Arctic Oscillation and Polar Vortex Analysis and Forecasts

January 17, 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 neutral and is predicted to remain slightly positive over the next two weeks.
  • The weakly positive AO is reflective of mostly negative pressure/geopotential height anomalies in the central Arctic and on the Eurasian side of the Arctic and mixed pressure/geopotential height anomalies in the mid-latitudes. With predicted increasing heights over Greenland and Iceland, the North Atlantic Oscillation (NAO) is predicted to be mostly negative this week.  However next week heights are predicted to fall near Greenland and Iceland, forcing the NAO in a slow positive trend. 
  • Mid-tropospheric closed lows/negative geopotential height anomalies accompanied by cold surface temperatures will dominate the weather this week in Alaska, Europe and East Asia. In contrast ridging/positive geopotential height anomalies accompanied by mild surface temperatures will dominate the weather in Canada, the lower 48 United States (US), Northern Europe and Western Asia.
  • Starting next week a sudden stratospheric warming (SSW) /polar vortex (PV) weakening is predicted to begin, which will likely have a significant impact on the Northern Hemisphere (NH) tropospheric circulation in the coming weeks.
  • Strong poleward heat flux in the stratosphere over East Asia will both displace the stratospheric PV to the north slope of Asia and cause large height rises over Alaska and Northwest Canada.
  • The immediate response from the PV weakening will likely be increasing tropospheric ridging/positive geopotential height anomalies over northwestern North America directly under the stratospheric ridge and a deep tropospheric polar low under the displaced stratospheric PV over the north slope of Asia.
  • Rising heights over Alaska and Northwest Canada will help force troughing/ negative geopotential height anomalies accompanied by a cooling trend downstream over the Eastern US. Counterclockwise circulation around the polar low over the north slope of Siberia will transport North Atlantic maritime air into western Eurasia including Europe resulting in a trend to milder temperatures across the region.
  • East Asia tends to be cold leading up to SSW/PV weakenings with the cold weather easing post the SSW.  Therefore the next couple of weeks look to be cold across East Asia with a milder trend to follow.
  • I discuss below in more detail my expectations for the weather across the NH in the coming weeks within the framework of troposphere-stratosphere coupling.

Impacts

Back in the fall and early winter, multiple times in this blog I offered the winter of 2009/10 as the best analog for what I expected for this winter in regards to the stratospheric PV.  In 2009/10 there was a SSW/PV weakening in November and then a second larger event that culminated in a major mid-winter warming in early February.  This winter there was a SSW/PV weakening in November and now all the weather models predict a second SSW/PV weakening to begin next week.  The PV weakening will continue to evolve past next week and will likely peak in early February but much uncertainty still exists.  But if the weather model forecasts verify then at least in regards to the stratospheric PV, the winter of 2009/10 will likely to turn out to be the best analog for winter 2016/17 in the observational record.  My expectations for the similarity in the stratospheric PV behavior were based on the similar snow advance index (SAI) values observed in October 2009 and 2016.  In contrast winter 2009/10 there was an El Niño and the quasi-biennial oscillation (QBO) was in an easterly phase.   This winter is a La Niña winter and the QBO is in a westerly phase.  But in spite of the similarities in the stratospheric circulation between the two winters, their have been important differences in the tropospheric circulation including a much more negative AO so far in winter 2009/10.

Since late December I have discussed our experimental PV forecast model that was predicting a PV weakening for the second half of January.  That forecast was at least two weeks ahead of the Global Forecast System (GFS) model forecast of a PV weakening.  The PV forecast model predicts that the stratospheric PV will be generally weak through mid-February.  If the model is qualitatively correct then the upcoming PV weakening could be a prolonged event, which in my opinion complicates the forecast.  If the predicted poleward heat flux for next week immediately resulted in a major mid-winter warming (MMW where the winds reverse from westerly to easterly at 60°N and 10 hPa) then the forecast should be relatively easier.  Changes in the atmospheric circulation associated with the MMW would propagate to the surface within two weeks and likely last through the end of winter.  However at least for now that is not what I am anticipating.  As we saw last winter even if the PV weakens, the poleward heat flux can remain active leading to warm episodes at the surface even if the PV weakening results in short-lived colder weather.

The near term forecast related to the SSW/PV weakening does look straightforward.  A pulse of poleward heat transport next week is predicted to displace the stratospheric PV from the North Pole towards the north slope of Siberia next week. The poleward heat flux is also predicted to warm the stratosphere over the western Arctic (closer to Eastern Siberia and Alaska) leading to ridging/positive geopotential height anomalies over Alaska and Northwest Canada.  This is a similar stratospheric PV pattern that was observed at the end of November though this time the ridging is predicted to be of greater amplitude and more extensive than back in November.  In November the downstream cooling from the ridging was focused in western North America.  However I expect any downstream cooling from the upcoming ridging to be focused in the Eastern US.

Back in November the displacement of the PV towards northern Eurasia resulted in very cold temperatures in Western Asia due to strong troughing across the region.  This may yet occur but at least initially the weather models are predicting increasingly westerly flow across western Eurasia.  For much of the winter, ridging across and to the east and west of Northern Europe has blocked maritime air from penetrating into Western and Northern Asia resulting in a cold winter so far.  However with the stratospheric PV predicted to be displaced towards northwestern Eurasia, geopotential heights are predicted to fall across the region.  Lower heights across northwestern Eurasia and predicted rising heights across southwest Eurasia will result in strengthening westerlies across the region.   This will allow mild maritime air to overspread Europe and Western Asia.  I do think that this pattern is transitory and I discuss below in the Longer Term section my expectations for how the pattern across both continents may evolve.

Near Term Conditions

1-5 day

The AO is currently neutral (Figure 1), reflective of mixed geopotential height anomalies in the Arctic and across the mid-latitude ocean basins  (Figure 2).  However lowering heights in the central Arctic will force a positive AO trend while geopotential height anomalies are predicted to rise this week across Greenland and Iceland this week forcing the NAO in a negative trend.

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 17 January 2017 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 17 January 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 south of Iceland extend eastward across the United Kingdom (UK), Scandinavia and over to the Urals and south across much of Western Asia (Figure 2). South and west of the ridging, a cut off low/negative geopotential height anomalies cover much of the remainder of Europe (Figure 2). Northwesterly flow between high pressure to the north and low pressure to the south is transporting cold air into Central, Western and Southern Europe (Figure 3). Further north across Scandinavia and east over to northwest Asia including western Siberia, under the ridging/positive geopotential height anomalies, temperatures are relatively milder (Figure 3). Northerly flow between another couplet of ridging/positive geopotential height anomalies in Western Siberia and a cutoff low/negative geopotential height anomalies in southcentral Siberia (Figure 2) is advecting colder temperatures into East Asia (Figure 3).  Troughing/negative geopotential height anomalies in Southcentral Asia (Figure 2) are resulting in below normal temperatures (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 17 January 2017 at 00Z. Note the high heights south of Northern Europe, the Urals, East Asia and Canada with low heights in Europe, the Mediterranean, Southern Siberia, Alaska and the Southwestern US.

Deep troughing/negative geopotential height anomalies over Alaska are contributing to downstream ridging/positive geopotential height anomalies over most of Canada and the US lower 48 states  (Figure 2).  The resultant westerly and southwesterly flow is transporting mild temperatures across much of Canada and the US (Figure 3).  One notable exception is Alaska where well below normal temperatures are predicted under the negative heights (Figure 3).  Another exception are seasonable temperatures predicted in the Southwestern US (Figure 3) as northerly flow upstream of a trough swinging through the region transports some colder air (Figure 2).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 18 – 22 January 2017. Note the warm temperatures across much of the US, Canada and Northern Europe with cold temperatures in Western, Central and Southern Europe, much of East Asia and Alaska. The forecast is from the 00Z 17 January 2017 GFS ensemble.

Some new snowfall is predicted in Southern Europe and East Asia under the troughing/negative geopotential height anomalies (Figure 4).  In contrast snowmelt is predicted in the regions of ridging/positive geopotential height anomalies including Northern and Eastern Europe, Southern Canada and the Western US (Figure 4). Though not showing up in the figure, new snowfall is predicted for northern New England.

Figure 4.  Forecasted snow depth anomalies (mm/day; shading) from 18 – 22 January 2017. Note the snowfall over parts of Northwest Asia and northern Canada with snowmelt in Northern Europe, Southern Canada and the Northern US. The forecast is from the 00Z 17 January 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to trend slightly positive next week (Figure 1).  The positive trend in the AO is a result of falling geopotential heights on the Eurasian side of the Arctic and mostly positive geopotential height anomalies in the mid-latitude ocean basins, (Figure 5a). And with negative geopotential height anomalies predicted near Iceland, the NAO is likely to either trend or become mostly positive this period as well.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 23 – 27 January 2017. (b) Same as (a) except averaged from 28 January – 1 February 2017. The forecasts are from the 17 January 2017 00z GFS ensemble. 

The tropospheric circulation this period will start to reflect changes in the stratospheric PV circulation.  A deep polar low will form over the north slope of Western Siberia this period, reflecting the displacement of the stratospheric PV over the same region.  In fact negative geopotential height anomalies are predicted over all of northern Eurasia from northern Scandinavia to Eastern Siberia (Figure 5a). In contrast, ridging/positive geopotential height anomalies will stretch across southern Eurasia from Europe to East Asia (Figure 5a).  Strong westerly flow between the low heights to the north and high heights to the south will transport mild, maritime air across Northern Europe, northwestern Asia and southern Siberia (Figure 6). One ridge axis over Western Asia (Figure 5a) will turn the flow more northerly downstream over northern Siberia.  This will insulate northern Siberia from the mild air and allow below normal temperatures to stretch across all of northern Siberia (Figure 6).  Similarly another ridge axis over China (Figure 5a) will turn the flow more northerly downstream over Northeast Asia (Figure 6).    Two other exceptions to the predicted mild weather are in Southern Europe and southcentral Asia where troughs/negative geopotential height anomalies are predicted (Figure 5a) to result in below normal temperatures (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 23 – 27 January 2017. Note the warm temperatures across much of the US and Canada, Northern Europe and Southern Siberia with cold temperatures in Central and Western Europe, Northeastern Asia and Alaska. The forecast is from the 00Z 17 January 2017 GFS ensemble.

Meanwhile over North America strong height rises in the polar stratosphere over Alaska and Northern Canada will be reflected in the troposphere with ridging/positive geopotential height anomalies across almost all of Canada (Figure 5a). Strong ridging over Canada will force height falls for much of the US lower 48 (Figure 5a).  With rising heights, temperatures will be above to possibly much above normal in Canada (Figure 6).  Though heights will rise over Alaska as well, residual troughing will force southwesterly flow upstream over Western Canada cutting off any Arctic air moving southward out of Canada into the US resulting in mild temperatures for the US as well (Figure 6).  However troughing/negative geopotential height anomalies exiting the mountain Western US may bring seasonable to below normal temperatures to the intermountain West of the US (Figure 6).   Negative geopotential height anomalies over Alaska (Figure 5a) will persist below normal temperatures (Figure 6), though not as cold as the previous period.

Figure 7. Forecasted snow depth anomalies (mm/day; shading) from 23 – 27 January 2017. Note the new snowfall over Siberia, Northern Europe and Western Canada with snowmelt in Southeastern Canada and Europe. The forecast is from the 00Z 17 January 2017 GFS ensemble.

With troughing predicted across all of northern Eurasia new snowfall is possible across all of northern Eurasia including Scandinavia and Siberia (Figure 7). In western North America troughing will support new snowfall as well in Western Canada and the Western US (Figure 7).

11-15 day

With mostly negative pressure/geopotential height anomalies predicted over the central and Eurasian side of the Arctic and positive pressure/geopotential height anomalies over the North American side of the Arctic, a neutral to slightly positive AO is predicted for this period (Figure 1). Negative pressure/geopotential height anomalies are predicted for Greenland and Iceland and therefore the NAO is predicted to be slightly positive as well.

The couplet of troughing/negative geopotential height anomalies across northern Eurasia and ridging/positive geopotential height anomalies over southern Eurasia is predicted to persist this period (Figure 5b). In addition the Southern European trough is predicted to weaken considerably this period (Figure 5b).  Westerly flow between the low heights to the north and high heights to the south will transport mild, maritime air across most of Europe and Western Asia (Figure 8).   Weak ridging/positive geopotential height anomalies over the Ural Mountains will both block the flow of mild, maritime air and turn the flow more northerly downstream over Eastern Asia (Figure 5b).  Troughing over Siberia and northerly flow over East Asia will support widespread cold temperatures across those regions (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 28 January – 1 February 2017. Note the warm temperatures for Europe, Alaska and Canada with cold temperatures in much of East Asia and the Southeastern US.   The forecast is from the 00Z 17 January 2017 GFS ensemble.

The re-arrangement of the tropospheric circulation across North America, reflective of the circulation in the polar stratosphere, should be complete this period.  Strong ridging/positive geopotential height anomalies over Alaska and northern Canada (Figure 5b) will mirror strong ridging in the stratosphere over the region.  The ridging over northwestern North America will force troughing/negative geopotential height anomalies downstream over the Eastern US (Figure 5b).  Warm temperatures are predicted under the ridging/positive geopotential height anomalies for Alaska and Canada (Figure 8) while cold temperatures are predicted under the troughing/negative geopotential height anomalies over the Eastern US (Figure 8).

Figure 9.  Forecasted snow depth anomalies (mm/day; shading) from 28 January – 1 February 2017. Note the snowfall over Northern Europe, East Asia, Central Canada with snowmelt in the Western US and Southeastern Europe. The forecasts are from the 00Z 17 January 2017 GFS ensemble.

Once again with troughing predicted across northern Eurasia new snowfall is possible across northern Eurasia including Scandinavia and Siberia (Figure 9). Also with cold air widespread across East Asia new snowfall is predicted for East Asia including China (Figure 9). With cold air predicted to return to Eastern Canada and the Eastern US so does the potential for new snowfall (Figure 9). In contrast with ridging and warmer temperatures, snowmelt is predicted for the Western US and Europe (Figure 9).

Longer Term

30–day

The latest polar cap geopotential height anomalies (PCHs) plot (Figure 10) shows near normal tropospheric and stratospheric PCHs.  Over the next week or so the PCHs are predicted to turn briefly colder consistent with the positive AO forecast in both the troposphere and stratosphere (Figure 1).  However the current tropospheric circulation of high heights/pressures over northern Eurasia and low heights/pressures over the northern North Pacific sector is highly favorable for more active upward Wave Activity Flux (WAFz) or poleward heat transport.  And increased WAFz/poleward heat transport is predicted for next week (Figure 11) that is strong enough to warm the stratospheric PCHs back to positive for the first time since early December.  Also for the first time since early December the stratospheric AO is predicted to turn negative (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 17 January 2017 GFS ensemble.

The WAFz pulse predicted for next week does result in an SSW/weak PV as warming and ridge building occurs over Alaska and northwest Canada (Figure 12).  This displaces the stratospheric PV off the North Pole first towards northwest Asia and then closer to northern Scandinavia (Figure 12). 

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 17 January 2017 GFS ensemble.

The near term the impact of the predicted troposphere-stratosphere coupling should be relatively straightforward.  Across eastern North America, as often is the case, the increase in WAFz/poleward heat transport will coincide with a relatively mild period.  However the resultant SSW and ridge building over northwestern North America will favor increasing troughing and colder temperatures over the Eastern US with time.  For reasons that I don’t understand even though it often turns milder across eastern North America during periods of active WAFz/poleward heat transport, across East Asia it often turns colder.  More Arctic outbreaks are predicted for East Asia for next week and the cold weather should continue until the SSW peaks and then I would expect it to turn milder in East Asia.  However on the western side of Eurasia, including Europe, the initial impact from the SSW/weakening PV will be a turn to milder weather.  Because the PV is near the north slope of Siberia, the counterclockwise flow around the PV is westerly.  This favors enhanced westerlies across Europe and a turn to milder weather.

Figure 12. (a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere averaged from 23 – 27 January 2017. (b) Same as (a) except for forecasted 10 mb geopotential heights and temperature anomalies across the Northern Hemisphere averaged from 28 January – 1 February 2017. The forecasts are from the 00Z 17 January 2017 GFS operational model.

If the SSW/weak PV would peak relatively quickly, I believe that would make longer range predictions simpler but as of now I am anticipating a more prolonged event.  The stratospheric ridge centered near Alaska looks quite impressive to me and I expect this to be a fairly stable feature in the stratosphere.  Therefore I expect the general pattern across North America to be one of a western ridge/eastern trough with the cold temperatures focused further east than in the first half of the winter for much of February and possibly even early March.  However because it does appear that the SSW/weak PV event looks to continue to evolve for some time, more pulses of WAFz/poleward heat transport are possible that could punctuate the overall cold period with milder interludes.  This would result in potentially more transitory weather than if a major MMW would occur this month, which currently is not predicted by the weather models.

For Europe, the immediate impact of the SSW/weak PV is for the weather to turn milder.  However if there are subsequent pulses of WAFz/poleward heat transport this should further weaken the stratospheric PV and nudge it closer to Europe.  This could either turn the winds more northerly across Europe or even force the stratospheric PV into Europe.  Both of these scenarios would reverse the weather once again across Europe and result in much colder temperatures.

Over the next few weeks I will be looking for one of three possible outcomes.  The first, my personal favorite, is a PV split, which I believe favors snowstorms.  No sign of it yet in the weather models but multiple WAFz/poleward heat transport pulses would make this scenario increasingly likely.  The second possible outcome is for the stratospheric PV to be bodily displaced into Europe.  This would likely focus the largest negative temperature departures across western Eurasia including Europe. The third possible outcome is for the PV to be displaced into the northern North Atlantic.  This would likely result in strong cross polar flow into eastern North America and focus the largest negative temperature departures across eastern Canada and the Eastern US.

There are of course many other alternative scenarios but rather than describe each one I think just about every other scenario would lead to a milder weather pattern for both the US and Europe.

Surface Boundary Conditions

Arctic Sea Ice

Arctic sea ice extent experienced yet another pause in growth this week and remains at 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 but sea ice anomalies are much more impressive on the North Atlantic side (Figure 13). 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 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 in the coming weeks.  In contrast 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.   With the deep polar low predicted over Alaska, I expect sea ice to grow more quickly in the Bering Strait with the cold temperatures predicted in the region next week before milder temperatures return.

Figure 13. Observed Arctic sea ice extent on 16 January 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 have warmed over the past week relative to normal and are currently above temperatures required to qualify as La Niña conditions. 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 16 January 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 near Alaska with cooler than normal waters from Japan to the Pacific Northwest.  Well above normal waters extend across the subpolar North Atlantic near Greenland and north of Iceland. 

The Madden Julian Oscillation (MJO) is predicted to enter phase one this week (Figure 15).  Phase one favors troughing and cold temperatures in eastern North America.  The models predict troughing but not cold temperatures.  However the forecast of MJO phase one is transient and is predicted to enter phase two by next week, a phase that favors cold in the Western US.  I don’t believe that the MJO is having a significant impact on North American weather.  If the eastern North American trough is as transient as the MJO phase followed by a cold Western US, then I will be more open to the idea that the MJO is a key player in the predicted pattern change.

Figure 15. Past and forecast values of the MJO index. Forecast values from the 00Z 17 January 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.  This could possibly be strengthening the North Pacific jet leading to high amounts of rainfall along the US West Coast.

Northern Hemisphere Snow Cover

Warmer temperatures in the Eastern US have caused snow cover in North America to come off of decadal highs and Northern Hemisphere snow cover off of near decadal highs for the date. Eurasian snow cover could expand further especially with cold temperature predicted for East Asia and snowfall for China.

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, favors a strengthened 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.