Arctic Oscillation and Polar Vortex Analysis and Forecasts

September 18, 2017

Special blog on winter 2016/2017 retrospective can be found here - http://www.aer.com/winter2017

Special blog on winter 2015/2016 retrospective can be found here - http://www.aer.com/winter2016

Dr. Judah Cohen from Atmospheric and Environmental Research (AER) recently embarked on an experimental process of regular research, review, and analysis of the Arctic Oscillation (AO). This analysis is intended to provide researchers and practitioners real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns.

With the start of fall I will be transitioning to a fall/winter schedule, which is once every week starting sometime in October.   Precipitation forecasts will be replaced by snow accumulation forecasts.  Also there will be more emphasis on ice and snow boundary conditions (which are both now in their seasonal advance) and their influence on hemispheric weather.

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Summary

  • Currently mixed positive pressure/geopotential height anomalies cover the Arctic basin with mixed pressure/geopotential height anomalies across the mid-latitudes of the Northern Hemisphere (NH).  This is resulting in a neutral Arctic Oscillation (AO).  With overall positive pressure/geopotential height anomalies across Greenland and Iceland the North Atlantic Oscillation (NAO) is slightly negative.
  • Strong blocking/high pressure currently centered between Iceland and Scandinavia is predicted to remain the dominant circulation feature at high latitudes over the next two weeks biasing the AO/NAO negative to possibly strongly negative.
  • As is typical when the AO is negative, Europe is currently characterized by troughing/negative geopotential height anomalies and cool temperatures, which is likely to persist for another week.
  • The region with the strongest relationship with the AO is Siberia and with a persistent negative AO predicted, cold temperatures are predicted for much of Northern Asia for the next two weeks.  This should also support a rapid start to the snow advance season for the region.
  • The Eastern United States (US), which is often cool when the AO is negative, is just the opposite with an overall warm week predicted for the region.
  • The next week or so North America will be characterized by an amplified troughing/negative geopotential height anomalies and cool temperatures in the Western US and ridging/positive geopotential height anomalies and warm temperatures in the Eastern US and Canada. 
  • However as the AO turns more negative the ridging will weaken across Eastern North America and the pattern will likely transition to western ridging/eastern troughing with a concomitant reversal in temperature anomalies at the end of September/beginning of October.
  • Early in the period, ridging will result in warm temperatures for East Asia but eventually troughing encroaching from the west will bring with it cooler temperatures.

Impacts

Many important markers are currently being set indicating the atmosphere is beginning in earnest the transition from summer to winter.  There are four features that I am monitoring closely over the coming weeks and months to gauge the evolving atmospheric circulation pattern and resultant weather across the NH.  The first is the nascent stratospheric polar vortex (PV).  The PV has returned to the NH polar stratosphere.  Much recent research including my own has shown that the relative strength of the PV if not forces, certainly leads prolonged periods of temperature anomalies across key regions of the NH.  A strong PV is related to relatively milder temperatures across the mid-latitudes of the NH while a weak PV is related to relatively colder temperatures across the mid-latitudes of the NH.  This relationship is strongest in mid-winter.  Early signs are that the PV will start off relatively weak similar to last fall.  This is somewhat surprising because increasing greenhouse gases favor colder stratospheric temperatures and hence a stronger PV.  Poleward heat flux or vertical wave activity flux is predicted to be unusually active in the coming two weeks, which is likely the reason for the predicted relatively weak start to the PV.  The active poleward heat flux is also likely related to the second feature that I will be following - high latitude blocking.

The negative AO state is often a manifestation of strong high latitude blocking while the positive AO often reflects a lack of high latitude blocking.  The predicted negative AO in the coming two weeks is a result of predicted strong high latitude blocking with the dominant block predicted to reside in the region of Scandinavia and the Barents-Kara seas.  In the near term this will lead to a cold and snowy period across most of Siberia.  Blocking in this region is favorable for weakening the stratospheric polar vortex and will likely lead to weakening of the PV over the next two weeks.  If similar blocking occurs later on during the late fall and early winter it will favor a sudden stratospheric warming (SSW).  SSW in the winter often precedes extended periods of severe winter weather across the continents of the NH.

The third feature is Arctic sea ice extent. The minimum in Arctic sea ice extent is achieved this time of year and if the minimum has not already been reached it should occur relatively soon. The past two blogs I suggested the possibility that the sea ice minimum could be similar to the years 2008 and 2010 and that is looking likely.  Sea ice extent is extremely low compared to climatology but will not be a new record low.  The largest anomalies are in the North Pacific side of the Arctic in the Beaufort Sea.  This pattern matches recent Septembers.  Typically, the largest anomalies migrate with the progression of fall to the North Atlantic side of the Arctic.  It is my opinion that low sea ice favors high latitude blocking but the nature of the blocking is regionally dependent.   For example, low sea ice in the Barents-Kara Seas favors blocking in the northwest Eurasia sector resulting in cold temperatures in parts of Asia.

The fourth feature is Siberian snow cover.  My, along with my colleagues and others, research has shown that extensive Siberian snow cover in the fall favors a trough across East Asia with a ridge to the west near the Urals.  The atmospheric circulation pattern favors more active poleward heat flux, a weaker PV and cold temperatures across the NH.  With a predicted strong negative AO in the coming weeks, snow cover is likely to advance relatively quickly heading into October.  It is very early in the snow season but recent falls have been snowy across Siberia and therefore I do expect another upcoming snowy fall across Siberia.  Though admittedly, recent Siberian snow cover as a predictor of winter temperatures has been mixed.

Near Term Conditions

1-5 day

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

Figure 1. (a) The predicted daily-mean near-surface AO from the 00Z 18 September 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.

The lynchpin of the circulation pattern across Eurasia is strong ridging/positive geopotential height anomalies between Iceland and Scandinavia with an extension to northwest Siberia (Figure 2).  The strong block is contributing to three troughs/negative geopotential height anomalies across northern Eurasia.  The first is troughing/negative geopotential height anomalies across Western and Central Europe (Figure 2) resulting in seasonable to below normal temperatures for Western Europe (Figure 3). Further east, weak ridging/positive geopotential height anomalies dominate Southeastern Europe and southwestern Asia (Figure 2) resulting in above normal temperatures across Eastern Europe and southwestern Asia (Figure 3).  Ridging/blocking in the Barents-Kara seas is forcing toughing/negative geopotential height anomalies one centered over the Urals and a second over northcentral and northeastern Siberia (Figure 2) favoring below normal temperatures for Western and Central Siberia (Figure 3). Further downstream, ridging/positive geopotential height anomalies centered near Kamchatka (Figure 2) are resulting in southwesterly flow and above normal temperatures for Eastern Siberia (Figure 3).   Finally ridging/positive geopotential height anomalies over Southeast Asia (Figure 2) are resulting in above normal temperatures for East Asia (Figure 3).

Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 18 September 2017. Note the high heights over Northern Europe, Western Asia and Eastern North America with low heights over Europe, Siberia and western North America.

Deep troughing/negative geopotential height anomalies in the eastern North Pacific extends east into Western Canada and the Northwestern US while forcing ridging/positive geopotential height anomalies in eastern North America (Figure 2).  This is resulting in seasonable to below normal temperatures for southwestern Canada and the northwestern US (Figure 3).  The ridging further east is contributing to seasonable to above normal temperatures for Central and Eastern Canada and the Central and Eastern US (Figure 3).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 September 2017. Note the warm temperatures across Southeast Europe, Eastern Asia, the Central US and Canada with cool temperatures across Europe, Northern Asia, Southwestern Canada and the Northwestern US. The forecast is from the 00Z 18 September 2017 GFS ensemble.

Southerly flow will bring additional precipitation to Southeastern Europe (Figure 4). Troughing is predicted to result in new rainfall and snowfall for Southwestern Canada and the Northwestern US (Figure 4). More rainfall is predicted for the monsoon regions of Asia and the Southwestern US (Figure 4).  Hurricane Jose could bring heavy rainfall to the southeast New England (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/day; shading) from 19 – 23 September. Note the above normal rainfall over Eastern Europe, Southeast Asia and the Western US and Canada.  The forecasts are from the 00Z 18 September 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to remain negative next week, as geopotential heights turn mostly positive across the Arctic (Figure 5a). Similarly, with positive geopotential height anomalies stretching across Greenland and Iceland (Figure 5a), the NAO will likely be negative as well.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 24 – 28 September 2017. (b) Same as (a) except averaged from 29 September – 3 October 2017. The forecasts are from the 18 September 2017 00z GFS ensemble. 

Deep troughing/negative geopotential height anomalies are predicted to consolidate south of Greenland and Iceland forcing strong ridging/positive geopotential height anomalies downstream over Northern and Eastern Europe and Western Asia (Figure 5a). High heights will result in above normal temperatures for Northern and Eastern Europe (Figure 6).  However underneath the ridging, residual troughing in Western and Central Europe (Figure 5a) will persist below normal temperatures (Figure 6). Downstream of the ridging, troughing/negative geopotential height anomalies are predicted to persist across Central Siberia and extend across Southern Siberia (Figure 5a). Low heights and northerly flow will result in below normal temperatures for Western Asia and much of Siberia (Figure 6).  Ridging/positive geopotential height anomalies are predicted to persist across Southeast Asia (Figure 5a) resulting in above normal temperatures for East Asia (Figure 6).  Ridging/positive geopotential height anomalies in Eastern Siberia (Figure 5a) favor above normal temperatures in the region (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 September 2017. Note the warm temperatures across Northern and Eastern Europe, Eastern Siberia and much of Canada and the Central US with cool temperatures in Western and Central Europe, Central and Northern Asia and the Northwestern US. The forecasts are from the 00Z 18 September GFS ensemble.

Much of North America will be dominated by ridging/positive geopotential height anomalies with the exception of the Western US where troughing/negative geopotential height anomalies are predicted to persist (Figure 5a). Temperatures are predicted to be seasonable to warm across Canada and the Eastern US (Figure 6). Western US troughing will help maintain seasonable to below normal temperatures in the Western US this period (Figure 5a). 

Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 24 – 28 September 2017. Note rainfall over Siberia, Southeast Asia, Alaska, Central Canada and the US. The forecasts are from the 00Z 18 September 2017 GFS ensemble.

Predicted troughing will support additional rainfall and snowfall across Siberia (Figure 7). Additional rainfall is predicted for the monsoon regions of Southern Asia (Figure 7).  Predicted troughing will support additional rainfall and snowfall across western North America (Figure 7).

11-15 day

Positive pressure/geopotential height anomalies across the Arctic (Figure 5b) will favor a negative bias in the AO (Figure 1). Similarly, with positive geopotential height anomalies stretching across Greenland and Iceland (Figure 5b), the NAO will likely be negative as well.

Troughing/negative geopotential height anomalies are predicted to persist south of Greenland and Iceland this period helping to anchor ridging/positive geopotential height anomalies across Northern Europe (Figure 5b). Persistent ridging favors above normal temperatures across Northern Europe this period (Figure 8). However south across the remainder of Europe, the lack of any geopotential gradient (Figure 5b) and weak flow favors near seasonable temperatures (Figure 8).  Further downstream, persistent troughing/below normal geopotential height anomalies stretching from Western Siberia and east across Southern Siberia (Figure 5b) favor seasonable to below normal temperatures across much of Siberia (Figure 8).  Ridging/positive geopotential height anomalies are also predicted to persist across Eastern Siberia (Figure 5b) once again favoring seasonable to above normal temperatures for the region (Figure 8). Further south, troughing/negative geopotential height anomalies are predicted to reach East Asia (Figure 5a) favoring mostly below normal temperatures for East Asia (Figure 8).  

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 29 September – 3 October 2017. Note the warm temperatures across Northern Europe, Eastern Siberia, and much of Canada and the Western US with cool temperatures in Western, Central and East Asia. The forecasts are from the 00Z 18 September 2017 GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to cover most of North America this period (Figure 5b). With high heights and west to southwesterly flow covering much of the continent, widespread above normal temperatures are predicted for much of Western and Northern Canada and the Western US (Figure 8).  However nascent troughing in the Eastern US and Southeastern Canada (Figure 5b) could favor more seasonable temperatures (Figure 8).

Figure 9.  Forecasted precipitation anomalies (mm/day; shading) from 29 September – 3 October 2017. Note the above normal rainfall over the monsoon regions of Asia and the Eastern US.  The forecasts are from the 00Z 18 September 2017 GFS ensemble.

Additional rainfall is predicted for the monsoon regions of Southern Asia (Figure 9).  Renewed troughing and/or southwesterly flow could bring additional rainfall to the Eastern US (Figure 9).

Longer Term

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows cold/below normal PCHs in the stratosphere but normal to above normal PCHs in the troposphere (Figure 10).   The forecast is for continued and strengthening warm PCHs in the troposphere, which would favor high latitude blocking. The forecast of normal to above normal PCHs in the troposphere favors seasonable to seasonably cool temperatures across Northern Asia, Europe and/or the Eastern US in the upcoming weeks.  The PCH forecast supports the general persistence of this pattern even into early October.

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 18 September 2017 GFS ensemble.

Surface Boundary Conditions

SSTs/El Niño/Southern Oscillation

Equatorial Pacific sea surface temperatures (SSTs) remain normal to slightly below normal in much of the tropical and subtropical Pacific (Figure 11).  Earlier in the summer, El Niño was predicted for this winter, though that is looking less likely now with neutral El Niño/Southern Oscillation (ENSO) conditions or even La Niña more likely this winter.  SSTs across the globe remain well above normal though below normal SSTs exist across parts of the NH mid latitudes oceans in both the North Pacific and the North Atlantic oceans.

Figure 11. The latest weekly-mean global SST anomalies (ending 17 September 2017). Data from NOAA OI High-Resolution dataset.

Currently no phase of the Madden Julian Oscillation (MJO) is favored.  Over the next two weeks the forecast is still for no phase of the MJO to be favored (not shown).