Arctic Oscillation and Polar Vortex Analysis and Forecasts

February 13, 2017

Special blog on winter 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.

I plan  on updating the weather discussion every Monday.  Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.

Summary

  • The Arctic Oscillation (AO) is currently negative but is predicted to turn positive by the weekend and remain positive week through the end of February.  However I consider the AO forecasts as low confidence.
  • The current negative AO is reflective of positive pressure/geopotential height anomalies on the North Atlantic side of the Arctic with negative pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic sector. The North Atlantic Oscillation (NAO) is also currently negative because of positive height anomalies near Iceland and Greenland.  However, the NAO is also predicted to remain mostly positive over the next two weeks.
  • The positive AO trend is consistent with more active vertical Wave Activity Flux (WAFz)/poleward heat transport.  As often is the case, the upcoming period of more active poleward heat transport will coincide with relatively mild temperatures for much of North America and Europe but cold temperatures for East Asia.
  • The more active poleward heat transport will also result in the third sudden stratospheric warming (SSW)/weak polar vortex (PV)/negative stratospheric AO event of the winter.
  • The SSW/weak PV event will likely peak around the end of the month.  I expect that the circulation anomalies associated with the weak PV will likely descend from the stratosphere to the troposphere starting at the end of February.
  • Changes in the tropospheric circulation the end of February and early March will possibly include a negative trend in the tropospheric AO and a cooling trend in temperatures for eastern North America and/or Europe.

Impacts

The predicted pattern of ridging/positive geopotential height anomalies over western Eurasia with troughing/negative geopotential height anomalies in the North Pacific is favorable for more active poleward heat transport.  Consistent with this pattern a new round of active upward Wave Activity Flux (WAFz)/poleward heat flux is predicted over the next two weeks and will likely have an important influence on Northern Hemisphere (NH) weather. Active WAFz/poleward heat transport favors a positive trend in the tropospheric AO accompanied by colder temperatures over the Arctic and milder temperatures in eastern North America and Europe.  One region that often bucks the milder NH mid-latitude trend during active poleward heat transport is East Asia. 

I once asked in the blog why is it that temperatures in East Asia are often cold during periods of active poleward heat transport opposite of the NH average.  After some thought I think that it is related to the atmospheric circulation that favors periods of active poleward heat transport.  It has been shown, by myself and others, that the pattern that is most favorable for active poleward heat transport is ridging/positive geopotential height anomalies over western Eurasia with troughing/negative geopotential height anomalies in the North Pacific.  That pattern often results in southwesterly to westerly flow of mild maritime air over Europe and North America but northerly flow over East Asia.  So even though in theory any combination of poleward transport of warm temperatures and equatorward transport of cold temperatures results in increased WAFz/poleward heat transport, it is likely that the atmospheric pattern associated with surges of warm temperatures over North America and/or Europe with cold surges over East Asia best projects onto the climatological pattern when wave energy escapes the troposphere and is absorbed in the stratosphere.

On the flip side more active WAFz/poleward heat transport favors a negative trend in the stratospheric AO accompanied by warmer temperatures over the Arctic and colder temperatures across the mid-latitudes.  However, following the peak in the SSW/weak PV events, the circulation anomalies present first in the stratosphere descend with time from the stratosphere to the troposphere.  Therefore, I expect as early as the very end of February but more likely early March, a negative trend in the tropospheric AO and colder temperatures in eastern North America and Europe.  However, given how mild temperatures are ahead of the cooling trend and the lateness in the winter season the full impacts of the SSW/weak PV event will likely be muted or damped compared to the impacts of a similar event earlier in the winter.

This pattern of a SSW/weak PV event in mid-winter to be quickly followed by active poleward heat transport and yet another SSW/weak PV event reminds me of last winter and I think there may be similarities to the end of this winter to last winter including an early final warming in the polar stratosphere and a delayed spring, at least regionally.

Near Term Conditions

1-5 day

The AO is currently negative (Figure 1), reflective of positive geopotential height anomalies across the North Atlantic side of the Arctic and negative geopotential height anomalies across the mid-latitudes of the North Atlantic basin (Figure 2).  Geopotential height anomalies are mostly positive near Iceland and Greenland (Figure 2), and therefore the NAO is negative as well.

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

Strong ridging/positive geopotential height anomalies stretches from eastern Greenland and Iceland and across much of Europe (Figure 2) resulting in predicted mild temperatures this week for Europe and east into Northwest Asia (Figure 3).  Downstream of the European ridging, troughing/negative geopotential height anomalies extend south from the North Pole across all of Western Asia and into the Middle East as well as east across all of Siberia (Figure 2).  Negative geopotential height anomalies combined with northerly flow is predicted to result in below normal temperatures for much of Southwestern Asia and east across Central and Eastern Siberia and even Northeast Asia (Figure 3).  Southwesterly flow between the Ural trough and ridging/positive geopotential height anomalies across East Asia (Figure 2), is transporting mild temperatures across Central and Southeast Asia, including much of China (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 13 February 2017 at 00Z. Note the high heights over Europe, Southern Asia and Western North America with low heights over the North Pacific, the Northeastern US and Western Asia. 

Another trough/negative geopotential height anomalies extend from the North Pole south across Alaska and much of the North Pacific basin (Figure 2).  This is forcing ridging/positive geopotential height anomalies across Western Canada and the Western United States (US) (Figure 2).  Positive geopotential height anomalies and southwesterly flow of mild air are predicted to bring mild temperatures to Alaska, the Western US and much of Canada (Figure 3).  The ridging across western North America is contributing to troughing/negative geopotential height anomalies stretching across Southeast Canada and the Eastern US (Figure 2) resulting in seasonable temperatures for Southeast Canada and the Northeastern US (Figure 3). 

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 14 – 18 February 2017. Note the warm temperatures across Northern Europe, China and much of North America with cold temperatures in Southwestern Asia, Canada and Northeastern Asia. The forecast is from the 00Z 13 February 2017 GFS ensemble.

The negative geopotential height anomalies anchored over Western Asia will help support new snowfall for Western Asia and Southern Siberia (Figure 4). An active Jet Stream will also help support new snowfall across Eastern Canada and Northern New England (Figure 4).  In contrast ridging and mild air will support snowmelt for Northern and Eastern Europe, Western Canada and the Northwestern US.

Figure 4.  Forecasted snow depth anomalies (mm/day; shading) from 14 – 18 February 2017. Note the snowfall over parts of Siberia, Western Asia and Alaska with snowmelt in Europe and Western Canada.  The forecasts are from the 00Z 13 February 2017 2016 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to turn positive next week (Figure 1) as negative geopotential height anomalies previously over the Pole and Western Asia retrograde into the North Atlantic side of the Arctic (Figure 5a).  With negative geopotential height anomalies stretching from Greenland to Iceland the NAO may is predicted to be positive as well.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 19 – 23 February 2017. (b) Same as (a) except averaged from 24 – 28 February 2017. The forecasts are from the 13 February 2017 00z GFS ensemble.

The tropospheric circulation will begin to reflect changes in the stratospheric circulation this period.  With the stratospheric PV is once again displaced towards Northwest Asia and Northern Europe, will promote troughing/negative geopotential height anomalies across the same region in the troposphere (Figure 5a).  This will further suppress ridging/positive geopotential height anomalies south across Southern Europe, the Mediterranean and Southwest Asia (Figure 5a).  Strong westerly flow between low heights across Scandinavia and high heights across Southern Europe will transport maritime air and mild temperatures across Europe, Western Asia and even Central Asia (Figure 6). Northerly flow between ridging/positive geopotential height anomalies in Central Asia and troughing/negative geopotential height anomalies in Eastern Siberia (Figure 5a) will transport colder temperatures across Eastern Siberia and East Asia (Figure 6).  Persistent ridging/positive geopotential height anomalies across East Asia (Figure 5a) will bring above normal temperatures to Southeast Asia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 February 2017. Note the warm temperatures across much Europe, Western and Central Asia, China and much of North America with cold temperatures in East Asia. The forecasts are from the 00Z 13 February 2017 GFS ensemble.

Ridging/positive geopotential height anomalies across Northern Canada in the stratosphere will promote ridging in the troposphere across Alaska and Northern Canada as well (Figure 5a).  Meanwhile a trough/negative geopotential height anomalies will undercut the ridging in Alaska and come ashore along the US West Coast (Figure 5a).  This in turn will force ridging/positive geopotential height anomalies across the Eastern US (Figure 5a). High heights and southwesterly flow will support widespread mild temperatures across Canada and the US (Figure 6).

Figure 7. Forecasted snow depth anomalies (mm/day; shading) from 19 – 23 February 2017. Note the snowfall over Siberia, Western Canada and Alaska with snow melt across Eastern Europe, Western Asia and the Northeastern US. The forecasts are from the 00Z 13 February 2017 GFS ensemble.

With geopotential heights lowering across Scandinavia, new snowfall is predicted (Figure 7).  In contrast, further east in the mild air new snowmelt is predicted across Eastern Europe and Western Asia (Figure 7).  Milder air will also support snowmelt across Southern Canada and the Northern US (Figure 7).

11-15 day

Predicted negative pressure/geopotential height anomalies across Northern Europe and the North Atlantic side of the Arctic this period (Figure 6b) will continue to favor a positive AO. With negative pressure/geopotential height anomalies predicted across Greenland and Iceland, the NAO will also likely remain positive this period (Figure 1).

Little change is predicted in the general circulation pattern across Eurasia this period (Figure 5b). The general pattern of above normal geopotential heights across most of Southern Europe and Southern Asia with below normal geopotential heights across Northwestern Eurasia and Eastern Siberia is predicted to persist (Figure 5b).  Persistent westerly flow between low heights across Scandinavia and high heights across Southern Europe and Southern Asia will transport maritime air and mild temperatures across Europe, Western Asia and Central Asia (Figure 8).  One exception could be Scandinavia and far Northwestern Asia where northerly flow could transport colder temperatures across the region (Figure 8).  Northerly flow between ridging/positive geopotential height anomalies in Central Asia and troughing/negative geopotential height anomalies in Eastern Siberia (Figure 5b) will continue to transport colder temperatures across Eastern Siberia and East Asia (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 February 2017. Note the warm temperatures across western North America, Europe, Western and Central Asia with cold temperatures in Eastern Asia, Eastern Siberia and Eastern Canada.   The forecasts are from the 00Z 13 February 2017 GFS ensemble.

Ridging/positive geopotential height anomalies across are predicted to consolidate across Alaska and the Gulf of Alaska (Figure 5b).  This will force troughing/negative geopotential height anomalies downstream over eastern North America (Figure 5b).  Above normal temperatures are likely under the ridging in Alaska, Western Canada and across the Southern US (Figure 8).  Troughing and northerly flow is predicted to result in colder temperatures across Eastern Canada, which could filter into the Northeastern US (Figure 8).

Figure 9.  Forecasted snow depth anomalies (mm/day; shading) from 24 – 28 February 2017. Note the snowfall over Scandinavia, Western and Eastern Canada with snow melt over Western and Central Asia. The forecasts are from the 00Z 13 February 2017 GFS ensemble.

Colder temperatures are likely to support new snowfall across Northern Europe, Eastern Canada and the Northern US (Figure 9).  Meanwhile a persistent mild pattern favors additional snowmelt across Eastern Europe, Western and Central Asia (Figure 9).

Longer Term

30–day

The latest plot of the polar cap geopotential heights (PCHs) shows one SSW/weak PV event followed by downward propagation concluding with another cycle beginning, or as some like to joke - rinse, repeat (Figure 10). A new SSW/weak PV event can be seen by the positive/warm stratospheric PCHs predicted by the end of he week right through the end of February.  The SSW/weak PV event is also consistent with the predicted negative stratospheric AO (Figure 1).

Figure 10. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecast is from the 00Z 13 February 2017 GFS ensemble.

The stratospheric PV is currently near normal in strength and in position.  But the PV is predicted to slide over to northwest Eurasia with warm temperatures overspreading the polar stratosphere (Figure 11), a sign that the PV is undergoing a disruption. This pattern only amplifies by month’s end (Figure 11). The weakening stratospheric PV (Figure 11), is a result of predicted above normal poleward heat flux/WAFz over the next two weeks (Figure 12).  

Figure 11. (a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 19 – 23 February 2017. (b) Same as (a) except averaged from 24 – 28 February 2017. The forecasts are from the 00Z 13 February 2017 GFS operational model.

Though it is not apparent from the PCH plot, I do expect the circulation anomalies in the polar stratosphere to propagate into the troposphere as early as the end of February and more likely in early March.  The PV disruption should favor a return to a more wintry pattern both across eastern North America and Europe.  As has been the case previously this winter, warming across Northern Canada and positive geopotential height anomalies should favor ridging/blocking across Northern Canada and a suppressed Jet Stream across the US.  When this pattern has occurred previously this winter it has not been able to force a sustained and persistent period of cold temperatures across eastern North America and I see little reason to expect differently as winter 2016/17 concludes, given the record warmth predicted this week across North America and the strengthening sun.  Instead I expect temperatures to return to more seasonable levels and perhaps the pattern could become more favorable for new snowfall, as storms take a more southerly trajectory across North America.

Figure 12. 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 13 February 2017 GFS ensemble.

The displacement of the stratospheric PV towards Northern Europe has also occurred previously this winter.  Northerly flow around the displaced PV has brought periods of cold temperatures to Europe and I expect a repeat of this scenario sometime in March.  Also usually East Asia is colder leading up to a SSW and milder following a SSW, therefore I expect a reversal to milder weather across East Asia in March.  No strong suggestions of the colder scenario for with the US or Europe from the weather models but as I have noted repeatedly in the blog the weather models have a difficult time with downward propagation of warm PCHs from the stratosphere to the troposphere and I expect the same over the coming weeks.  Therefore I expect large model uncertainty and forecast volatility through at least the end of the month.

Of course the alternative is for the SSW/weak PV event to have little impact on the tropospheric circulation with the mild pattern continuing right into spring.

Surface Boundary Conditions

Arctic Sea Ice

Arctic sea ice extent advance this week stalled once gain this week as warm air surged into the central Arctic.  Therefore, Arctic sea ice extent remains at or near record low levels. Negative sea ice anomalies remain on both the North Atlantic side in the Barents-Kara Seas and on the North Pacific side in the Bering Strait (Figure 13). Recent research has shown that regional anomalies are important and the sea ice region most highly correlated with Eurasian temperatures is the Barents-Kara Seas region where low Arctic sea ice favors a strengthened Siberian high and cold Eurasian temperatures.  Low sea ice in the Barents-Kara seas is also thought to contribute to a weakened polar vortex/negative AO mid to late winter. Consistent with that theory a SSW/PV weakening is predicted for the third time this winter.  In contrast, negative sea ice anomalies on the North Pacific side of the Arctic have not been shown to perturb the stratospheric PV but are thought to be favorable for forcing cold temperatures in Canada and the US.

Figure 13. Observed Arctic sea ice extent on 12 February 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).

SSTs/El Niño/Southern Oscillation

Equatorial Pacific sea surface temperatures (SSTs) continue to be weakly cooler than average (Figure 14) but are currently above temperatures required to qualify as a minimal La Niña. It will be a close call as to whether the forecasts of La Niña will verify.

Figure 14. The latest weekly-mean global SST anomalies (ending 11 February 2017). Data from NOAA OI High-Resolution dataset. The tropical Pacific shows La Niña SST structure with cool waters near the equator in the eastern and central tropical Pacific. Warmer than normal waters also extend into the subtropical North Pacific and along the western coast of North America and along the East Asian coast.  Well above normal waters extend across the subpolar North Atlantic near Greenland and Iceland.

Currently the Madden Julian Oscillation (MJO) is currently entering Phase 8 and is predicted to achieve near record amplitude in phase 8 before weakening in Phase 1 (Figure 15).  Phases eight and one favor troughing (ridging) and cold (warm) temperatures in the Eastern (Western) US.  The problem is that over the next week the weather models predict the opposite pattern with a trough in the west and a ridge in the east accompanied by warm temperatures.  Still there is the potential for the MJO to constructively interfere with a weak PV that also favors troughing (ridging) and cold (warm) temperatures in the Eastern (Western) US before month’s end.

Figure 15. Past and forecast values of the MJO index. Forecast values from the 00Z 13 February 2017 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.

Colder than normal SSTs still persist across the mid-latitudes of the North Pacific.  I do believe that this likely strengthened the North Pacific jet leading to high amounts of rainfall along the US West Coast and milder temperatures in the Eastern US.  It is possible that this pattern could continue through the end of the winter.

Northern Hemisphere Snow Cover

Snow cover across the NH as a whole and Eurasian and North America separately continue in a holding pattern near decadal means. However with milder weather predicted, Eurasian and North American snow cover could begin to recede as is expected this time of the year regardless of the pattern.

Snow cover advance across Eurasia continued consistently above normal for the entire month of October.  Also because much of the advance has occurred at latitudes south of 60°N, the snow advance index is also well above normal.  Above normal snow cover extent, especially south of 60°N, favored a strengthened Siberian high, cold temperatures across northern Eurasia and a weakened polar vortex/negative AO this winter followed by cold temperatures across the continents of the NH.