” tripole SLP anomaly pattern (see Figure 8a) so favorable for disrupting the stratospheric PV. Still relatively high pressure near the Urals and northern North Atlantic is sufficient to force increased vertical transfer of Rossby wave energy (vertical wave activity flux or WAFz) and poleward heat flux. The WAFz plot in Figure 5 shows active WAFz throughout November and December. The positive WAFz is mostly confined to the troposphere in November but the individual pulses reach the mid-stratosphere throughout December.

Figure 4. Observed average surface temperature anomalies ((°C; shading) for November 10, - December 15, 2017.

The next two paragraphs are the same as in the retrospective for 2018/19 but I feel are important for winter 2017/18. I have believed for a while now that conventional thinking of the behavior of the stratospheric PV as binary was overly simplistic and too limiting when trying to understand tropospheric influence, i.e., a strong PV and a weak or disrupted PV that is only really considered when a major mid-winter warming (MMW) is observed, which is identified when the zonal mean zonal wind reverses from westerly to easterly at 60°N and 10 hPa. I was able to find new ways to characterize PV behavior when graduate student Marlene Kretschmer from the Potsdam Institute for Climate Impact Research visited me at AER. In our first collaborative paper we identified seven main clusters or categories of PV behavior from the strongest PV in cluster one and the weakest PV in cluster seven (see Kretschmer et al. 2018a). In a follow up paper instead of applying cluster analysis at 10hPa (the middle stratosphere) we applied it at 100 hPa (the lower stratosphere) where we identified five clusters (see Kretschmer et al. 2018b). Again, cluster one was the strongest PV and cluster five the weakest, but we also identified a new weak cluster (cluster four). We differentiated cluster four from five in that in cluster five the WAFz is absorbed in the polar stratosphere and in cluster four the WAFz is reflected off of the stratospheric PV back into the troposphere.

Figure 5. Observed daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N from October 1, 2018 through March 31, 2019.

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. Though it was not included in the paper, our analysis did show that in about two weeks-time the cold temperatures overspread the Eastern US as well. Also, the tropospheric response is usually delayed relative to the WAFz pulses and the response can be long 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 West Pacific Oscillation (WPO). 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.

Again, as for winter 2018/19, I have not done a detailed analysis but the information in the provided figures are at least broadly consistent with my ideas. The WAFz pulses during December are reflective and bouncing off of the stratospheric PV. Each positive pulse in December is quickly followed by negative values. To be clear a negative value does not necessarily mean downward WAFz but instead relatively less upward WAFz and from the plot I cannot differentiate. As I just mentioned reflective WAFz results in blocking/high pressure as well as warming near Alaska with downstream troughing and cold temperatures across Central and Eastern North America first in the stratosphere and quickly followed in the troposphere. In Figure 6, I present the geopotential heights for both 10 hPa from December 1-21, 2017 and for 500 hPa from December 21, 2017 – January 10, 2018. Figure 6a closely matches the “reflective” cluster 4 for the stratospheric PV (see Figure 1 from Kretschmer et al. 2018b) with positive geopotential height anomalies centered near Alaska and Eastern Siberia with downstream negative geopotential heights across central and eastern North America that extend into the North Atlantic. The stratospheric PV itself is displaced towards the Barents-Kara Seas though no weakening is evident.

The general circulation pattern in the stratosphere of ridging near Alaska with downstream toughing across eastern North America can be seen replicated in the troposphere with ridging/positive geopotential height anomalies across western North America and downstream troughing/negative geopotential height anomalies across eastern North America (Figure 6b). Also, a region of negative geopotential height anomalies were observed in East Asia both in the stratosphere and the troposphere. Probably the biggest difference was the strong blocking centered near the Urals in the troposphere and not the stratosphere. If, as I argue that the Ural blocking is in part forced by high snow and low sea ice, this would make sense since the forcing is bottom up and not top down, though I am not as convinced as I was in late 2018.

Figure 6. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for December 1- December 21, 2017 b) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from December 21, 2017 - January 6, 2018. c) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for January 7 - 17, 2018. d) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from January 7 - 17, 2018.

I do believe that of a reflective stratospheric PV disruption coupled with Ural blocking can explain the large-scale surface temperature anomalies from late December through early January (Figure 7a). The hemispheric temperature anomalies are consistent with the temperature anomalies associated with a reflective stratospheric PV disruption as seen in Figure 5a of Kretschmer et al. 2018b. Warm anomalies in Alaska and the West Coast of North America (with the exception of Western Canada) that extend into Eastern Siberia with cold anomalies widespread across central and eastern North America. Europe is warm as well with downstream regional cold temperatures in Central and East Asia. Right underneath the Ural block, temperatures are well above normal. But the temperature pattern across Eurasia is also consistent with the Ural blocking with cold advection downstream of the block on northerly flow across part of Central and East Asia and warm advection upstream of the block on southerly flow across Europe.

Figure 7. Observed average surface temperature anomalies ((°C; shading) for a) December 16, 2017 - January 6, 2018 and b) January 7 - 17, 2018.

The cold in eastern North America was intense and historical. It was the coldest New Year’s Eve in New York’s Times Square celebration in 100 years. Here in Boston the temperature remained below 20°F for seven straight days. That happened only once before in 1917. The historic cold ended with an historic blizzard the end of the first week of January 2018 known as the “bomb” cyclone due to the record drop in central pressure during its development along the US East Coast.

Though I along with colleagues developed the troposphere-stratosphere-troposphere coupling framework as seen in Cohen et al. (2007) to characterize absorptive PV disruptions a more concise version can be seen with this reflective PV disruption in the polar cap geopotential height anomalies (PCHs) plot (Figure 8). A tropospheric precursor can be seen in early December, characterized by warm/positive PCHs confined to the troposphere. Upward propagation takes place as seen in the WAFz plot in mid- to late-December (Figure 5). For the PV disruption of late December, the upward propagation can even be seen in the PCHs, with warm PCHs extending upward from the troposphere to the stratosphere (Figure 8). The pulses of WAFz result in positive/warming of the stratospheric PCHs in late December. Finally, the downward propagation can be seen in late December and into early January with the return of warm/positive PCHs to the troposphere.

Figure 8. Observed daily polar cap geopotential height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies from October 1, 2018 through March 31, 2019.

Midwinter

Though the intense cold in the Eastern US moderated after the first week of January there does seem to be one last reflective PV disruption the second week of January. The WAFz exhibits the typical dipole structure of positive WAFz anomalies immediately followed by negative WAFz anomalies the second and into the third week of January (Figure 5). From January 7-17, 2018, the polar stratosphere is characterized by a stretched PV along an axis from Central Asia into Eastern North America with warming/ridging centered near Alaska (but closer to the Aleutians; Figure 6c). A similar pattern is seen in the mid-troposphere with troughs in Siberia and eastern North America with ridging centered near Alaska and extending down the length of western North America (Figure 6d). There is another region of strong ridging/blocking centered over the Urals and Scandinavia (Figure 6d). As I argue earlier in December 2017 and in November and December 2018 this ridging does not have a counterpart in the stratosphere and is more likely related to large negative sea ice anomalies in the Barents-Kara Seas and is being forced from the bottom and not the top of the atmosphere, but I admit for now this remains speculative. Finally, temperatures are below normal in the Eastern US and parts of Central and East Asia with overall relatively mild temperatures in Alaska, western North America and Europe (Figure 7b).

A more classical troposphere-stratosphere-troposphere (T-S-T) coupling event related to an absorptive reflective PV disruption commenced immediately on the heels of the T-S-T related to the reflective PV disruption of late December and possibly again in mid-January. The first stage of the T-S-T event is the tropospheric precursor characterized by positive/warm PCHs in the troposphere throughout January through early February. Observed January 2018 SLP anomalies closely matches the “tripole” SLP anomaly pattern with relatively high pressure near Scandinavia/Urals (in January 2018 there is no high pressure near Scandinavia as there was in November and December 2018) 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 (Figure 9).

Figure 9. a) Regression of November SLP anomalies (hPa) onto October monthly mean, October Eurasian SCE (contouring) and December meridional heat flux anomalies at 100 hPa, averaged between 40-80°N (shading). b) Observed sea level pressure anomalies (hPa; shading) across the Northern Hemisphere from January 1 - 31, 2018.

I would argue that during the month of January the pulses of WAFz transitioned from reflective to absorptive. The two WAFz pulses in late January and early February impacted the stratospheric PV differently than the previous pulses. First, previous positive WAFz anomalies were quickly followed by negative WAFz anomalies but the WAFz anomalies remained continuously positive from about the third week in January through the second week of February (Figure 5). Second, the core of the stratospheric PV was previously impervious to warming in December and early January but is vulnerable to warming in late January and early February. In Figure 10, I show the 10 hPa geopotential heights for the from January 18 through February 11, 2019, the day before an MMW was first achieved, and the impact of the WAFz is different than earlier in the fall and winter. In previous WAFz pulses the stratospheric PV stretched and bounced around, but the core of the PV remained intact and impervious to warming. After January 18 the WAFz resulted in warming of the North Pacific side of the polar stratosphere and the stratospheric PV is now displaced towards Greenland. Also, there is now a second area of warming appearing over the North Atlantic (Figure 10). Admittedly the polar warming does not look that impressive in Figure 10, but it is mostly a function of the averaging period and the warming looks more impressive in later (see below Figure 13). The transition from reflective to absorptive can also be seen in the plot of the PCHs. Prior to the second week of February, stratospheric PCHs were mostly cold/negative but then turn positive from the second week of February (Figure 8).

Figure 10. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from January 18 - February 11, 2018. b) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from January 18 - February 11, 2018.

Often in the blog I discuss how if WAFz is resulting in a negative AO, high pressure and warming in the polar stratosphere then it is likely resulting in the opposite, a positive AO, low pressure and cooling in the polar troposphere. Also shown in Figure 10 are the 500 hPa geopotential heights for January 18 through February 11, 2019. In general, mostly positive heights cover the mid-latitudes with the exception of East Asia. Temperatures are also relatively mild, including much of the US and Europe (Figure 11). The biggest region of below normal temperatures is Asia with cold temperatures stretching from Central to East Asia (Figure 11). I also often mention that western North America and East Asia are frequently cold during these active WAFz/positive AO periods, so it is not surprising that it is cold across a large portion of Asia and in Western Canada during this period. Still continued blocking centered on the Urals forced troughing downstream with cold temperatures across Asia.

Figure 11. Observed average surface temperature anomalies ((°C; shading) for January 18 - February 11, 2018.

The strong upward WAFz absorbed in the stratosphere at the end of January and early February weakened the stratospheric PV and can be seen in the warm/positive PCHs from the second week of February through mid-March (Figure 7). In Figure 12a I show the 10 hPa geopotential heights mid- to late- February. Warming entered the polar stratosphere from Siberia and the North Atlantic resulting in a PV split (see below Figure 19) with one daughter vortex located over Europe with the second daughter over Western Canada. The clear dominant daughter vortex is over Canada (the European daughter vortex doesn’t even contour on my plot but can be seen in the animation below in Figure 19).

Figure 12. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for February 12 - 28, 2018. b) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from February 12 - 28, 2018. c) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for March 1 - 31, 2018. d) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from March 1 - 31, 2018.

Again, in the blog I have often discussed that in my opinion there exists an initial tropospheric response and a delayed tropospheric response to stratospheric PV disruptions. In Figure 12b I show the 500 hPa geopotential heights for mid- to late- February and in Figure 13a the surface temperatures for mid- to late- February, which I consider the immediate or initial response. In the second half of February troughs with related cold temperatures exist in the troposphere directly beneath the two stratospheric daughter PVs one in Europe and the other in Western Canada that extends southwestward into the Western US. Across Europe, possibly more importantly, the flow is persistently from the east both in the stratosphere and in the troposphere, drawing cold Siberian air westward into Europe. On the flip side a region of high pressure/ridging exists in the polar stratosphere on the Eurasian side of the Arctic centered over the Laptev Sea (Figure 12a). In the troposphere blocking/ridging is centered nearby over the Barents-Kara Seas (Figure 12b) with relatively mild temperatures across the Arctic (not shown) and the adjacent North Slope of Asia (Figure 13a). In the stratosphere the Eastern US is under strong southwesterly flow and some ridging (Figure 12a). In the troposphere strong ridging centered over the Eastern US (Figure 12b) provides strong southwesterly flow of air resulting in very mild to record warm temperatures for the send half of February (Figure 13a).

Figure 13. Observed average surface temperature anomalies ((°C; shading) for a) February 12 - 28, 2018 and b) for March 1 - 31, 2018.

Late winter

The delayed response to the stratospheric PV split/disruption can be seen in March and even into April. Describing the circulation features in the stratosphere and troposphere and assigning attribution, I believe is a challenge and I will try not to use language that suggests that the stratospheric circulation is driving tropospheric circulation. I will begin the discussion of the late winter with a figure that I showed in the recap of winter 2018/19.

In Figure 14, I show the weekly temperature response for weeks one through three during and after a significant stratospheric PV disruption. This figure was included in the original Kretschmer et al. 2018a paper but was removed at the request of the reviewers. Soon following the PV disruption, the most widespread negative temperature departures are found across northern Eurasia with much weaker anomalies across North America. However, over time the temperature anomalies weaken across Eurasia but strengthen across North America so that by week three the greatest negative and most expansive temperature departures are found across central and eastern North America with weaker and more regional temperature anomalies across Eurasia. I think that the hemispheric temperature anomalies were broadly consistent with Figure 14 in February, March and April 2018. Negative temperature anomalies were initially more widespread across Eurasia in February with less extensive, but intense cold anomalies over western North America with very mild temperatures in eastern North America. However, during March, the cold anomalies spread into the Eastern US while the negative temperature anomalies weakened across Eurasia. This was even more apparent in April.

Figure 14. Composite mean of detrended lagged near surface temperature for cluster 7 (weak PV) using a) a lag of 0-7 days, b) a lag of 7-14 days and c) a lag of 14-21 days. In all panels significant values (P<0.05) are indicated with hatches.

In March, the North American daughter vortex retrograded to the west across the Arctic and into Siberia though some residual area of low geopotential heights remained centered near the Aleutians (Figure 12c). The major region of high pressure/warming migrates from the Laptev Sea to Greenland (Figure 12c). Similarly, in the troposphere, the dominant area of low geopotential heights is over Siberia and there is strong blocking/high pressure over Greenland (Figure 12d). Blocking over Greenland during the month of March favors troughing/low geopotential heights upstream over the Northeastern US and downstream over Europe. In the troposphere there is strong ridging/high pressure stretching from Japan to Alaska and centered over the Dateline with no truly similar feature in the stratosphere (Figure 12c&d). Ridging in this region is consistent with the Madden Julian Oscillation (MJO), which was in phases two and three in early March, and La Niña both of which favor ridging near the Aleutians. Ridging near the Aleutians favors downstream troughing in western North America and upstream troughing in East Asia.

Underneath the troughs/low geopotential heights, temperatures are below normal. The greatest negative departures are found in Northern Siberia and northwest Russia associated with the dominant hemispheric trough that month (Figure 13b). But below normal temperatures were also present in Europe, East Asia, the Northern US and Southern Canada (Figure 13b) also associated with mid-tropospheric troughs. In contrast underneath the ridges/high geopotential heights, temperatures are above normal. Temperatures were above normal in Southern Asia, Alaska, Greenland and Northeastern Canada (Figure 13b).

Normally I don’t discuss April in my winter retrospectives but in 2018, it is my impression that April was unusually winter-like with anomalous cold and snow and that it was a continuation of the atmospheric response to the stratospheric PV disruption from mid-February. In Figure 15, I show the typical PCH plot used in the weekly blog from April 1. I include it because it suggests that the warm/positive PCHs spawned at the tropopause from the stratospheric PV disruption back in February, produced another big “drip” or descent of warm/positive PCHs into the lower troposphere in early April that persisted for much of the month. This produced blocking/high pressure near Alaska that drove cold air downstream across much of Canada and the Eastern US (Figure 16). Again, I do think that the surface temperature anomaly pattern is consistent with Figure 14, immediately following the stratospheric PV disruption of mid-February 2018, the greatest negative temperature anomalies are observed in Eurasia but over time those deep negative anomalies transition to North America.

Figure 15. Observed daily polar cap geopotential height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies from March 2, 2018 through April 1, 2018 and predicted daily polar cap geopotential height standardized anomalies from April 2 - 17, 2018.

Like the PV disruption back in late December and early January in eastern North America, the PV disruption in mid-February triggered memorable winter weather but this time on a larger scale that spanned the hemisphere. In the runup to the PV disruption, intense cold pooled across Siberia that extended into East Asia and brought record cold to Tokyo. Following the PV disruption, easterly flow that extended the depth of the atmosphere over western Eurasia syphoned the cold air in Siberia towards Europe bringing record cold and heavy snows. The full force of the cold and snow made it to the United Kingdom in March resulting in record cold and heavy snows as well and was labelled the “Beast from the East.” In North America, the PV disruption initially brought record cold to Southwestern Canada and the Northwestern US but record warmth to the Eastern US. But with time the cold was drawn to the east by four consecutive Nor’easters that brought high winds, heavy snows and coastal flooding to the Northeastern US. The cold in an anomalous sense deepened in April resulting in numerous cold temperature records and heavy snows.

Figure 16. Observed average surface temperature anomalies ((°C; shading) for April 1 - 30, 2018.

Observed winter circulation

In my opinion the extensive Eurasian snow cover in October coupled with low sea ice in the Barents-Kara Seas in late fall and early winter favored the strengthened and westward displaced Siberian high during November, December and January, the active upward WAFz, from mid-November through mid-February, the weak PV in late December and mid-February to mid-March accompanied by the drip of warm/positive PCHs from the stratosphere into the troposphere and high latitude blocking including Greenland blocking in March.

Whereas the timing of the large PV disruption in 2019 matched composites using snow cover extent, the timing of the large PV disruption in 2018 better matches composites using sea ice extent (SIE). In Furtado et al. (2016) we compare the timing of PV disruptions using October Eurasian SCE and November SIE and November SIE favors later PV disruptions than October SCE (see Figure 2). The WAFz pulses of late January and early February and the PV disruption of early to mid-February better matches the composites using sea ice rather than snow cover (see Figure 2 of Furtado et al. 2016).

In Figure 17, I show the winter mean (December-February) circulation in the mid-stratosphere (10 hPa) and mid-troposphere (500 hPa). The impact of the stratospheric PV disruption/split in February is evident on the winter mean anomalies. Positive height anomalies cover the polar stratosphere with negative height anomalies across the mid-latitudes especially across Siberia and eastern North America. However, the pattern is not as classically negative AO as in winter 2018/19 with the positive height anomalies weaker and shifted closer to Alaska possibly due to the lateness in the main PV disruption and the greater influence on the stratospheric PV from reflective PV disruptions earlier in the winter. Also, in contrast to winter 2018/19, the disrupted stratospheric did translate into a similar pattern in the troposphere. Positive heights dominate the Arctic with the largest positive anomalies centered over the Urals and the Laptev Sea and troughs/low heights over East Asia, Europe and eastern Canada.

Figure 17. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for December 1, 2017 - February 28, 2018. b) Observed average 500 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from December 1, 2017 - February 28, 2018.

Finally, I include two extended animations of the stratospheric PV. The first is from December 1, 2017 through January 15, 2018 that shows the variability in the PV related to the “reflective” stratospheric PV disruption in Figure 18. The second is from January 24, 2018 through February 28, 2018 that shows the variability in the PV related to the “absorptive” stratospheric PV disruption or PV split in Figure 19. The first PV disruption is characterized by stretching while the second shows a clean split of the stratospheric PV.

Figure 18. Polar vortex animation. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for December 1, 2017 - January 15, 2018.

Figure 19. Polar vortex animation. a) Observed average 10 hPa geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for January 24 - February 28, 2018.

Winter forecasts

The main predictors in the AER winter forecast are October Eurasian SCE, the Arctic sea ice anomaly, the SLP anomaly across northern Eurasia in the fall and El Niño. In November I published the AER winter forecast on the National Science Foundation website. That forecast was relatively mild across the Southwestern US and Eastern New England with cold in much of the Northern and Eastern US based on above normal Eurasian SCE and below normal SIE. I also published the NH version of the forecast in the blog in late November (Figure 20). Snow cover was high and sea ice low so therefore the model temperature forecast was consistent with a weak polar vortex and a negative AO. Also, La Niña contributed to colder temperatures across western North America in the model. The model also predicted cold temperatures across Northern Asia, Northern Europe, the Northern and Eastern US and Western and Central Canada. Much of the remainder of the NH was predicted to be mild. The observed winter temperatures are also shown in Figure 20. The model forecast was good though the model was too cold in Eastern Europe and the Southeastern US.

Figure 19. Predicted December, January and February 2017/18 surface temperature anomalies from the a) NMME suite of models and b) C3S suite of models both initialized November 1, 2018, c) The observed surface temperature anomalies for December, January and February 2017/18. d) Predicted December, January and February 2017/18 surface temperature anomalies from the AER statistical model initialized November 8, 2017. Smoothing was applied to the statistical model and observed surface temperature anomalies.

I also include the North American multi-model ensemble (NMME - an ensemble of North American models) forecast for NH temperatures and the European model ensemble (C3S) in Figure 20. As is the case every winter now, the dynamical models predict almost universal above normal temperatures across the NH continents. And once again the NMME and C3S winter temperature forecasts are in general too warm. The one region where the models predict closer to normal or slightly below normal temperatures is in Western Canada and this is related to the predicted La Niña. I attribute the consistent model error of being too warm to model deficiencies in simulating high latitude surface-atmosphere coupling. And similar to winter 2018/19 the occurrence of a polar vortex disruption is of little consequence in the dynamical models. Thus model error is discussed in more detail in Cohen et al. 2018.

Concluding remarks

I believe that the winter of 2017/18, like winter 2018/19 demonstrated the importance of the polar vortex on our winter weather and the benefit of using predicted behavior of the polar vortex in winter forecasts. I believe that 2017/18 provided particularly good examples of the different categories of stratospheric PV disruptions on our weather, the better known and studied absorptive PV disruptions in February 2018 and the lesser known and studied reflective stratospheric PV disruption in December 2017. I also believe recognition of these categories of stratospheric PV disruptions allowed longer lead time predictions of the impact on hemispheric weather.

I also believe that Arctic boundary forcings are the best available predictors of the possible behavior of the polar vortex. I am not going to get into the details here but ENSO, the Madden Julian Oscillation (MJO) and the QBO are all proposed as influencers of the stratospheric PV and all were in different phases in winter 2017/18 and winter 2018/19. Eurasian snow cover and Barents-Kara sea ice were in the same phase though I readily admit two years is too small of a sample to draw conclusions from but still an interesting footnote.

Finally, winter 2017/18 provided a nice example of winter whiplash. I know best for my region where record, even historic cold in January was followed by record and historic warmth in February. I argued the idea of weather whiplash related to Arctic boundary forcings and the behavior of the stratospheric PV in Cohen 2016.