Arctic Oscillation and Polar Vortex Analysis and Forecasts

February 21, 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 positive and is predicted to remain positive through the end of February and then turn negative in early March.
  • The current positive AO is reflective of negative pressure/geopotential height anomalies across the Arctic and mostly positive pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is also currently positive because of negative height anomalies near Iceland and Greenland.  However, the NAO is also predicted to trend negative in early March.
  • The current positive AO is consistent with more active vertical Wave Activity Flux (WAFz)/poleward heat transport.  As often is the case, the ongoing period of more active poleward heat transport is coinciding with mild temperatures for much of North America and Europe but cold temperatures for East Asia.
  • The more active poleward heat transport is forcing the third sudden stratospheric warming (SSW)/weak polar vortex (PV)/negative stratospheric AO event of the winter and even possibly an unprecedented second major mid-winter warming (reversal of the wind from westerly to easterly at 60°N and 10 hPa) of the month (two major warmings in the same month have never been observed).
  • The SSW/weak PV event will likely peak either the end of February or early March.  The circulation anomalies associated are predicted to descend from the stratosphere to the troposphere in early March.
  • Predicted changes in the tropospheric circulation in early March include a negative trend in the tropospheric AO and a cooling trend in temperatures for North America and/or Europe. 
  • I expect this trend to a more negative AO and cooler temperatures across the Eastern US and Europe to continue well into March, though the pattern for much of this winter argues against a persistently cold pattern especially in the Eastern US.

Impacts

An SSW/weak PV event culminated in a major mid-winter in warming in the first days of February.  There was a clear downward propagation of circulation anomalies associated with the weak polar vortex within a week’s time.  The downward propagation included warm polar cap heights and a negative AO in the troposphere.  It has been demonstrated in the scientific literature that the tropospheric response to a major warming where the circulation anomalies clearly descend into the troposphere, persist for up to four to six weeks.  These were my expectations as well but from the polar cap geopotential heights (PCHs) plot it is clear that the tropospheric response lasted only on the order of one week.  This was quickly followed by another period active upward Wave Activity Flux (WAFz)/poleward heat transport that favors a cold tropospheric PCH (and the Arctic is as cold as it has been in a year), a positive AO and mild temperatures across the mid-latitude Northern Hemisphere (NH) continents.  However the response in the stratosphere to the active WAFz is just the opposite and PCHs are currently warming in the stratosphere and the stratospheric AO is negative.  I expect that the tropospheric circulation will respond to the latest SSW/weak PV event in the coming weeks but the weather models seem unsure of the robustness of the tropospheric response.

This  “rinse, repeat” of tropospheric-stratosphere coupling reminds me of last February and March.  In early February 2016 there was an SSW/weak PV event (though not as robust as in early February 2017) with a relatively short duration tropospheric response quickly followed by more active WAFz, a positive tropospheric AO and mild temperatures across the NH continents.  The active WAFz did result in a record weak PV in March and another round of downward propagation of warm tropospheric PCHs and a negative tropospheric AO.   Why accelerated troposphere-stratosphere coupling has occurred the past two winters, in my opinion is an important question and I don’t have a good answer.  Though both winters were characterized by low Barents-Kara sea ice, strong blocking near the Urals/Barents-Kara seas that is so favorable for WAFz/poleward heat transport.

It is currently relatively cold in the central Arctic and very mild across the NH continents or a “cold Arctic/warm continents” pattern. However, the third SSW/weak PV event of the winter is currently occurring that is predicted to peak the very end of February or possibly early March.  Following the peak in the SSW/weak PV event, I expect that the circulation anomalies present first in the stratosphere will descend with time from the stratosphere to the troposphere.  Therefore, I anticipate a negative trend in the tropospheric AO accompanied by warmer temperatures in the Arctic and colder temperatures across the NH continents or a “warm Arctic/cold continents” pattern.  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 early in the winter.  However two dominant features this winter that have favored milder temperatures have been a strong North Pacific jet that has repeatedly pumped mild air across the US and high pressure/blocking across Northern Europe that has deflected the cold air south across Southern Europe and the Mediterranean.  Therefore it is certainly plausible that those circulation features will continue to be a moderating influence on temperatures across the US and Northern Europe through the end of winter and into early spring.

Near Term Conditions

1-5 day

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

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 21 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.

In the stratosphere the PV is displaced near northern Scandinavia and occupies the North Atlantic sector of the Arctic.  A reflection of the stratospheric PV exists in the troposphere with deep troughing/negative geopotential height anomalies stretching from Greenland to the Urals (Figure 2).  The troughing/negative geopotential height anomalies extend even further east into Western and Central Siberia and off the Kamchatka Peninsula (Figure 2).  Further south strong ridging/positive geopotential height anomalies stretches from south of Iceland across much of Europe and much of the southern half of Asia (Figure 2).  Strong westerly flow squeezed between low heights to the north and high heights to the south is resulting in mild temperatures for most of Europe, Central and Southern Asia (Figure 3).  Under the elongated stretch of negative geopotential height anomalies across Scandinavia and far northern Asia, temperatures are predicted to be below normal (Figure 3).  Some weak troughing/negative geopotential height anomalies across the Arabian Peninsula and Northern India (Figure 2), are resulting in below normal temperatures across the region (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 21 February 2017 at 00Z. Note the high heights over Europe, Southern Asia and Eastern North America with low heights over Scandinavia, the Urals, the eastern North Pacifica and western North America. 

North America is divided into two with troughing/negative geopotential height anomalies across western North America and strong ridging/positive geopotential height anomalies in eastern North America (Figure 2).  Under the troughing, seasonably cool temperatures are predicted in Western Canada and the US while under the ridging, above to well above normal temperatures are predicted in Eastern Canada and the US  (Figure 3).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 22 – 26 February 2017. Note the warm temperatures across Europe, Central and eastern North America with cold temperatures in Northern Europe, Northern Siberia, Western Canada and the Northwestern US. The forecast is from the 00Z 21 February 2017 GFS ensemble.

New snowfall is predicted in regions with negative geopotential height anomalies including Scandinavia, Alaska and Western Canada and the US Northern Plains while under the ridging snow melt is predicted in Western Asia, the Northeastern US and Southeastern Canada (Figure 4).

Figure 4.  Forecasted snow depth anomalies (mm/day; shading) from 22 – 26 February 2017. Note the snowfall over Scandinavia, Eastern Asia, Alaska, Canada and the US Northern Plains with snowmelt in Western Asia and the Northeastern US.  The forecasts are from the 00Z 21 February 2017 2016 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to remain positive next week though it is predicted to trend towards neutral (Figure 1) as geopotential height anomalies across the Arctic turn more mixed though positive geopotential height anomalies will continue to dominate the mid-latitudes (Figure 5a).  With geopotential height anomalies near Greenland and Iceland also predicted to be mixed, the NAO will likely trend towards neutral as well.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 27 February – 3 March 2017. (b) Same as (a) except averaged from 4 – 8 March 2017. The forecasts are from the 13 February 2017 00z GFS ensemble. 

With the stratospheric PV predicted to remain between the Urals and Scandinavia will anchor troughing/negative geopotential height anomalies across northwest Eurasia (Figure 5a).  Another trough/negative geopotential height anomalies are predicted across Central Siberia with a trough extending south towards East Asia (Figure 5a).  To the south ridging/positive geopotential height anomalies will persist across much of Europe and Central and Southern Asia (Figure 5a).  This favors a mild, westerly flow across much of the Eurasian continent similar to last period, with above normal temperatures for most of Europe and Western Asia (Figure 6).  Exceptions to the mild pattern are predicted in the regions of troughing/negative geopotential height anomalies across Scandinavia, Central and Eastern Siberia and even Northeast Asia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 27 February – 3 March 2017. Note the warm temperatures across much Europe, Western and Central Asia, China and much of the US with cold temperatures in Scandinavia, Siberia and Canada. The forecasts are from the 00Z 21 February 2017 GFS ensemble.

The troughing/negative geopotential height anomalies across western North America are predicted to deepen and expand eastward this period suppressing the ridging/positive geopotential height anomalies to the south and east across the Southeastern US (Figure 5a). This will re-orient the temperature anomaly pattern from west-east to north-south with predicted temperatures to be below normal for much of Canada and above normal temperatures across the Southern and Eastern US (Figure 6).

Figure 7. Forecasted snow depth anomalies (mm/day; shading) from 27 February – 3 March 2017. Note the snowfall over Siberia, Canada and Alaska with snowmelt across Central Asia. The forecasts are from the 00Z 21 February 2017 GFS ensemble.

New snowfall is predicted in the colder air across northern Eurasia and Alaska, Canada and even possibly the Northern US (Figure 7).  In contrast, ongoing snowmelt is predicted in Western Asia where mild temperatures persist (Figure 7).

11-15 day

Positive pressure/geopotential height anomalies are predicted start to stretch from the North Atlantic across the central Arctic and into the North Pacific this period (Figure 6b). With positive pressure/geopotential height anomalies the Arctic including Greenland and Iceland should bias both the AO and the NAO negative this period (Figure 1).

Little change is predicted in the general circulation pattern across Eurasia this period (Figure 5b). The general pattern of deep negative geopotential height anomalies beneath the stratospheric PV across northwestern Eurasia with a second trough in Central Siberia and above normal geopotential heights across most of Europe and the southern two thirds of Asia, is predicted to persist (Figure 5b).  Therefore, the pattern of below normal temperatures across Northern Europe and northern Siberia and above normal temperatures across much of the rest of Europe and Asia is predicted to continue (Figure 8).  Some exceptions could be Western Europe where ridging across Greenland and Iceland could favor troughing and cooler temperatures and East Asia where northerly flow could transport some colder air from Siberia across the region (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 4 – 8 March 2017. Note the warm temperatures across Eastern Europe, Western and Central Asia with cold temperatures in Eastern Asia, Northern Siberia and Canada.   The forecasts are from the 00Z 21 February 2017 GFS ensemble.

Below normal geopotential heights are predicted to continue to stretch across northern North America including Canada, Alaska and even the far Northern US while above normal geopotential heights stretch across the Southern US (Figure 5b). This pattern favors normal to below normal temperatures for Canada, Alaska and the Northern one third of the US and above normal temperatures for the Southern two thirds of the US (Figure 8).

Figure 9.  Forecasted snow depth anomalies (mm/day; shading) from 4 – 8 March 2017. Note the snowfall over Northwestern Asia, Western and Eastern Canada with snow melt over Central Asia. The forecasts are from the 00Z 21 February 2017 GFS ensemble.

With cold air entrenched across the northern tier of the continents new snowfall is expected across Northern Asia, Canada and possibly the Northeastern US (Figure 9).  Meanwhile mild temperatures will continue to favor snowmelt across Central Asia (Figure 9).

Longer Term

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows above normal PCHs in the stratosphere and below normal PCHs in the troposphere (Figure 10). The opposite PCHs in the stratosphere and troposphere are consistent with the negative stratospheric AO but positive tropospheric 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 21 February 2017 GFS ensemble.

The warm stratospheric PCHs and negative stratospheric AO are symptoms of yet another SSW/weak PV event.  This is the third event of the winter and potentially the second major warming in the span of a month.   The stratospheric PV is already displaced towards northwest Eurasia and is predicted to remain there over the next two weeks as warm air from lower altitudes spirals towards the North Pole (Figure 11).   With the PV displaced towards Eurasia, ridging/high pressure is stretched across Northern Canada.  The weakening stratospheric PV (Figure 11) is a result of above normal poleward heat flux/WAFz, which is predicted to peak this week (Figure 12). 

Figure 11. (a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 27 February – 3 March 2017. (b) Same as (a) except averaged from 4 – 8 March 2017. The forecasts are from the 00Z 21 February 2017 GFS operational model.

I do expect the circulation anomalies in the polar stratosphere to propagate into the troposphere in early March.  Previous PCH plots showed more clearly the downward propagation than today’s plot and future plots will determine if the trend continues or not.  The PV disruption should favor a return to a more wintry pattern both across North America and Eurasia.  As has been the case previously this winter, warming across Northern Canada and positive geopotential height anomalies should favor ridging/blocking across Alaska and the adjacent North Pacific.  This is consistent with the forecasts of the tropospheric circulation for this week.  But just has occurred previously this winter, the ridging near Alaska is predicted to slide west towards Eastern Siberia.  This has kept any cold air outbreaks brief across eastern North America this winter as troughing off the US West Coast pumped mild maritime air across the North American continent.  A repeat is certainly possible once more in March and the weather models are predicting that troughing across the US will be focused in the west favoring any cold shots into the Eastern US to be only glancing.

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 21 February 2017 GFS ensemble.

Currently the pattern is mild across Europe and at least initially with the stratospheric PV parked near Scandinavia, will favor strong westerly flow across the region and mild temperatures.  But in an attempt to not throw the baby out with the bath water my expectations for the coming weeks remain consistent with previous atmospheric responses to SSW/PV disruptions including warming PCHs in the troposphere a tendency towards a negative AO/NAO and relatively cold temperatures in Europe and/or the Eastern US.  But I do have to acknowledge that there are forces in play that I do not fully understand or recognize that have been unfavorable to a sustained negative AO and cold temperatures in the Eastern US and Northern Europe.  And those forces still may dominate the hemispheric atmospheric circulation through the end of March.

Surface Boundary Conditions

Arctic Sea Ice

Colder temperatures in the Arctic have allowed sea ice extent to advance this week and Arctic sea ice extent is currently not at a record low for the date. Still negative sea ice anomalies remain on both the North Atlantic side in the Barents-Kara Seas and on the North Pacific side mostly in the Sea of Okhotsk (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 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 ongoing for the third time this winter.  In contrast, negative sea ice anomalies on the North Pacific side of the Arctic, especially in the Sea of Okhotsk, have not been shown to perturb the stratospheric PV but are thought to be favorable for forcing cold temperatures in Western Canada.

Figure 13. Observed Arctic sea ice extent on 20 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 warm and are close to normal (Figure 14). It will be a close call as to whether the forecasts of a La Niña winter will verify.

Figure 14. The latest weekly-mean global SST anomalies (ending 19 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 in phase one and is predicted to transition fairly quickly to phases two and three while weakening (Figure 15).  Phases two and three favor generally cold temperatures across the US but the signals are not strong.

Figure 15. Past and forecast values of the MJO index. Forecast values from the 00Z 21 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.

The colder than normal SSTs that have persisted across the mid-latitudes of the North Pacific seem to weakening.  I do believe that the stripe of cold temperatures 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 with warming SSTs, the forcing for this pattern is weakened.

Northern Hemisphere Snow Cover

Snow cover across the NH as a whole is beginning its seasonal decline. Eurasian snow cover remains near decadal means but North American snow cover is near decadal lows and comparable to last winter. However with the arrival of colder temperatures, North American snow cover could experience a temporary advance before resuming its seasonal decline.

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.  The PV has been weak this winter but cold temperatures have been mostly confined to northern Asia and Europe.