November 12, 2018
Special blog on winter 2016/2017 retrospective can be found here - http://www.aer.com/winter2017
Special blog on winter 2015/2016 retrospective can be found here - http://www.aer.com/winter2016
Dr. Judah Cohen from Atmospheric and Environmental Research (AER) recently embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO). This analysis is intended to provide researchers and practitioners real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns.
With the start of spring we transitioned to a spring/summer schedule, which is once every two weeks. Snow accumulation forecasts will be replaced by precipitation forecasts. Also, there will be less emphasis on ice and snow boundary conditions and their influence on hemispheric weather.
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The AO/PV blog is partially supported by NSF grant AGS: 1657748.
Summary
- The Arctic Oscillation (AO) is currently negative and is predicted to remain near negative for the next two weeks.
- The current negative AO is reflective of mostly positive pressure/geopotential height anomalies across the Arctic and mixed pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently positive with weak pressure/geopotential height anomalies across Greenland and Iceland but positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic and is predicted to trend negative over the next two weeks.
- Currently, troughing/negative geopotential height anomalies are confined to far Western Europe with ridging/positive geopotential height anomalies in Central and Eastern Europe. Ridging and southwesterly flow is bringing mild temperatures to much of Europe including the United Kingdom (UK). However over the next two weeks the ridging/positive geopotential height anomalies are predicted to consolidate first over Northern Europe and then drift west towards Greenland. This will allow colder temperatures to filter from Siberia first across Southern Europe and then slowly migrate north.
- Troughing/negative geopotential height anomalies across Central Asia are bookended by ridging/positive geopotential height anomalies in West and East Asia. However over time the pattern is predicted to rotate westwards. This will result initially in relatively cold temperatures for Western Siberia and Central Asia that will slide west towards Europe over the next two weeks. Initially relatively mild temperatures across East Asia are predicted to be replaced by colder temperatures as troughing returns to the region.
- Currently ridging/positive geopotential height anomalies and above normal temperatures dominate Alaska and Western Canada with troughing/negative geopotential height anomalies with normal to below normal temperatures east of the Rockies. However the pattern is predicted to transition to increasing ridging/positive geopotential height anomalies across the North American Arctic and troughing/negative geopotential height anomalies across the United States (US) with a return to more seasonable temperatures.
- In the Impacts section, I discuss blocking in the high latitudes of the North Atlantic that could have significant implications for the long term weather.
Impacts
It’s that time of year again, a sign of the season like holiday decorations that winter is approaching, when I dust off the six step model how October Siberian snow cover extent can influence the weather across the Northern Hemisphere (NH) in winter (Figure i.). The first step is the advance of snow cover across Siberia during the month of October. If the snow cover advance is sluggish then the next anticipated step is a weakened and more contracted Siberian high and mild temperatures across Siberia. Alternatively if Siberian snow cover advances more rapidly then the next anticipated step is a strengthened and more expansive Siberian high and cold temperatures across Siberia.

Figure i. Conceptual model for how fall snow cover modifies winter circulation in both the stratosphere and the troposphere; case for extensive snow cover illustrated: 1. Snow cover increases rapidly in the fall across Siberia, when snow cover is above normal diabatic cooling helps 2. to strengthen the Siberian high and leads to below normal temperatures. 3. Snow forced diabatic cooling in proximity to high topography of Asia increases upward flux of energy in the troposphere, which is absorbed in the stratosphere. 4. Strong convergence of wave activity flux indicates higher geopotential heights, a weakened polar vortex and warmer temperatures in the stratosphere. 5. Anomalous geopotential heights and winds appear to propagate down from the stratosphere into the troposphere all the way to the surface. 6. Dynamic pathway culminates with strong negative phase of the Arctic Oscillation at the surface.
The advance of snow cover across Siberia was mixed this past October with a sluggish start for the first three weeks or so but then a strong finish to end the month. I have been debating with myself which to weight more the slow start or the fast finish. Also it could be that the schizophrenic character of snow cover advance may both be an indication of a schizophrenic type winter but hard to represent in a seasonal average. But more important than my own interpretation of the response to the snow cover advance is the actual atmospheric response to the snow cover advance. To me over the course of last week but especially on Friday the atmospheric response became quite clear – the response consistent to a rapid advance of Siberian snow cover resulting in a strengthened and expanded Siberian high.
After discovering the strong statistical relationship between October Siberian snow cover extent and winter weather in the Eastern US (though admittedly since then the relationship has weakened), I focused, with the help of many collaborators, my research on trying to understand the physical mechanism of this empirical relationship which is summarized in Figure i. As it turns out for step two, the Siberian high response, is not just any generic contraction or expansion but the response in the northwest quadrant is most important. If rapid snow cover advance yields a northwest expansion of the Siberian this favors increased vertical transfer of energy from the troposphere to the stratosphere while a more anemic advance of snow cover yields a retreat of the Siberian High in its northwest quadrant which then favors decreased vertical transfer of energy from the troposphere to the stratosphere or step three in the model. The northwestward expansion of the Siberian often but not always resembles Scandinavian blocking.
One way we represented this northwestward expansion of the Siberian high in Cohen et al. 2014 was by regressing October snow cover extent anomalies on to November sea level pressure (SLP) anomalies and regressing December anomalies of vertical energy transfer from the troposphere to the stratosphere on to November SLP anomalies (Figure ii). Those two different regression patterns show incredible similarity and we argued supports the physical connection between snow cover advance in October, the northwestward expansion of the Siberian high in November and more active vertical energy transfer in December. This would then lead to a sudden stratospheric warming/weakened polar vortex most likely in January and eventually a negative AO/NAO and widespread severe winter weather across the NH.

Figure ii. a) Regression of November SLP anomalies (hPa) onto Ocotber monthly-mean Eurasian SCE (contouring) and onto December meridional heat flux anomalies at 100 hPa, averaged between 40-80°N (shading). b) Estimated sea level pressur eanomlies based on GFS analysis and 16 day forecast from the 00Z 9 November 2018 GFS. c) Same as b) but from the 00Z 12 November 2018 GFS.
Included in Figure ii is the GFS estimation of monthly mean November 2018 SLP anomalies from this past Friday. The resemblance between the two is striking and estimated November 2018 SLP anomalies could be described as “textbook” for disrupting the stratospheric PV. This pattern is a precursor to the large PV disruptions similar to last February and the expected response in the atmospheric circulation most closely resembles the negative NAO or AO with Greenland blocking. So if Scandinavian blocking most often precedes a significant PV disruption then Greenland blocking is most often follows a PV disruption as seen in Figure iii (especially panels c) and d) prior to PV splits) from Cohen and Jones 2011. Therefore the temperature response is strongest across northern Eurasia consistent with a negative NAO/AO with more uncertainty across North America as we showed in Kretschmer et al. 2018.

Figure iii. Sea level pressure (SLP) anomalies (hPa) (a) averaged 45 to 0 days prior to PV displacements, (b) averaged 0 to 45 days after PV displacements, (c) averaged 45 to 0 days prior to PV splits, and (d) averaged 0 to 45 days after PV splits. Colored shading represents anomalies, solid contours show the full values of the SLP field, and dashed contours represent 90% and 95% confidence levels.
While Scandinavian blocking is often the precursor of a significant stratospheric PV disruption, Greenland blocking is the symptom or the tropospheric response to the PV disruption. The GFS is now suggesting that the blocking over Scandinavia will slide west towards Greenland, shifting the monthly means from Scandinavia towards Greenland (Figure ii). In that scenario presumably the threat of a PV disruption and the longer term risk of severe winter weather across the NH but especially northern Eurasia would subside while the risk of short term risk of severe winter weather across Northern Europe and the Eastern US would increase with the longer term prospects more in doubt.
It seems to me that the models will likely struggle with the placement of the blocking in the North Atlantic. This needs to be watched carefully with different scenarios possible based on the exact location of the blocking, but high latitude blocking on either side of the North Atlantic raises the risk of severe winter weather for both the Eastern US and northern Eurasia including Europe.
Near Term Conditions
1-5 day
The AO is currently negative (Figure 1), reflective of mostly positive geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). Geopotential height anomalies are slightly positive across Iceland and Greenland (Figure 2), however mostly positive geopotential height anomalies across the mid-latitudes of the North Atlantic are contributing to a positive NAO. During the week the NAO will trend to negative as positive geopotential heights slowly consolidate closer to Greenland.

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 12 November 2018 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 12 November 2018 GFS ensemble. Gray lines indicate the AO index from each individual ensemble member, with the ensemble-mean AO index given by the red line with squares.
Troughing/negative geopotential height anomalies are limited to Western Europe with ridging/positive geopotential height anomalies stretching across much of Europe (Figure 2). High heights and/or southwesterly flow will bring relatively mild temperatures for most of Europe (Figure 3). Ridging/positive geopotential height anomalies are predicted to persist this period in West and East Asia sandwiching troughing/negative geopotential height anomalies geopotential height anomalies in Central Asia (Figure 2). This will yield widespread above normal temperatures across the Middle East, northwest and East Asia with normal to below normal temperatures in Western Siberia and Central Asia (Figure 3). However the troughing in Central Asia curves to the southwest (Figure 2) bringing some of the cold air into southwest Asia as well (Figure 3).

Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 12 November 2018.
The dominant circulation pattern across North America is ridging/positive geopotential height anomalies in Alaska and western North America and above normal temperatures with troughing/negative geopotential height anomalies downstream and below normal temperatures in central and eastern North America (Figures 2 and 3). 
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 13 – 17 November 2018. The forecast is from the 00Z 12 November 2018 GFS ensemble.
Troughing and cold temperatures will bring widespread new snowfall to Siberia and parts of Central Asia and even Turkey (Figure 4). Across North America, troughing and cold temperatures will bring new snowfall to southern Alaska, northwestern and Eastern Canada and the Northeastern US (Figure 4).

Figure 4. Forecasted snowfall anomalies (mm/day; shading) from 13 – 17 November 2018. The forecast is from the 00Z 12 November 2018 GFS ensemble.
Mid-Term
6-10 day
The AO is predicted to remain negative next week (Figure 1) with mostly positive geopotential height anomalies across the Arctic especially near Scandinavia (Figure 5a). And with weak positive geopotential height anomalies across Greenland to Iceland, the NAO will likely turn negative as well next week.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 18 – 22 November 2018. (b) Same as (a) except averaged from 23 – 27 November 2018. The forecasts are from the 12 November 2018 00z GFS ensemble.
Ridging/positive geopotential height anomalies are predicted to consolidate across Northern Europe allowing troughing/negative geopotential height anomalies to spread across Southern Europe this period (Figure 5a). This will result in normal to above normal temperatures for Northern Europe including the UK with normal to below normal temperatures across Southern and Central Europe (Figure 6). Consolidation of ridging/positive geopotential height anomalies to Scandinavia and to Eastern Siberia will allow troughing/negative geopotential height anomalies to expand out of Central Asia west towards Southeastern Europe and east into East Asia (Figure 5a). This will result in in normal to above normal temperatures being mostly confined to Northwest Asia, the Middle East and northeast Asia with normal to below normal temperatures across Central Asia that extends southwestward towards Southeastern Europe and east towards East Asia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 18 – 22 November 2018. The forecasts are from the 00Z 12 November 2018 GFS ensemble.
The circulation pattern across North America is predicted to mostly persist from the previous period with ridging/positive geopotential height anomalies across western North America including Alaska and troughing/negative geopotential height anomalies downstream into eastern North America (Figure 5a). The resultant temperature anomalies across North America are predicted to be normal to below normal temperatures east of the Rockies and normal to above normal temperatures for the Western US, Western Canada and Alaska (Figure 6).

Figure 7. Forecasted snowfall anomalies (mm/day; shading) from 18 – 22 November 2018. The forecasts are from the 00Z 12 November 2018 GFS ensemble.
Troughing and cold air will bring the potential for new snowfall across large parts of Siberia, Central Asia, East Asia and Southeast Europe (Figure 7). With widespread cold air across North America, new snowfall is possible in Alaska, northwestern and Eastern Canada and even the Northeastern US (Figure 7).
11-15 day
With positive geopotential height anomalies predicted for the Arctic but especially near Greenland (Figure 5b), the AO is predicted to remain negative this period (Figure 1). With strong positive pressure/geopotential height anomalies across Greenland and Iceland, the NAO will is also predicted to remain negative and possibly even strongly negative this period (Figure 1).
Ridging/positive geopotential height anomalies are predicted to continue drifting westward towards Greenland this period (Figure 5b). Therefore troughing/negative geopotential height anomalies are predicted to become more widespread across Europe with normal to below normal temperatures, including the UK (Figure 8). One exception is predicted to be northern Scandinavia where above normal temperatures are predicted to linger (Figure 8) with persistent positive geopotential height anomalies (Figure 5b). Troughing/negative geopotential height anomalies are predicted to dominate northcentral and Western Asia and even East Asia (Figure 5b). Ridging/positive geopotential height anomalies are predicted for Eastern Siberia extending southwestwards towards southcentral Asia (Figure 5b). This pattern favors normal to below normal temperatures for much of Northern Asia including East Asia with normal to above normal temperatures for Southern Asia including the Middle East (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 23 – 27 November 2018. The forecasts are from the 00Z 12 November 2018 GFS ensemble.
As ridging/positive geopotential height anomalies migrates from Europe to Greenland, high heights are predicted to become more widespread across the North American Arctic with troughing/negative geopotential heights to the south across the US (Figure 5b). This will favor for now a continuation of the temperature patter with normal to below normal temperatures across much of the Eastern US and Eastern Canada and normal to above normal temperatures in the Western US and Western Canada (Figure 8).

Figure 9. Forecasted snowfall anomalies (mm/day; shading) from 23 – 27 November 2018. The forecasts are from the 00Z 12 November 2018 GFS ensemble.
Once again additional snowfall is possible across much of northern Eurasia including Siberia, Scandinavia Eastern and even Central Europe (Figure 9). Seasonable to cold temperatures across Alaska, Canada and even the Northern Rockies into the Northern Plains will also support potentially new snowfall (Figure 9).
Longer Term
30–day
The latest plot of the polar cap geopotential heights (PCHs) shows in general normal to above normal PCHs in the troposphere and normal to below normal PCHs in the stratosphere (Figure 10). The above normal PCHs in the lower troposphere are consistent with a predicted mostly negative AO the next two weeks (Figure 1). The below normal PCHs in the stratosphere are consistent with the positive stratospheric AO this week while the weakening cold stratospheric PCHs predicted for next week are consistent with the predicted negative trend in the stratospheric AO towards neutral for next week (Figure 1).

Figure 10. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecasts are from the 00Z 12 November 2018 GFS ensemble.
The plot of Wave Activity Flux (WAFz) or poleward heat transport shows a relatively robust pulse of energy during last week that is now ending (Figure 11). This relatively strong pulse of WAFz is disturbed the PV causing it to stretch and extend into eastern North America (Figure 12). It is my opinion that this stretching of the PV is related to the cold air outbreak this week across the region.

Figure 11. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. The forecast is from the 00Z 12 November 2018 GFS ensemble.
Another pulse of WAFz is predicted for next week (Figure 11). The next pulse is not as strong as last week’s pulse but the amplitude could increase with subsequent model forecasts. I expect that next week’s pulse will once again perturb the stratospheric PV and cause the PV to stretch along an axis from Western Siberia into Eastern North America for the end of November (Figure 12). It is my opinion that this configuration of the stratospheric PV can dislodge more cold air from the North American Arctic across eastern North America either at the very end of November or early December after a moderation in temperatures predicted for next week. However the core of the cold in the stratosphere extends from Eastern Canada back towards the Western US and could be a sign the focus of the cold temperatures is centered more to the west than the current cold air outbreak but this is admittedly highly speculative on my part.

Figure 12. (a) Analyzed 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 12 November 2018. (b) Same as (a) except forecasted averaged from 23 – 27 November 2018. The forecasts are from the 00Z 12 November 2018 GFS operational model.
As I discussed in the Impacts section, blocking in the North Atlantic is conducive to more pulses of WAFz and disruption of the stratospheric PV. However the placement of the blocking either centered near Scandinavia or Greenland has important implications for the strength of the WAFz pulses and the likelihood of a significant stratospheric PV disruption. I expect the models to struggle with the tropospheric blocking feature, the WAFz and the behavior of the stratospheric PV, making for large uncertainty of the weather beyond a week.

Figure 13. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for December 2018. The forecasts are from the 12 November 2018 CFS.
I include in this week’s blog the monthly 500 hPa geopotential heights (Figure 13) and the surface temperatures (Figure 14) forecast for December from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). For what it’s worth the CFS has been fairly consistent in predicting a positive Pacific North American (PNA) pattern or a classic El Niño pattern. The forecast for the troposphere is ridging for Europe, Barents-Kara Seas, East Asia, near Alaska and Western Canada with troughs across Western Europe, East Asia, the Aleutians and the Eastern US (Figure 13). This pattern favors mild temperatures for Europe, the Middle East, East Asia and western North America with cold temperatures for Northern Asia, Eastern Canada and the Eastern US (Figure 14). Though based on the latest weather model runs I am highly skeptical of the forecast for Europe.

Figure 14. Forecasted average surface temperature anomalies (°C; shading) across the Northern Hemisphere for December 2018. The forecasts are from the 12 November 2018 CFS.
Surface Boundary Conditions
Arctic Sea Ice
Arctic sea ice growth rate continues at a good clip but still remains well below normal. However the negative sea ice anoamlies are mostly confined to two regions - the Chukchi-Beaufort and Barents-Kara Seas (Figure 13). Normal to above normal sea ice in and around Greenland and the Candian Archipelagos may favor a positve winter NAO. Based on recent research low sea ice anomalies in the Chukchi and Bering seas favors cold temperatures in central and eastern North America while low sea ice in the Barents-Kara seas favor cold temperatures in Central and East Asia, however this topic remains controversial. Recent research has shown that regional anomalies that are most highly correlated with the srtatospheric PV are across the Barents-Kara seas region where low Arctic sea ice favors a weaker winter PV. However it is looking like the the greatest negative anomalies are emerging in the Barents-Kara Seas and this may be the region most favored for ridging/blocking during the winter months.

Figure 15. Observed Arctic sea ice extent on 11 November 2018 (white). Orange line shows climatological extent of sea ice based on the years 1981-2010. Image courtesy of National Snow and Ice Data Center (NSIDC). Snow and Ice Data Center (NSIDC).
SSTs/El Niño/Southern Oscillation
Equatorial Pacific sea surface temperatures (SSTs) anomalies have continued to warm and have crossed the threshold into El Niño conditions (Figure 13), and the forecast is for likely weak to possibly moderate El Niño conditions for this winter. Observed SSTs across the NH remain well above normal especially in the North Pacific though below normal SSTs exist regionally. Well above normal SSTs in the northern North Pacific near Alaska are reminiscent of the "blob" winters of 2013/14 and 2014/15 and could support mid-tropospheric ridging in the coming months. Cold SSTs south of Iceland and in the subtripcs of the North Atlantic with above normal SSTs in the mid-latitudes are supposed to favor a positive winter NAO.

Figure 16. The latest weekly-mean global SST anomalies (ending 8 November 2018). Data from NOAA OI High-Resolution dataset.
Currently the Madden Julian Oscillation (MJO) is in phase 24 and is predicted to quickly weaken to where no phase is favored (Figure 14). Therefore little influence is expected from the MJO over the next two weeks.

Figure 17. Past and forecast values of the MJO index. Forecast values from the 00Z 5 November 2018 ECMWF model. Yellow lines indicate individual ensemble-member forecasts, with the green line showing the ensemble-mean. A measure of the model “spread” is denoted by the gray shading. Sector numbers indicate the phase of the MJO, with geographical labels indicating where anomalous convection occurs during that phase. Image source: http://www.atmos.albany.edu/facstaff/roundy/waves/phasediags.html
Northern Hemisphere Snow Cover
Snow cover advance continues its climb out of the basement across Eurasia and now is near decadal means. Snow cover advance could pick up more speed as cold temperatures start spreading to the west towards Europe this week. Above normal snow cover extent this past October, favors a strenghtened Siberian high, cold temperatures across northern Eurasia and a weakened polar vortex/negative AO this upcoming winter followed by cold temperatures across the continents of the NH.

Figure 18. Observed Eurasian (top) and North American (bottom) snow cover extent through 11 November 2018. Image source:
http://www.star.nesdis.noaa.gov/smcd/emb/snow/HTML/snow_extent_plots.html
The rate of North American snow cover advance continues at a blistering rate and remains near decadal highs. With continued cold air across Canada and the Northern US, North America snow cover will likely continue further. If snow and cold establish a foothold across Canada this fall, it could support an early start to winter across the Northern US.
