Arctic Oscillation and Polar Vortex Analysis and Forecasts

March 21, 2017

Special blog on winter 2015/16 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 spring I will be transitioning to a spring/summer schedule, which is once every two weeks.  Snow accumulation forecasts will be replaced by precipitation accumulation forecasts.  Also there will be less emphasis on ice and snow boundary conditions (which are both now in their seasonal decline) and their influence on hemispheric weather.

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Summary

  • The Arctic Oscillation (AO) is currently positive and is predicted to trend negative this week towards neutral and then remains near neutral or slightly positive next week.
  • The current positive AO is reflective of negative pressure/geopotential height anomalies across the Arctic and mixed pressure/geopotential height anomalies across the mid-latitudes. With negative geopotential height anomalies across Greenland and Iceland, the North Atlantic Oscillation (NAO) is also currently positive.  The NAO is also predicted to trend similar to the AO over the next two weeks.
  • The positive AO is consistent with a relatively mild pattern across the Northern Hemisphere (NH) including much of North America, Europe and Northern and Eastern Asia.
  • Some exceptions to the mild pattern are eastern most North America and Central Asia.
  • Increased blocking in the North Atlantic will contribute to the AO/NAO heading back to neutral and is predicted to result in a cooling trend across western Eurasia, including Europe and could extend the relatively cool temperatures across eastern-most North America.
  • The polar vortex (PV) is weak and will remain so until the final warming (where the PV disappears until next fall) when stratosphere-troposphere coupling ends.
  • With the cessation of stratosphere-troposphere coupling longer term weather predictability becomes more challenging.  Still I expect a mostly mild spring across the NH and any relatively cool temperatures to be regional.

Impacts

Today is the first full day of spring and temperatures across much of the Northern Hemisphere (NH) are seasonally appropriate.  Temperatures are currently seasonable and even above seasonable levels for large portions of Eurasia and North America.  Exceptions are below normal temperatures in the interior of the continents, i.e., Central Asia and Central Canada.  The relative mild pattern for the NH is related to a positive AO and below normal geopotential heights encircling the Arctic.  Negative geopotential height anomalies dominating the Arctic basin favors positive geopotential height anomalies further south across continents and westerly winds.  High heights and westerly flow contribute to mild temperatures across the NH.

Enhancing the warmth across the NH continents are troughs/negative geopotential height anomalies in the eastern ocean basins.  A trough in the eastern North Pacific is advecting/transporting mild, maritime air across North America.  Similarly, a trough in the eastern North Atlantic is advecting/transporting mild, maritime air across Eurasia.  The trough in the eastern North Pacific is predicted to remain mostly in place over the next couple of weeks.  However, the trough in the eastern North Atlantic is predicted to yield to blocking or ridging/positive geopotential height anomalies in the region.  With blocking/ridging across the eastern North Atlantic/Northern Europe will deflect the flow of mild, maritime air from flowing across Eurasia.  This should allow for a cooling trend of temperatures relative to normal across western Eurasia including Europe over the next couple of weeks.

Longer term, the global environment favors mild temperatures for the coming months.  The Arctic remains exceptionally warm, aided by record or near record low sea ice; land and ocean temperatures also remain near record warm temperatures.  The only boundary forcing that I can think of that may contribute to relatively cool temperatures is snow cover across the NH.  The amount of snow covering (in mass not extent) the NH is the highest observed over the past twenty years or so.  I hope to be able to tweet out this week a graph of the snow mass this winter across the NH.  A deep snowpack across Canada and especially Siberia will retard warming at least across the high latitudes of the continents that may be transported episodically to lower latitudes.

Near Term Conditions

1-5 day

The AO is currently positive (Figure 1), reflective of negative geopotential height anomalies across the Arctic especially the central Arctic and mixed 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 March 2017 GFS ensemble. (b) The predicted daily-mean near-surface AO from the 00Z 21 March 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.

Troughing/negative geopotential height anomalies stretches across the northern North Atlantic to Northern Europe and then south from Scandinavia to the Arabian Peninsula (Figure 2).  To the south and west ridging/positive geopotential height anomalies covers the rest of Europe (Figure 2).  Strong westerly flow between low heights to the north and high heights to the south will bring mild temperatures to most of Europe this week (Figure 2).  One exception will be Western Europe as a closed low/negative geopotential height anomalies south of Iceland drops south towards Spain during the week (Figure 2) resulting in below normal temperatures for Western Europe (Figure 3). Further east, the trough/negative geopotential height anomalies across Western Asia (Figure 2) are predicted to slowly move eastward bringing with them below normal temperatures across Central Asia (Figure 3).  However much of the rest of Asia is predicted to be dominated by ridging/positive geopotential height anomalies dominating including Siberia and East Asia (Figure 3) resulting in above normal temperatures for Siberia and East Asia (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 21 March 2017 at 00Z. Note the high heights over Europe, Northern Asia and the US with low heights over Northern Europe, Western Asia and much of eastern Asia. 

Troughing/negative geopotential height anomalies stretch across Alaska and south into the Gulf of Alaska and just west of the United States (US) West Coast (Figure 2).  This is contributing to ridging/positive geopotential height anomalies downstream over Western Canada and much of the US (Figure 2) resulting in normal to above normal temperatures for much of Canada and the US (Figure 3).  An exception to the overall mild pattern is easternmost North America as an area of troughing/negative geopotential height anomalies extending south from the Arctic dives south towards Southeast Canada and the Northeastern US (Figure 2).  Northerly flow and low heights will result in below normal temperatures for eastern-most Canada and the Northeastern US (Figure 2).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 22 – 26 March 2017. Note the warm temperatures across Europe, Western Asia, Siberia, Western and Central Canada and the US with cold temperatures in Central Asia, Southern Alaska, Eastern Canada and the Northeastern US. The forecast is from the 00Z 21 March 2017 GFS ensemble.

A closed low centered near Spain will bring heavy precipitation to Southwestern Europe this week (Figure 4).  Southwesterly flow will bring above normal precipitation to western North America as well as the Mississippi Valley (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/day; shading) from 22 – 26 March 2017. Note the rainfall over Southwestern Europe, Western Canada and the US and the Mississippi Valley.  The forecasts are from the 00Z 21 March 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to trend slowly negative next week but remain mostly positive (Figure 1), as geopotential height anomalies remain mostly negative across the Arctic (Figure 5a). However, with positive geopotential height anomalies stretching from Greenland to Iceland (Figure 5a), the NAO may have a more negative bias than the AO.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 27 – 31 March 2017. (b) Same as (a) except averaged from 1 – 5 April 2017. The forecasts are from the 21 March 2017 00z GFS ensemble. 

Ridging/positive geopotential height anomalies are being forced east across Northern Europe by troughing/negative geopotential height anomalies ejecting eastward off the North American continent (Figure 5a) resulting in seasonable temperatures across Northern Europe (Figure 6).  Blocking across Scandinavia favors troughing and lower heights further south across Southern Europe (Figure 5a) accompanied by normal to below normal temperatures for Central and Southern Europe (Figure 6). Meanwhile the stratospheric PV is predicted to be displaced towards northwestern Asia with a tropospheric reflection of troughing/negative geopotential height anomalies over the same region (Figure 5a).  Therefore northerly flow and low heights will result in below normal temperatures for Western Asia (Figure 6). To the east of the Western Asia troughing, ridging/positive geopotential height anomalies are predicted for much of East Asia (Figure 5a) resulting in above normal temperatures for East Asia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 27 – 31 March 2017. Note the warm temperatures across Northern Europe, Northern and Eastern Asia, and much of Canada and the US with cold temperatures in Western Europe, Western Asia, the Canadian Maritimes and New England. The forecasts are from the 00Z 21 March 2017 GFS ensemble.

Persistent troughing/negative geopotential height anomalies in Alaska and the Gulf of Alaska favor ridging/positive geopotential height anomalies downstream over much of Canada and the US (Figure 5a).  Southwesterly flow and high heights are predicted to result in widespread normal to above normal temperatures across most of the US and Canada (Figure 6). One exception will be the Canadian Maritimes and New England, as northerly flow behind departing troughing/negative geopotential height anomalies (Figure 5a) will persist normal to below normal temperatures across the Canadian Maritimes and New England (Figure 6).

Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 27 – 31 March 2017. Note the rainfall over Southwestern Europe, Western Canada and the US and the Mississippi Valley.  The forecasts are from the 00Z 21 March 2017 GFS ensemble.

Energy and troughing will eject eastwards out of the eastern North Pacific across the US Rockies and into the Plain states bringing with it above normal precipitation to the Western and Central US (Figure 7). Troughing in Southern Europe and Western Asia may bring above normal precipitation as well (Figure 7).

11-15 day

With pressure/geopotential height anomalies predicted to remain mostly negative in the Arctic (Figure 6b) will likely bias the AO positive (Figure 1). Similarly with negative pressure/geopotential height anomalies predicted to return across Greenland and Iceland, the NAO will likely remain neutral to positive this period as well (Figure 1).

Little change is predicted in the circulation pattern this period across Eurasia.   With the stratospheric PV predicted to persist across northwest Asia, tropospheric troughing/negative geopotential height anomalies are predicted to be focused in Western Asia with persistent bookending ridging/positive geopotential height anomalies across Northern Europe and East Asia (Figure 5b). Therefore temperatures are predicted to be normal to below normal across much of western Eurasia due to low heights and northerly flow (Figure 8).  In contrast, high heights and westerly flow (Figure 5b) are predicted to continue the streak above normal temperatures for East Asia (Figure 8).  Similarly high heights will likely result in above normal temperatures across Scandinavia  (Figure 8) however further south, troughing under the Scandinavian block will likely transport some of the colder air to the east into Southern Europe (Figure 8).  

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 1 – 5 April 2017. Note the warm temperatures across Eastern Asia, Western and Central Canada and the Western and Central US with cold temperatures in Europe, Western Asia, the Canadian Maritimes and New England.   The forecasts are from the 00Z 13 March 2017 GFS ensemble.

Troughing/below normal geopotential heights are predicted to persist south of Alaska forcing ridging/above normal geopotential heights across much of North America (Figure 5b). This pattern favors normal to above normal temperatures for most of the US and Canada (Figure 8). One predicted exception is continued normal to below normal temperatures for Eastern-most Canada and for New England (Figure 8) forced by troughing//negative geopotential height anomalies extending south from the Arctic and northerly flow (Figure 8).

Figure 9.  Forecasted precipitation anomalies (mm/day; shading) from 1 – 5 April 2017. Note the rainfall over the US and especially the Mississippi Valley.  The forecasts are from the 00Z 21 March 2017 GFS ensemble.

Persistent energy ejecting eastward will favor continued troughing in the Southwestern US bringing with it above normal precipitation to the Central US and the Mississippi Valley (Figure 9).

Longer Term

30–day

The stratospheric AO is predicted to be near normal or slightly above normal into early April (Figure 1).  The latest plot of the polar cap geopotential heights (PCHs) shows negative/cold PCHs in the stratosphere (Figure 10) consistent with the slightly positive stratospheric AO.  Still with the return of sunshine to the Polar Regions, the stratospheric PV will continue to weaken consistent with climatology.  Tropospheric PCHs are mostly near normal with the exception of the lower troposphere where PCHs are slightly more negative/colder (Figure 10), consistent with the positive bias to the tropospheric AO.

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 March 2017 GFS ensemble.

Despite the disappearing stratospheric PV there is some stratosphere-troposphere coupling occurring. Vertical Wave Activity Flux (WAFz)/poleward heat transport is predicted to be active the final week of March (Figure 11).  This latest round of active WAFz/poleward heat transport will displace the stratospheric PV from Northeast Asia to Northwest Asia for the end of March and the beginning of April (Figure 12).  This is predicted to help anchor a polar low near the northwest slope of Eurasia producing a mostly northerly flow across western Eurasia including Europe and a colder pattern. 

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 21 March 2017 GFS ensemble.

I thought that the active vertical Wave Activity Flux (WAFz)/poleward heat transport predicted may result in the final warming (where the stratospheric PV disappears from the NH) but this is not predicted by the weather models.  Still the end the stratospheric PV until next fall is near (there is a good probability that the PV will not return to the North Pole this spring) along with the season of stratosphere-troposphere coupling and its application to longer range forecasts.

Figure 12. (a) Forecasted 10 mb geopotential heights (dam; contours) and temperature anomalies (°C; shading) across the Northern Hemisphere for 27 – 31 March 2017. (b) Same as (a) except averaged from 1 – 5 April 2017. The forecasts are from the 00Z 21 March 2017 GFS operational model.

Surface Boundary Conditions

SSTs/El Niño/Southern Oscillation

Equatorial Pacific sea surface temperatures (SSTs) have warmed from earlier in the winter and remain near normal (Figure 13).  SSTs are noticeably warmer in the eastern equatorial Pacific and if the warming continues in the spring, El Niño is possible next winter.

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

Currently no phase is favored for the Madden Julian Oscillation (MJO) and the MJO is predicted to remain weak for the next two weeks (not shown).  I don’t expect the MJO to have much influence on North American weather in the near term.