Arctic Oscillation and Polar Vortex Analysis and Forecasts

November 13, 2017

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 fall I will be transitioning to a fall/winter schedule, which is once every week starting sometime in October.   Precipitation forecasts will be replaced by snow accumulation forecasts.  Also there will be 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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Summary

  • The Arctic Oscillation (AO) is currently neutral but is predicted to be in a negative trend next week and remain negative through week two.
  • The current neutral AO is reflective of mostly mixed pressure/geopotential height anomalies across the Arctic and mostly mixed pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently neutral as well with mostly weak pressure/geopotential height anomalies across Greenland and Iceland.
  • Strong ridging/positive geopotential height anomalies are predicted to remain anchored near the dateline.  This will force troughing/negative geopotential height and cold temperatures across much of Siberia and East Asia. 
  • Ridging near the dateline will also force troughing/negative geopotential height across Alaska and the Gulf of Alaska accompanied by cold temperatures in Alaska and northwest Canada.
  • Normally this circulation pattern would result in a mild pattern across the Eastern United States (US), however strengthening ridging/positive geopotential height anomalies across Greenland (often referred to as Greenland blocking) will force troughing/negative geopotential height both across the Eastern US and Europe next week through the end of the month accompanied by seasonably cold temperatures.
  • Troposphere-stratosphere coupling has been relatively quiet, allowing for the stratospheric polar vortex (PV) to strengthen. Longer term, ridging/blocking across northern Eurasia could eventually initiate more active troposphere-stratosphere coupling resulting in a disruption of the stratospheric PV but as of yet, I see no signs of more active coupling.
  • Due to the lack of coupling, the stratospheric PV is predicted to remain stronger than normal for the remainder of November and into early December.  This needs to be monitored as it could potentially contribute to milder temperatures across the NH as soon as early December.
  • October Eurasian snow cover extent was above normal, Arctic sea ice extent is well below normal and will continue to be below normal for the entire winter.  Also, strong blocking is predicted across the high latitudes of the Eurasian sector in the coming weeks.  All three factors favor colder temperatures across at least parts of the Northern Hemisphere (NH) mid-latitude continents during the winter months. 
  • The AER winter forecast for the United States is tentatively scheduled to be published on the NSF website this week: https://www.nsf.gov/news/special_reports/autumnwinter/predicts.jsp

Impacts

What keeps me up at night?  The first is a strong positive anomaly in the 500 hPa geopotential height field floating around the northern North Pacific on either side of the dateline.  Regardless of everything else that is going on in the atmosphere, if that positive geopotential height anomaly (shown as sea level pressure anomally in Figure i) comes east into the Gulf of Alaska that will force a trough/negative geopotential height anomalies east of the Rockies and cold temperatures in the Eastern US.  Alternatively if those same positive height anomalies drift just a little further west and allows a trough/negative geopotential height anomalies to reside in the Gulf of Alaska with cold temperatures in western North America while mild Pacific air in all likelihood floods eastern North America with well above normal temperatures.  And to be honest I find it a difficult challenge to predict which of the two scenarios is most likely with polar opposite impacts downstream on the weather of North America.

Figure i. Estimate of November sea level pressure monthly anomaly.  Largest anomaly is positive anomaly centered east of the dateline near Alaska.

A La Niña is once again looking most likely for this winter.  La Niña is thought to favor a more westward shift of those positive geopotential height anomalies allowing troughing/negative geopotential height anomalies across western North America from the Pacific Northwest of the US north through Western Canada and Alaska with milder temperatures east of the Rockies across much of eastern North America.  However, I have spent the better part of a decade carefully following and documenting trends in various climate variables.  Related to this upcoming winter there are three relevant decadal trends: one is a cooling trend in temperatures in the Eastern US (Figure ii; right), the second is a positive trend in pressure/heights in the eastern North Pacific (Figure ii; left) and third a cooling trend in equatorial eastern Pacific sea surface temperatures that resemble a La Niña.  And if the trend is your friend this winter, this would favor the positive height anomaly extending into the eastern North Pacific driving cold air into eastern North America.

Figure ii. Trend in sea level pressure (December and January left) and surface temperature (Januaey and February right) over the period 1990/91-2015/16.  Figure taken from Kretscmer et l. 2017 BAMS published online http://journals.ametsoc.org/doi/pdf/10.1175/BAMS-D-16-0259.1.

The second item causing giving me restless nights is the strengthening stratospheric polar vortex.  Currently the troposphere and stratosphere are uncoupled.  The stratospheric PV is predicted to strengthen while the tropospheric circulation at high latitudes is predicted to become increasingly perturbed.  I actually prefer to call high latitude low pressure or centers of negative geopotential height anomalies in the troposphere polar lows rather than the polar vortex.  Rarely in the troposphere is there just one low center sitting atop the North Pole as is found in the stratosphere but rather there two or more low centers simultaneously present and often over the continents.  The troposphere and stratosphere can remain uncoupled for long periods in the winter but that is often the exception and at some point the stratosphere and troposphere couple are likely to couple.  How the troposphere and stratosphere couple is an open question in my mind.  One possibility is that the perturbed or disturbed state of the troposphere is transferred to the stratosphere and then boomerangs back into the troposphere, resulting in a relatively cold winter across the mid-latitudes of the NH.  The other possibility is that the mostly zonal circulation associated with the predicted strong stratospheric PV descends from the stratosphere to the troposphere resulting in a relatively mild winter across the mid-latitudes of the NH.    Both scenarios seem plausible to me yet the outcomes again would be polar opposite in regards to resultant sensible weather.

Near Term Conditions

1-5 day

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).  Geopotential height anomalies are relatively weak near Iceland and Greenland (Figure 2), and therefore the NAO is also neutral.

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 13 November 2017 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 13 November 2017 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.

Ridging/positive geopotential height anomalies in the eastern North Atlantic between Iceland and Great Britain are contributing to troughing/negative geopotential height anomalies in Scandinavia and Central Europe (Figure 2). However during the week the ridging in the North Atlantic will penetrate into Europe flattening out the flow and resulting in westerly winds and in milder temperatures for Western (including the UK) and Central Europe (Figure 3). However some residual troughing will form a cutoff low near Italy and northerly flow upstream of the trough axis across Italy will result in below normal temperatures for southwestern Europe (Figure 3).  Further east, mostly zonal (westerly) flow will result in above normal temperatures for Eastern Europe and Western Asia (Figure 3).  Meanwhile building positive geopotential height anomalies across the Kara and Laptev Seas (Figure 2) is forcing intensifying troughing/negative geopotential height anomalies further south across Siberia and East Asia resulting in below normal temperatures for most of Siberia and East Asia (Figure 3).  One exception is ridging/positive geopotential height anomalies and southerly flow in Eastern Siberia (Figure 2) that is bringing above normal temperatures to the region (Figure 3).   Finally, some weak ridging across Southeast Asia (Figure 2) will result in above normal temperatures for the region (Figure 3).

Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 13 November 2017. Note the high heights the Kara-Laptev Seas, the eastern North Pacific, Alaska and much of the US with low heights over Europe, Siberia and Northern Canada. 

Strong ridging/positive geopotential height anomalies extend across Eastern Siberia and western Alaska (Figure 2).  This is not only resulting in above normal temperatures for much of Alaska (Figure 3) but also helping to support troughing/negative geopotential height anomalies downstream in the Gulf of Alaska and across Western Canada (Figure 2). Northerly flow is contributing to below temperatures in Western Canada (Figure 3).  Upstream of the Gulf of Alaska trough, ridging/positive geopotential height anomalies dominate much of Southeastern Canada and the US (Figure 2) resulting in above normal temperatures for Southeastern Canada and much of the US (Figure 3).  One exception is the US East Coast where residual troughing and close to normal geopotential heights are resulting in seasonable temperatures (Figure 3). 

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 14 – 18 November 2017. Note the warm temperatures across Northern Europe, Western Asia, Alaska and the Southwestern US with cold temperatures across Western and Central Europe, Western and Central Siberia, Western Canada and the Eastern US. The forecast is from the 00Z 13 November 2017 GFS ensemble.

Troughing will result in additional snowfall for Scandinavia, northwest Asia, much of Siberia, Alaska and Canada (Figure 4).  In contrast, southerly flow and warmer temperatures will result in snowmelt across parts of Eastern Siberia and parts of Southern Canada (Figure 4).

Figure 4.  Forecasted snowfall anomalies (mm/day; shading) from 14 – 18 November 2017. Note the snowfall over Northern Europe, Siberia, Canada and Alaska with snowmelt over Southern Canada.  The forecasts are from the 00Z 13 November 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to drop into negative next week (Figure 1) as positive geopotential height anomalies spread across the Arctic with the exception of the Beaufort Sea (Figure 5a). And with positive geopotential height anomalies building across Greenland and Iceland, the NAO will descend into negative territory 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 19 – 23 November 2017. (b) Same as (a) except averaged from 24 – 28 November 2017. The forecasts are from the 13 November 2017 00z GFS ensemble. 

The ridging/positive geopotential height anomalies previously in the Laptev Sea will make its way to Greenland this period favoring troughing/negative geopotential height anomalies downstream across much of Europe (Figure 5a). Low heights and northerly flow will result in normal to below normal temperatures for much of Europe and the UK (Figure 6). Meanwhile some residual ridging/positive geopotential height anomalies are predicted to remain in Western Asia this period (Figure 5a). Southwesterly flow between the European trough and Western Asia ridge will bring mild temperatures to Eastern Europe and Western Asia (Figure 6).  Persistent ridging/positive geopotential height anomalies over the Kara and Laptev Seas will force troughing/negative geopotential height anomalies to the south across Siberia and East Asia (Figure 5a).  Northerly flow will allow below normal temperatures to become more widespread for Western and Central Siberia and into East Asia as well (Figure 6). Continued ridging/positive geopotential height anomalies centered near the dateline and extending into Eastern Siberia (Figure 5a) will maintain the streak of above normal temperatures for Eastern Siberia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 November 2017. Note the warm temperatures across Eastern Europe, Western Asia, Eastern Canada and the Western US with cold temperatures in Siberia and Northwestern Canada, Alaska and the Eastern US. The forecasts are from the 00Z 13 November GFS ensemble.

Across North America, ridging/positive geopotential height anomalies centered near the dateline will pull troughing/negative geopotential height anomalies across western Canada last period westward across Alaska and into the Gulf of Alaska (Figure 5a).   This will force ridging/positive geopotential height anomalies across Western Canada and the Western US (Figure 5a) and above normal temperatures (Figure 6).  However the rising geopotential heights in Greenland will force troughing/negative geopotential height anomalies in Eastern Canada and the Eastern US (Figure 5a) with normal to below normal temperatures for Southeastern Canada and much of the Eastern US (Figure 6).  

Figure 7. Forecasted snowfall anomalies (mm/day; shading) from 19 – 23 November 2017. Note new snowfall over Northern Europe, the Alps, Siberia, Alaska, Canada and the Northeastern US with snowmelt in Southwestern Canada. The forecasts are from the 00Z 13 November 2017 GFS ensemble.

Cold temperatures and troughing will support the potential for additional snowfall across much of Northern Eurasia including Siberia, Scandinavia, Eastern Europe, the Alps and even East Asia. In North America cold temperatures and troughing will support new snowfall in Northern and Eastern Canada and even the Northeastern US (Figure 7). On the flip side, southerly flow and warm temperatures will support snowmelt in Southwestern Canada (Figure 7).

11-15 day

Positive geopotential height anomalies will continue to dominate much of the Arctic (Figure 5b) maintaining the AO in negative territory this period (Figure 1). With positive pressure/geopotential height anomalies persisting across Greenland and Iceland, the NAO will likely remain negative as well (Figure 1).

Ridging/positive geopotential height persisting across Greenland will continue to favor troughing/negative geopotential height anomalies across Northern Europe as well this period with ridging/positive geopotential height anomalies across across Southern Europe (Figure 5b).  This nearly classic negative NAO pattern favors normal to below normal temperatures across Northern Europe (including the UK) and into northwestern Asia with normal to above normal temperatures for Southern Europe and Southwest Asia (Figure 8).  Once again this period, ridging/positive geopotential height anomalies are predicted on the Eurasian side of the Arctic from the Barents to the Chukchi Seas (Figure 5b). This will continue to favor troughing/negative geopotential height anomalies across much of Southern Siberia and into East Asia (Figure 5b) with concomitant temperatures below normal across much of Siberia and East Asia (Figure 8). One exception is predicted for Eastern Siberia where ridging/positive geopotential height anomalies coupled with southerly flow (Figure 5b) favor normal to above normal temperatures for Eastern Siberia (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 November 2017. Note the warm temperatures across Southern Europe, Southwestern Asia, and the Western US with cold temperatures in Northern Europe, Siberia, Northwestern Canada, Alaska and the Eastern US.   The forecasts are from the 00Z 13 November 2017 GFS ensemble.

Little change is predicted in the circulation across North America with strong positive geopotential height anomalies centered near the dateline anchoring troughing/negative geopotential height anomalies across Alaska and the Gulf of Alaska (Figure 5b).  This favors normal to below normal temperatures across Alaska and Northwestern Canada (Figure 8).  Further upstream ridging/positive geopotential height anomalies are predicted to persist across Western and Central Canada and the Western US (Figure 5b) with widespread normal to above normal temperatures across Western Canada and much of the Western US (Figure 8).  However with persistent Greenland blocking predicted troughing/negative geopotential height anomalies are favored in Southeastern Canada and the Eastern US (Figure 5b) with seasonably cold temperatures predicted in Southeastern Canada and the Eastern US (Figure 8).

Figure 9.  Forecasted snowfall anomalies (mm/day; shading) from 24 – 28 November 2017. Note new snowfall over Siberia, Northern Europe, Alaska and Canada.  The forecasts are from the 00Z 13 November 2017 GFS ensemble.

Troughing and cold temperatures will support expansion of NH snow cover extent with the potential of additional snowfall across Europe, Siberia, East Asia, Canada and even possibly the Northern US this period (Figure 9).

Longer Term

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows cold/below normal PCHs in the stratosphere but warm/above normal PCHs in the troposphere (Figure 10).  The forecast of warm positive PCHs in the troposphere is consistent with the predicted negative AO trend and a turn to colder weather across much of the NH continents but especially Asia.

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 00Z13 November 2017 GFS ensemble.

The forecast of Wave Activity Flux (WAFz) or poleward heat transport predicts a continuation of the relatively quiet period for the next two weeks (Figure 11).  This results in colder stratospheric PCHs (Figure 10) and a strengthening stratospheric PV over the next two weeks (Figure 12).  This can also be seen in the positive stratospheric AO (Figure 1).  The negative AO/NAO pattern is not one that favors more active and WAFz or poleward heat transport so the quiet period could persist well into December.

Figure 11. Observed and predicted daily vertical component of the wave activity flux (WAFz) standardized anomalies, averaged poleward of 40-80°N. The forecasts are from the 00Z 13 November 2017 GFS ensemble.

Last week I discussed two possible scenarios but actually after some more thought I believe that there are three: one where the troposphere and stratosphere remain uncoupled (the circulation in each sphere are divergent or opposite) and two where the troposphere and stratosphere become coupled (where the circulations are similar or mirror each other).  There are of course other possible scenarios but they are all variations of the three presented.  The first scenario could result where the WAFz or poleward heat transport remains quiet for an extended period.   In this scenario little changes in both the troposphere and the stratosphere with the troposphere remaining perturbed while a relatively strong PV persists in the stratosphere.  A good example of this scenario is winter 1995/96 and to a lesser degree winter 2010/11.  The other two scenarios involve troposphere and stratosphere coupling and a period of more active WAFz or poleward heat transport.  The first is where the WAFz becomes active but instead of being directed towards the North Pole it is directed to the equator.  This would result in a further strengthening of the stratospheric PV that eventually couples to the troposphere leading to an extended period of a positive tropospheric AO, mostly zonal flow and relatively mild conditions across the NH mid-latitude continents. The second is where the WAFz becomes active which is directed towards the North Pole leading to a weakening of the stratospheric PV.  Initially this could result in a relatively mild period for the NH but eventually the PV disruption couples with the troposphere resulting in increased high latitude blocking and a relatively cold pattern across the NH mid-latitude continents.  It is hard to know which scenario is favored but the easterly Quasi-Biennial Oscillation (QBO) should favor the latter scenario if and when the WAFz becomes more active.

Figure 12. (a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 19 – 23 November 2017. (b) Same as (a) except averaged from 24 – 28 November 2017. The forecasts are from the 13 November 2017 00z GFS ensemble. 

I was hoping to include longer-range temperature forecasts with all updates of the blog using the monthly temperature anomaly forecast from the Climate Forecast System (CFS) but failed to generate the plots today.  From what I can see from other websites, the CFS is predicting a very mild pattern for much of the NH with the exception of Canada especially Eastern Canada.  If the troposphere and stratosphere couple with a strong positive AO in both, this prediction or some variant theme is possible but I am skeptical.

Surface Boundary Conditions

Arctic Sea Ice

Large swaths of the North Pacific side of the Arctic basin are still ice-free as well as in the Barents-Kara Seas (Figure 13). The largest negative anomalies in sea ice extent are in the Beaufort Sea.  This is different from last November when the largest negative anomalies where in the Barents-Kara Seas.  The past two winters the strongest blocking at high latitudes was centered near the Barents-Kara Seas (the Urals in 2015/16 and Scandinavia in 2016/17).  Recent sea ice anomalies suggest a different scenario for the winter of 2017/18 with blocking focused in the North Pacific sector.  Admittedly this is a speculative inference on my part but something to watch in the coming months.

Figure 13. Observed Arctic sea ice extent on 12 November 2017 (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).

Recent research has shown that regional anomalies are important and the sea ice region most highly correlated with the winter AO is the Barents-Kara Seas region where low Arctic sea ice favors a negative winter AO and a cold Eurasia.  But as I just noted sea ice in the Barents-Kara-Laptev Seas is currently more extensive compared to last year at this time and blocking in this region with downstream cold air outbreaks across Eurasia may not be as favored for this upcoming winter.

SSTs/El Niño/Southern Oscillation

Equatorial Pacific sea surface temperatures (SSTs) remain normal to slightly below normal in much of the tropical and subtropical Pacific (Figure 15). El Niño/Southern Oscillation (ENSO) forecasts for this winter are that a La Niña is likely this winter.  ENSO forecasts have been struggling for much of the year but equatorial Pacific SSTs have recently cooled and I think that the winter forecast for La Niña is looking better.  Observed SSTs across the globe remain well above normal though below normal SSTs exist regionally across parts of the NH mid latitudes oceans in both the North Pacific and the North Atlantic oceans.

Figure 14. The latest weekly-mean global SST anomalies (ending 12 November 2017). Data from NOAA OI High-Resolution dataset.

Once again no phase of the Madden Julian Oscillation (MJO) is favored and the forecast is for no phase to be favored over the coming two weeks (not shown).  Therefore I do not expect that the MJO will be a significant influence on North American weather in the coming weeks.

Northern Hemisphere Snow Cover

October Eurasian snow cover extent was above normal relative to long-term averages (Figure 16). Above normal Eurasia snow cover favors a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter with cold temperatures across the continents of the NH.  The most likely timing of a weakened stratospheric AO followed by a negative AO is January.  Though despite the above normal snow cover extent, the rate of the snow cover advance for October was slower than the average rate of advance relative to the past two decades.  I do think that the slower rate of advance of snow cover favors a relatively strong stratospheric PV in the near term.  For November snow cover advance has been lethargic across Eurasia coincident with a relatively mild pattern.  However with an increasingly negative AO, snow cover extent has begun to advance once again and I do expect the trend in Eurasian snow cover advance to continue.

Figure 15. a) Observed Eurasian (snow cover extent through 12 November 2017. b) same as a) but for North Maerican snow cover. Image source: http://www.star.nesdis.noaa.gov/smcd/emb/snow/HTML/snow_extent_plots.html

North American snow cover is also above normal and in contrast to recent sluggishness in Eurasia has accelerated in November.  North American snow cover is currently at decadal highs and is in contrast to last November when snow cover extent was at decadal lows.  Further snow cover could advance is likely especially in eastern North America but the explosive growth this month is probably over.  Still more extensive snow cover could support colder Arctic air masses in the coming weeks.