JUDAH COHEN: IMPLICATIONS OF WIDESPREAD COLD & SNOW ACROSS SIBERIA

Arctic Oscillation and Polar Vortex Analysis and Forecasts

October 13, 2016

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.

With the critical month of October starting this weekend, I plan on resuming a weekly update to the weather discussion.  However publication may be somewhat erratic over the coming weeks.  Subscribe to our email list or follow me on Twitter (@judah47) for notification of updates.

Summary

  • The Arctic Oscillation (AO) is currently strongly negative and is predicted to remain negative over the next two weeks.  The AO is predicted to trend positive towards neutral through the end of next week and then reverse and trend further negative.
  • The current negative AO is reflective of positive pressure/geopotential height anomalies in the Arctic especially north of Alaska and on the European side of the Arctic and negative pressure/geopotential height anomalies across the mid-latitudes of the Northern Hemisphere (NH) especially in the North Pacific sector. The North Atlantic Oscillation (NAO) is currently also negative but because the positive height anomalies in the North Atlantic side of the Arctic are closer to Scandinavia than Iceland/Greenland, the NAO is predicted to fluctuate around zero over the next two weeks.
  • With pressure/geopotential height anomalies predicted to remain positive across much of the Arctic and pressure/geopotential height anomalies predicted to remain negative across the NH mid-latitudes, the AO is likely to remain in negative territory into the foreseeable future.
  • With the AO firmly negative for the remainder of the next two weeks, this favors below normal temperatures across large portions of northern Eurasia including Europe and Siberia. However because North America will be dominated by mostly positive pressure/geopotential height anomalies, temperatures will average above normal much of next week in eastern North America.  The following week with heights predicted to build once more near Greenland lower heights and temperatures are predicted for eastern North America for week two.
  • Eurasian snow cover is above normal but has currently stalled.  However both low sea ice and a persistent negative AO are favorable for the resumption of a more accelerated advance of Siberian snow cover. High Eurasian snow cover and low Arctic sea ice in the Barents-Kara seas favor first, a strengthening Siberian high and then a weakened polar vortex in winter.
  • The polar vortex both in the stratosphere and the troposphere look to remain relatively weak for the remainder of October, if this trend continues severe winter weather is likely for widespread portions of northern Eurasia, including Europe and East Asia, and the eastern United States (US).
  • I plan on posting on the NSF website a preliminary US winter forecast next Wednesday: https://www.nsf.gov/news/special_reports/autumnwinter/predicts.jsp

Impacts

The tropospheric AO is currently strongly negative, which follows a strongly negative stratospheric AO at the end of September.  The stratospheric AO has recovered closer to neutral and the tropospheric AO is predicted to follow the stratospheric trend and recover closer to neutral.  However the Global Forecast System (GFS) predicts that both the stratospheric and tropospheric AO will trend negative the final week or so of October.  An overall negative AO favors widespread cold temperatures across the NH mid-latitudes, especially northern Eurasia and a rapid advance in snow cover especially Siberia and to a lesser extent Canada.  October Eurasian snow cover extent is a key predictor in our winter forecast and if our expectations are correct that Eurasian snow cover will be above normal for the month of October, this will bias our model colder for the winter forecast.  I am also impressed with the persistence of the cold temperatures across Siberia for the month of October.  The longer the cold temperatures persist across Siberia, the harder it becomes to dislodge and the more likely those cold Siberian temperatures will be a player in the hemispheric weather outside of Siberia.

 Arctic sea ice remains well below normal and possibly most importantly for longer-term weather, sea ice is below normal in the Barents-Kara seas.  This will also bias the winter forecast colder.  Also with the likelihood that the AO will be negative right through the end of October, October 2016 will probably rival other recent Octobers for the most negative October AO observed, which include October 2002 and 2009.  And though there is not a strong relationship between the October AO and the winter AO, the winter AO in 2002/03 and 2009/10 were both negative and cold temperatures were widespread across northern Eurasia and the Eastern US.  However the AO was much more negative in the latter winter and the evolution of the polar vortex was quite different.  It is still very early and the outcome of this winter still very much in doubt but in my opinion, the longer that the tropospheric and/or stratospheric polar vortex remains weak and the AO negative, this favors a colder winter solution.  I will be monitoring the strength of the polar vortex very closely in November and if the vortex strengthens considerably this needs to be considered in future winter forecast updates.

In the near term the atmospheric circulation across Eurasia looks fairly stable over the next two weeks. Positive geopotential height anomalies look almost stationary across northwestern Eurasia with negative geopotential height anomalies both to the south across much of Europe and to the east across much of Siberia.  This will favor widespread below normal temperatures from Europe and across northern Eurasia including much of Siberia.  The circulation across North America will be more transitory.  Initially a strong ridge/positive geopotential height anomalies north of Alaska has contributed to a deep trough/negative geopotential height anomalies and cold temperatures in the interior of the continent.  However the ridge north of Alaska is predicted to weaken, that combined with low heights in the Gulf of Alaska will allow geopotential heights to rise across much of North America and therefore temperatures are predicted to warm during week one. However in week two geopotential heights are predicted to rise near Greenland favoring lowering heights in the Eastern US with a concomitant drop in temperatures.

Recent and Very Near Term Conditions

The AO is currently strongly negative (Figure 1), reflective of mostly positive geopotential height anomalies both across the Arctic and negative geopotential height anomalies across the mid-latitudes especially in the North Pacific sector (Figure 2).  Geopotential height anomalies are mostly positive on the European side of the Arctic east of Iceland (Figure 2), and therefore the NAO is not as negative.

Figure 1. (a) The predicted daily-mean AO at 10 hPa from the 00Z 13 October 2016 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 13 October 2016 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 stretch across the European side of the Arctic including Northern Europe (Figure 2) resulting in mild temperatures for Scandinavia (Figure 3).  Underneath the Scandinavian ridging, a closed area of low pressure stretches from Spain to the Black Sea (Figure 2).  Negative geopotential height anomalies combined with northerly and easterly flow are bringing below normal temperatures to much of Central and Southern Europe (Figure 3).  Downstream of the Scandinavian ridging/positive geopotential height anomalies, troughing/negative geopotential height anomalies stretch from the Caspian Sea to Japan (Figure 2) resulting in extensive below normal temperatures across Northern Asia including much of Siberia (Figure 3). Weak ridging/positive geopotential height anomalies across China (Figure 2) are resulting in above normal temperatures for much of China and Central Asia (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 13 October 2016 at 00Z. Note the high heights north of Alaska and the European side of the Arctic and eastern North America with low heights over Europe, Siberia and central North America.

Two areas of ridging/positive geopotential height anomalies dominate North America, with one over eastern North America centered in the Canadian Maritimes and the other just north of Alaska (Figure 2).  Under the ridging, temperatures are above normal in eastern Canada and Alaska (Figure 3).  Between the two ridges, negative geopotential height anomalies stretches from the Canadian Arctic archipelagos into the US Upper Midwest (Figure 2) resulting in below normal temperatures for much of the interior of Canada and the US Plain States (Figure 3).

Figure 3. Analyzed surface temperature anomalies (°C; shading) 13 October 2016 at 00Z. Note the warm temperatures across Northern Europe, Northernmost Siberia and eastern North America with cool temperatures in Europe, Southern Siberia and central North America.

Deep negative geopotential height anomalies in the Gulf of Alaska and a strong southwesterly flow will bring copious amounts of precipitation to the US Pacific Northwest.  That moisture will be transported east bringing above normal precipitation to southern Canada as well (Figure 4). Strong ridging across Northern Europe will bring dry weather to much of Northern Europe, however precipitation will be above normal underneath the closed low across Southern Europe (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/day; shading) from 14 – 18 October 2016. Note the wet conditions across Southern Europe, Eastern Siberia, the US Pacific Northwest and Southern Canada. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

With the negative AO, snow cover will likely advance across the Northern Hemisphere at an accelerated pace.  Snow fall is predicted across much of Siberia and Northern Canada (Figure 5).  Snow melt is predicted across the Tibetan Plateau and southcentral Canada (Figure 5).

Figure 5. Forecasted snow depth anomalies (mm/day; shading) from 14 – 18 October 2016. Note the snowfall over parts of Siberia and Northwest Canada. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to remain negative all of next week though it well trend positive towards neutral (Figure 1).  The negative AO is a result of mostly positive geopotential height anomalies stretched across the entire Arctic (Figure 6a).  However since the region of positive geopotential height anomalies does not include Greenland with positive geopotential height anomalies further south across the North Atlantic, the NAO will be positive.

Figure 6. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 19 – 23 October 2016. (b) Same as (a) except averaged from 24 – 28 October 2016. The forecasts are from the 13 October 2016 00z GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to persist across Northern Europe, though shifted slightly to the east relative to the previous period (Figure 6a).  Widespread positive heights will persist above normal temperatures for Scandinavia (Figure 7).  However, the much of the rest of Europe will be under the influence of negative geopotential height anomalies and northerly flow (Figure 6a) that will transport below normal temperatures across Europe (Figure 7).  Meanwhile downstream of the northwestern Eurasia ridging/positive geopotential height anomalies, troughing/negative geopotential height anomalies will persist from the Caspian Sea to Northern Japan (Figure 6a).  Likewise temperatures will remain below normal across much of Northern Asia from the Caspian Sea to Northern Japan.  One exception will be Southern and Eastern China where building geopotential height anomalies will bring above normal temperatures across (Figure 7).

Figure 7. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 October 2016. Note the warm temperatures across much North America, Northern Europe and Southern China with cold temperatures in Europe and Siberia. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

The trough from the previous period over Central Canada will lift northeastward into the northern North Atlantic this period, while the low heights in the Gulf of Alaska will help to force rising geopotential heights across much of North America especially Canada (Figure 6a). The resultant positive heights and southwesterly flow across the continent will result in above normal temperatures for much of the US and Canada (Figure 7). One exception could be in the Northeastern US where a piece of the low heights from the Gulf of Alaska will traverse the US and settle in the Northeastern US (Figure 6a). Northerly flow and lower geopotential heights could keep temperatures closer to seasonable in parts of the Northeastern US (Figure 7).

Figure 8. Forecasted snow depth anomalies (mm/day; shading) from 19 – 23 October 2016. Note the snowfall over Siberia, Scandinavia and Northern Canada. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

With geopotential heights remaining low and temperatures cold, snow cover should continue to expand south across Siberia this period (Figure 8).  In contrast, rising geopotential heights and warming temperatures, snow cover will stop advancing across Canada with melting along the southern edge of the continuous snow cover (Figure 8).

3-4 week

Positive pressure/geopotential height anomalies are predicted to persist across the Arctic this period (Figure 6b). Widespread positive heights across the Arctic and negative heights across the mid-latitudes especially in the North Atlantic will likely force both the AO and the NAO further into negative territory heading in to the end of October (Figure 1).

Little change is predicted in the general circulation pattern across Eurasia this period (Figure 6b). The above normal geopotential heights across Scandinavia will persist, expand southward across Western Asia and weaken (Figure 6b).  Ridging anchored across northwest Eurasia will help to maintain negative geopotential height anomalies both across Europe and much of Northern Asia including most of Siberia (Figure 9).  Therefore, the pattern of widespread below normal temperatures both across Europe and Northern Asia including much of Siberia will persist (Figure 9).  Two exceptions could be above normal temperatures along the border of Eastern Europe and western Asia (Figure 9) under the positive geopotential height anomalies and above normal temperatures (Figure 9) in Southern and Eastern China where ridging/positive geopotential height anomalies is predicted to persist (Figure 6b).

Figure 9. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 October 2016. Note the warm temperatures across western North America and Northern Europe, with cool temperatures along the US East Coast, Europe and Southern Siberia.   The forecasts are from the 00Z 13 October 2016 GFS ensemble.

Rising geopotential heights both near southern Greenland and in the Gulf of Alaska will help to force troughing/negative geopotential height anomalies in the Northeastern US and Southeastern Canada (Figure 6b). This should favor temperatures averaging seasonable to below normal for most of the Eastern US (Figure 9). The above normal geopotential height anomalies in the Gulf of Alaska will extend into wester North America (Figure 6b) resulting in normal to above normal temperatures across all of western North America (Figure 9).

Figure 10. Forecasted snow depth anomalies (mm/day; shading) from 24 – 28 October 2016. Note the snowfall over much of Siberia, Scandinavia and Northern Canada. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

With the AO predicted to turn more negative this period, snow cover advance will likely continue across the Northern Hemisphere both across northern Eurasia, especially Siberia and across Northern Canada (Figure 10).

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows well above normal PCHs in the troposphere (Figure 8) consistent with a persistent negative AO. And as discussed earlier, the strongly positive PCHs seem to have descended from the stratosphere.  Low Arctic sea ice will continue to favor warm/positive PCHs.  Though there is some dampening of the warm/positive PCHs the warming is likely to amplify the last week of October consistent with a negative trending AO.

Figure 11. Observed and predicted daily polar cap height (i.e, area-averaged geopotential heights poleward of 60°N) standardized anomalies. The forecasts are from the 00Z 13 October 2016 GFS ensemble.

Though the stratospheric AO is currently negative it is predicted to trend positive towards neutral over the coming week.  However, the hemispheric atmospheric circulation next week (Figure 6a) does favor increased energy transfer from the troposphere into the stratosphere and this may lead to further weakening of the stratospheric PV and a return to a negative stratospheric AO both in the stratosphere and the troposphere (Figure 1).  A weak polar vortex in the troposphere and possibly the stratosphere and positive PCHs favor below normal temperatures across the NH mid-latitudes, especially across Northern Eurasia and the Eastern US. It is my opinion, as we head into November, monitoring the strength of the polar vortex in both the troposphere and the stratosphere will be the most important circulation index for anticipating winter weather variability.

Surface Boundary Conditions

Arctic Sea Ice

Arctic sea ice reached its annual minimum on September 10th. Despite that Arctic sea ice has been growing or over a month now, large swaths of the North Pacific side of the Arctic basin are still ice-free.  One area to note on the North Atlantic side is in the Barents-Kara Seas, where sea ice anomalies are also negative (Figure 11). 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. Given that sea ice is running below normal, this currently favors more extensive Siberian snow cover in the coming weeks, followed by a strengthened Siberian high and a weakened polar vortex/negative AO this upcoming winter.

Figure 12. Observed Arctic sea ice extent on 12 October 2016 (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 12). The cool anomalies (i.e., La Niña-like) are not terribly impressive and most ENSO forecasts are for either neutral or more likely weak La Niña conditions, as SSTs have cooled along the equator.   La Niña conditions favor a negative Pacific/North American (PNA) pattern that produces cold anomalies in the northwestern US and warm anomalies in the Southeastern US. In addition, the North Pacific SST pattern resembles more of a positive Pacific Decadal Oscillation (PDO) pattern, which favors a positive PNA opposite of La Niña. Therefore forcing from the Pacific SSTs seems to be contradictory or weak at best early in this fall season.

Figure 13. The latest weekly-mean global SST anomalies (ending 11 October 2016). 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.

We also note that northern North Atlantic SSTs are warmer than the same time last year.  Northern North Atlantic SSTs for the past few winters have been exceptionally cold, which I believe favors an enhanced longitudinal temperature gradient, a strengthened Jet Stream, a positive NAO and warm Europe.   Potentially significantly warmer SSTs this winter could favor opposite conditions this upcoming winter with a weakened longitudinal temperature gradient, a weakened Jet Stream, a negative NAO and colder Europe.  But it is still very early and there are many other complicating factors still unknown.  Even warmer SSTs are present in the northern North Pacific, which could also favor high latitude blocking this upcoming winter. But it is still very early and there are many other complicating factors still unknown.  Also air and ocean temperatures remain near record levels across the NH and without dynamically forced cold, the streak of warm weather will persist.

I will also begin soon presenting the phase of the MJO, however for now I will just mention that the ECMWF model shows no favored phase of the MJO in the foreseeable future.

Northern Hemisphere Snow Cover

Snow cover advance across Eurasia was rapid the first week of October but has since stalled the second week of October. This has lead so far to an above normal snow cover extent but a near normal snow advance index.  Therefore, it will be important to continue to monitor snow cover advance across Eurasia the remainder of the month.  The predicted negative AO and below normal heights across Siberia favor snow cover to continue advancing at a normal to above normal rate.  With the AO/NAO predicted to be negative through the end of October, confidence is growing that October Eurasian snow cover will be above normal once again this year.  Above normal 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.

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