December 10, 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 transition to a fall/winter schedule, postings are once every week. Precipitation forecasts will be replaced by snow accumulation forecasts along with more emphasis on ice and snow boundary conditions (which are both now in their seasonal advance) and their influence on hemispheric weather.
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The AO/PV blog is partially supported by NSF grant AGS: 1657748.
- The Arctic Oscillation (AO) is currently neutral but is predicted to first trend positive to positive values this week and then remain near neutral right up until the holidays.
- The current neutral AO is reflective of mixed pressure/geopotential height anomalies across the Arctic and mixed pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently neutral with weak positive pressure/geopotential height anomalies across Greenland and Iceland and mixed pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic and is predicted to remain close to neutral over the next two weeks.
- Troughing/negative geopotential height anomalies across Central Europe will bring near seasonable temperatures this week but in general ridging/positive geopotential height anomalies will dominate much of Europe over the next two weeks resulting in normal to above normal temperatures including the United Kingdom (UK) starting this weekend and into next week.
- This week troughing/negative geopotential height anomalies in the Laptev Sea with ridging/positive geopotential height anomalies to the south across Siberia first will bring relatively mild temperatures to Siberia with normal to below normal temperatures further south across Southern Asia, the Indian subcontinent and Southeast Asia. However, building ridging/positive geopotential height anomalies in the Barents-Kara Seas will force deepening troughing/negative geopotential height anomalies with cold temperatures across Northern Asia, especially Siberia.
- Deepening troughing/negative geopotential height anomalies across Alaska will bring increasingly cold temperatures to the state while building ridging/positive geopotential height anomalies and spreading above normal temperatures for much of the rest of the continent including much of Canada and the Lower 48 the next two weeks. However, this week some remnant troughing/negative geopotential height anomalies in the Eastern United States (US) will bring normal to below normal for this week.
- In the Impacts section, I discuss my expectations for the large-scale pattern and temperatures across the Northern Hemisphere as a result of the increasingly likely sudden stratospheric warming and stratospheric polar vortex (PV) displacement towards Eurasia.
I realized last week that since I have been following the behavior of the stratospheric PV very closely, we have experienced only major midwinter warmings (MMWs – where the zonal mean zonal wind at 10hPa and 60°N reverses from westerly to easterly) of the PV split variety and not displacement variety. I consider that a fortunate occurrence since it is my belief that for reasons not truly known to me that PV splits favor blockbuster snowstorms especially nor’easters more so than PV displacements. But because all the PV disruptions have been splits, this has probably contributed to a gap in my knowledge on PV displacements. I even created an animation of the PV disruption of the winter of 2003/04 that I don’t think I ever shared but show it now in Figure i. There seems to be a strong similarity to what is predicted at least through late December and the animation could provide some guidance how the stratospheric PV may evolve in the coming weeks. It will be interesting to follow how closely the stratospheric PV evolution resemble each other this winter and other winters with stratospheric PV displacements in the 2000’s continuing into January.
Figure i. Animation of the 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for December 2003 and January 2004.
After I wrote last week’s blog I thought why didn’t I compare the Northern Hemisphere surface temperature response to PV splits and displacements in my Cohen and Jones 2011 paper as I did with sea level pressure? Probably because following both PV splits and displacements the surface pressure pattern closely resembles the negative AO/NAO and the temperature pattern associated with the negative AO/NAO is well known so no need to research the obvious. The temperature pattern is often referred to as a quadropole with below normal temperatures across Northern Europe and the Eastern US (but really focused in the Southeast) with above normal temperatures across the Mediterranean, North Africa and across Northeastern Canada. Temperatures are also cold across much of northern Eurasia including Northeast Asia but especially Siberia (see Figure ii). I don’t feel the same about the AO/NAO as I did then. The back to back winters of 2013/14 and 2014/15 have changed my perspective on the AO/NAO. Still to first order, if a PV displacement that crosses the threshold of MMW status, then generally expected temperature pattern should resemble the negative AO temperature pattern from two to six possibly nine weeks after the MMW. By the way our polar vortex model predicts a possible MMW around December 27th and then the next best possible chance the end of the first week of January. But based on the ongoing and predicted disruptions of the PV, that whole period looks to be favorable for a MMW to occur.
Figure ii. Correlation of the December, January and February AO with December, January and February surface temperatures. Light, darker and darkest shading represents 90, 95 and 99% significance. Red are positive values and blue negative values. Ao was multiplied by -1 to display temperatures for the AO negative phase.
I am aware of one study that looked at the surface temperature pattern before and after PV splits and displacements, Mitchell et al. 2013. They found that temperatures were above normal across eastern North America the month prior to stratospheric PV displacements but below normal the ensuing month. The temperature pattern across Europe resembles that related to a positive NAO with relatively warm temperatures across Northern Europe but cold across Southern Europe. But after the PV displacement temperatures across much of Europe average above normal. Across Asia temperatures average below normal the month prior to a stratospheric PV displacement but above average the month subsequent to a stratospheric PV displacement. I have to admit seeing that Asia is relatively mild post a stratospheric PV displacement is a surprise to me though to be fair the temperature anomalies are mostly not significant across the mid-latitudes post a stratospheric PV displacement with significant temperature anomalies more widespread for stratospheric PV splits. Possibly the most significant regional temperature anomaly in all four months shown (prior and post stratospheric PV displacements and splits) is the warmth across North America the month prior to stratospheric PV displacement. The current predicted temperature pattern across North America is mild for mid to late December except for cold in Alaska consistent with the month prior to stratospheric PV displacements. The predicted temperature pattern across Eurasia with warm across Northern Europe but cold in Siberia is also consistent with the temperature pattern prior to a stratospheric PV displacement.
Figure iii. Surface temperature anomalies in the NH during vortex (left) displacement events and (middle) splitting events, 30 to 0 day period before the event, (middle) the 0–30-day period after the event. Statistically significant areas at the 95% level according to a Student’s t test are stippled. From Mitchell et al. Journal of Climate page 2668, 2013.
Still as I am learning there is quite a bit of event to event variability. I have included in Figure iii the stratospheric PV a week or longer prior to when an MMW was observed and then in Figure iv the observed temperature anomalies across the NH the month when an MMW was observed and the following one or two months. Winters included are 1998/99, 2000/01, 2001/02, 2003/04, 2005/06 (in the Cohen and Jones paper this is listed as PV split - the cold Europe and blockbuster February snowstorm along the mid-Atlantic are consistent with this - but at least the beginning resembles a PV displacement), 2006/07 and 2007/08. Prior to 1998, the most recent stratospheric PV displacement was in 1987. My personal preference is not to use analogs prior to the era of amplified Arctic warming (pre-1990) for current winters.
Figure iv. Observed 10 mb geopotential heights (contouring) and anomalies (shading) for a) December 1-7, 1998, b) December 1-10 2000, c) December 15-28, 2001, d) December 21-31, 2003, e) January 1-14, 2006, f) February 7-17, 2007 and g) February 7-17 2008.
In all of the stratospheric PV displacements since 1998 the stratospheric PV is displaced towards northern Eurasia with the exception of February 2007 when the stratospheric PV was displaced towards Greenland. Also, the flow around the stratospheric PV in all winters was directed from Siberia towards eastern North America (5 winters) or to Europe (2 winters). The predicted stratospheric PV displacement is consistent with all the previous stratospheric PV displacement of the past two decades.
Figure v. Observed surface temperature anomalies for a) December-February, 1998/99, b) December-February 2000/01, c) January-February, 2002, d) January-February 2006, e) January-February 2006, f) February-March 2007 and g) February-March 2008.
Looking at the surface temperature anomalies during and following all seven stratospheric PV displacements shows more variability. Temperatures across eastern North America are below normal four of seven two or three winter months during and following the stratospheric PV displacement. Temperatures across much of Europe are below normal really only once for the two or three winter months during and following the stratospheric PV displacement. Temperatures across Asia are below normal three of seven two or three winter months during and following the stratospheric PV displacement.
Many of the details with the predicted sudden stratospheric warming are still unknown including the configuration of the stratospheric PV and the amplitude or even if it will remain simply a displacement or transition to a split (though I don’t see signs of that currently). But the associated temperature variability is greatest and most robust across eastern North America whipsawing from well above normal to below normal. The temperature response across Eurasia is less robust but stratospheric PV displacements do not as a rule seem to favor an extended period of below normal temperatures across Europe following the displacement. While for Asia the signals are more mixed.
In January I do expect Arctic sea ice anomalies to become more of a factor. It is my opinion that low regional sea ice can help force colder temperatures while above normal sea ice contributes to milder temperatures downstream. Sea ice is below normal in the North Pacific side of the Arctic in the Bering and Chukchi Seas, above normal in and around Greenland and the Canadian Archipelagos and below normal in the Barents-Kara Seas. This could constructively interfere or amplify the temperature anomalies associated with a stratospheric PV displacement with cold in eastern North America, mild in Europe and cold in Asia. The temperature pattern across North America could also be further be reinforced by the El Niño.
Finally, I have discussed previously about Mother Nature seeming to foreshadow the weather (for example Novembers 2013 and 2014). North American snow cover was record extensive for the month of November and for the fall season. I would interpret that as a possible foreshadowing of a cold winter in eastern North America. I also think that Mother Nature has a sense of humor and also throws in head fakes (e.g., November 2009 and 2010) to keep us guessing (of course I am just anthropomorphizing the weather). For now, I feel more strongly that the record snow cover extent this past fall was foreshadowing the winter weather across Norther America.
Near Term Conditions
The AO is currently neutral (Figure 1), reflective of mixed geopotential height anomalies across the Arctic and mixed geopotential height anomalies across the mid-latitudes of the NH (Figure 2). Though during the week negative geopotential height anomalies are predicted to become more widespread across the Arctic forcing a positive AO. Geopotential height anomalies are weak across Iceland and Greenland with mixed geopotential height anomalies across the mid-latitudes of the North Atlantic (Figure 2) and therefore the NAO is also near neutral.
Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 3 December 2018 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 3 December 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.
Currently ridging/positive geopotential height anomalies centered in the eastern North Atlantic and Western Europe is forcing troughing/negative geopotential height anomalies across Central Europe with more ridging in Eastern Europe (Figure 2). This is resulting in normal to above normal temperatures for Western and Eastern Europe including the UK with normal to below normal temperatures in Central Europe (Figure 3). Ridging/positive geopotential height anomalies across Northern Asia with troughing/negative geopotential height anomalies geopotential height anomalies across Southern Asia (Figure 2) are predicted to yield widespread normal to above normal temperatures across much Siberia and the Middle East with close to seasonable or below temperatures for Northern India and Southeast Asia (Figure 3).
Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 10 December 2018.
Deepening troughing/negative geopotential height anomalies across Alaska are forcing downstream ridging/positive geopotential height anomalies in western North America and Central Canada (Figure 2) with more troughing/negative geopotential height anomalies in the Eastern US (Figure 3). This is predicted to result in normal to below normal temperatures in Alaska and the Eastern US with normal to above normal temperatures for much of Canada and the Western US (Figure 3).
Figure 3. Forecasted surface temperature anomalies (°C; shading) from 11 – 15 December 2018. The forecast is from the 00Z 10 December 2018 GFS ensemble.
Troughing and/or cold temperatures will bring widespread new snowfall to Siberia and Central Europe (Figure 4). Across North America, increase troughing and colder temperatures will bring new snowfall to Alaska and Western Canada while milder temperatures will result in snowmelt in the Mid-Atlantic US (Figure 4).
Figure 4. Forecasted snowfall anomalies (mm/day; shading) from 11 – 15 December 2018. The forecast is from the 00Z 10 December 2018 GFS ensemble.
The AO is predicted to remain near neutral next week (Figure 1) with mixed geopotential height anomalies across the Arctic (Figure 5a). And with weak and mixed geopotential height anomalies across Greenland, the NAO will likely be near neutral 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 16 – 20 December 2018. (b) Same as (a) except averaged from 21 –25 December 2018. The forecasts are from the 10 December 2018 00z GFS ensemble.
Ridging/positive geopotential height anomalies are predicted to dominate much of Europe this period (Figure 5a). This is predicted to result in a normal to above normal temperatures for much of Europe including the UK (Figure 6). Ridging/positive geopotential height anomalies centered around the Barents-Kara Seas and the Ural Mountains will support downstream troughing/negative geopotential height anomalies across Northern and Eastern Asia (Figure 5a) with normal to below normal temperatures for much of Siberia and East Asia (Figure 6). Persistent ridging/positive geopotential height anomalies across much of Southwestern Asia will result in normal to above normal temperatures across the Middle East (Figure 6). Meanwhile some of the troughing/negative geopotential height anomalies across Northern Asia will sink into southcentral Asia including Northern India and Pakistan (Figure 5a). More ridging to the east across Southeast Asia (Figure 5a) will favor normal to above normal temperatures across the region (Figure 6).
Figure 6. Forecasted surface temperature anomalies (°C; shading) from 16 – 20 December 2018. The forecasts are from the 00Z 10 December 2018 GFS ensemble.
Deep troughing/negative geopotential height anomalies across Alaska and the Gulf of Alaska will force ridging/positive geopotential height anomalies further east across much of North America (Figure 5a). The resultant temperature anomalies across North America are predicted to be normal to below normal temperatures across Alaska with normal to above normal temperatures in Canada and the US (Figure 6).
Figure 7. Forecasted snowfall anomalies (mm/day; shading) from 16 – 20 December 2018. The forecasts are from the 00Z 10 December 2018 GFS ensemble.
Troughing and cold air will bring the potential for new snowfall across much of Northern Asia, Scandinavia and expanding into Eastern Europe and Central Asia (Figure 7). Across North America, new snowfall is possible in Alaska and Canada (Figure 7). Predicted milder temperatures could result in snowmelt across Central Europe and the Northern US (Figure 7).
With mostly mixed geopotential height anomalies predicted for the Arctic (Figure 5b), the AO is likely to remain neutral this period (Figure 1). With weak pressure/geopotential height anomalies across Greenland, the NAO is also predicted to remain neutral this period as well (Figure 1).
Ridging/positive geopotential height anomalies are predicted to dominate Europe (Figure 5b). High heights and westerly flow across the continent is predicted to result in normal to above normal temperatures for much of Europe including the UK (Figure 8). Predicted ridging/positive geopotential height anomalies focused near the Barents Kara Seas are predicted to continue to force troughing/negative geopotential height anomalies to the east across Northern Asia with more ridging/positive geopotential height anomalies dominating much of Southern Asia (Figure 5b). This pattern favors normal to below normal temperatures for much of Siberia and Northeast Asia with normal to above normal temperatures for Southern Asia including the Middle East, the Indian subcontinent and Southeast Asia (Figure 8). Some weakness in the ridging across the Tibetan plateau could allow colder temperatures to filter to the east towards Nepal, Bangladesh and Myanmar (Figure 8).
Figure 8. Forecasted surface temperature anomalies (°C; shading) from 21 – 25 December 2018. The forecasts are from the 00Z 10 December 2018 GFS ensemble.
Troughing/negative geopotential height anomalies previously centered over Alaska are predicted to drift westwards towards the Dateline pulling ridging/positive geopotential height anomalies over North America westward to become more focused across Western Canada and allowing renewed troughing/negative geopotential heights in the Eastern US (Figure 5b). This will allow below normal temperatures previously confined to Canada to spread southeast into Western Canada with normal to above normal temperatures for Central and Eastern Canada and the US (Figure 8). The spreading southward of cold air out of Alaska could be the beginnings of an Arctic outbreak heading towards the Lower 48.
Figure 9. Forecasted snowfall anomalies (mm/day; shading) from 21 – 25 December 2018. The forecasts are from the 00Z 10 December 2018 GFS ensemble.
Once again additional snowfall is possible across much of northern Eurasia including Siberia, Northeast Asia and even Scandinavia (Figure 9). Seasonable to cold temperatures across Alaska and Canada and even the Northcentral and Northeastern US will also support potentially new snowfall (Figure 9). Mild temperatures could result in snowmelt across Eastern Europe (Figure 9).
The latest plot of the polar cap geopotential heights (PCHs) shows in general near normal PCHs in the troposphere and in the stratosphere except for normal to below normal PCHs in the lower troposphere (Figure 10). The below normal PCHs in the lower troposphere are consistent with a predicted positive AO this week (Figure 1). However, stratospheric PCHs are predicted to turn warm/positive starting next week consistent with the predicted sudden stratospheric warming. The above normal PCHs in the stratosphere are consistent with the negative stratospheric AO for next week (Figure 1). The warm PCHs are predicted to propagate downward fairly quickly into the troposphere, though I am skeptical of the timing and may be delayed relative to forecast shown below.
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 10 December 2018 GFS ensemble.
The plot of Wave Activity Flux (WAFz) or poleward heat transport a very active next two weeks with the most amplified pulse at the end of the period (Figure 11). The ongoing pulses of WAFz are predicted to further disrupt the polar vortex and cause at least a minor sudden stratospheric warming in the polar stratosphere.
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 10 December 2018 GFS ensemble.
Currently the stratospheric PV is centered near Greenland with warming centered near the Dateline (Figure 12). The ongoing pulses of WAFz are predicted to nudge the stratospheric PV closer to the Eurasian coast while the warming wrapping in from East Asia is predicted to amplify the warming near the Dateline and help it build across Alaska then Canada and even spreading towards the North Pole (Figure 12).
Figure 12. (a) Analyzed 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 10 December 2018. (b) Same as (a) except forecasted averaged from 21 – 25 December 2018. The forecasts are from the 00Z 10 December 2018 GFS operational model.
As discussed in the Impacts section a stratospheric PV displacement is looking more and more likely during the second half of December and the question is more will it result in a minor or major stratospheric warming. Following the peak of the stratospheric warming, I would expect the warm/positive PCHS to “drip” down into the troposphere. On average this dripping takes about two weeks. But in previous blogs I have discussed there is often and immediate response to a stratospheric warming and a delayed response and they could markedly differ. In the Impacts section I discuss the anticipated impacts from the predicted stratospheric warming. A sudden stratospheric warming usually leads to a warm Arctic not only in the stratosphere but also at the surface as well. And as I will discuss in my Fall AGU talk on Friday a warmer Arctic favors more severe winter weather in the Northeastern US.
Figure 13. Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere for January 2019. The forecasts are from the 10 December 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 January from the Climate Forecast System (CFS; the plots represent yesterday’s four ensemble members). The CFS forecast has become more variable of late. The forecast for the troposphere is ridging for Central Canada, Southwestern Europe and East Asia with troughs across Western Asia, Northern Asia, Southcentral Asia, Northeastern Canada and the northern North Pacific (Figure 13). This pattern favors mild temperatures for Europe, the Middle East, East Asia and much of North America with cold temperatures for Siberia, the Indian subcontinent, Northeast Asia and Northeastern Canada (Figure 14).
Figure 14. Forecasted average surface temperature anomalies (°C; shading) across the Northern Hemisphere for Januray 2019. The forecasts are from the 10 December 2018 CFS.
Surface Boundary Conditions
Arctic Sea Ice
Arctic sea ice growth rate continues at a slow rate and 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 strength of the stratospheric 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. I expect that the forecasts of lower heights and colder temperatures near Alaska will help sea ice grow in the Chukchi and Bering seas in the near term.
Figure 15. Observed Arctic sea ice extent on 9 December 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 remain warm and support El Niño conditions (Figure 13), and the forecast is for likely weak to possibly moderate El Niño conditions for this winter. The expectations have been for a Central Pacific El Niño however, the warmest SST anomalies are now near the South American coast more similar to a canonical El Niño, though uncertainty continues. 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 subtropcs of the North Atlantic with above normal SSTs in the mid-latitudes are thought to favor a positive winter NAO.
Figure 16. The latest weekly-mean global SST anomalies (ending 8 December 2018). Data from NOAA OI High-Resolution dataset.
Currently the Madden Julian Oscillation (MJO) is in phase 3 (Figure 14). However the MJO is expected to transition to phase 4 and then weaken where to where no pahse is favored over the next two weeks. MJO phases 3 through 4 favor Alaska troughing and ridging in the interior of the continent with mild temperatures and could be influencing the predicted circulation pattern across North America. The MJO phase 3 is thought to force a sudden stratospheric warming in about a month's time (see for example Garfinkel et al. 2012).
Figure 17. Past and forecast values of the MJO index. Forecast values from the 00Z 3 December 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
I included the latest snow cover extent maps below but I feel that there is likely an error and therefore I have not updated the text. Snow cover advance continues its climb across Eurasia and remains near decadal means. Snow cover advance could pick up more speed as cold temperatures start spreading east to East Asia this week and to the west towards Europe next 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 9 December 2018. Image source:
The rate of North American snow cover advance rate has slowed somewhat but remains near decadal highs. With continued cold air across Canada and the Northern US, North America snow cover will likely continue further this week. However predicted eventual milder temperatures can lead to a pull back of snow cover extent. The early advance of snow cover across Canada this fall, has likely contributed to an early start to winter across the Northern US.