Arctic Oscillation and Polar Vortex Analysis and Forecasts

June 19, 2017

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

  • Currently mostly negative 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 positive Arctic Oscillation (AO). And with mostly negative pressure/geopotential height anomalies across Greenland and Iceland the North Atlantic Oscillation (NAO) is also positive.
  • As expected during a positive NAO, ridging/positive geopotential height anomalies and above normal temperatures stretch across Europe, especially Western Europe.
  • Over the next week, the AO is predicted to trend further into positive territory.  This is related to generally higher heights and warm temperatures across Europe.  In week two, a generally negative trend in the AO may signal a lowering of heights and return to more seasonable temperatures.
  • The positive geopotential height anomalies across the eastern North Atlantic/Europe extend westward into the western North Atlantic/Eastern United States (US) also contributing to warm temperatures along the Eastern Seaboard. 
  • However, a trough/negative geopotential height anomalies currently in the Central US will soon reach the East Coast and bring a reduction in temperatures to more seasonable levels.  But with the AO still positive, ridging/positive geopotential height anomalies are predicted to return with a concomitant rise in temperatures by the weekend.
  • For week two, building ridging/positive geopotential height anomalies along the West Coast of North America will force troughing/negative geopotential height anomalies now in the Gulf of Alaska into the Eastern US along with a streak of seasonably cool temperatures.
  • For East Asia, the model forecast is for a trend towards increasing ridging/positive geopotential height anomalies and above normal temperatures in the next two weeks.
  • With a trough/negative geopotential height anomalies predicted to remain in the Central Arctic basin into the foreseeable future at least slightly favors ridging and warm temperatures for both Europe and the Eastern US.  However in my opinion, drier soils in Europe relative to the Eastern US favor overall relatively warm temperatures in Europe compared to the Eastern US.

Impacts

Much of the spring and even early June have been characterized by high latitude blocking especially in the northern North Atlantic, which has kept portions of western Eurasia, including Europe and eastern North America, including the Northeastern US, cool over the period. The block in the northern North Atlantic suppressed the Jet Stream across western Eurasian and eastern North America resulting in a Jet Stream more reminiscent of winter than late spring.  However, the models are predicting that a relatively deep low pressure will form and spin in the central Arctic basin for the foreseeable future.  This is a more hostile circulation pattern for high latitude blocking and instead favors more ridging/positive geopotential height anomalies in the mid-latitudes.  Also, the Jet Stream will recede to higher latitudes giving the overall circulation a more traditional summer configuration. Still these relationships are weaker in summer than winter and a deep low in the Arctic basin is not as a reliable indicator of a mild pattern in summer as it would be in winter.

Last blog I discussed whether the return of warmer temperatures will now set the stage for a streak of above normal temperatures for an extended period or not.   At least in the Eastern US the answer appears to be no.  Ridging/positive geopotential height anomalies are predicted to become well established along the west coast of North America.  This in turn will contribute to renewed troughing/negative geopotential height anomalies in eastern North America and the likelihood of seasonable to seasonably cool temperatures next week.  One particularly important feedback in summer is that between soil moisture and temperatures. Moister soils result in cooler temperatures and cooler temperatures result in moisture soils while drier soils result in warmer temperatures and warmer temperatures result in drier soils. Soils across the Eastern US remain relatively wet and are likely tilting the probabilities to troughing vs. ridging in the Eastern US.    At the end of the two-week period, the models are predicting more troughing in Western Europe as well.   However, I continue to believe that Western Europe is at higher risk for longer stretches of dry weather and above normal temperature in the coming months because soils are relatively drier in Europe compared to the US. 

I continue to believe that global conditions favor temperatures averaging above normal for most regions in across the NH mid-latitudes include the US and Europe.  Warm summers in the Western US have been the norm recently and this summer is looking like no exception.  For Europe the dry soils, in my opinion also favor a warmer than normal summer.  The summer forecast for the Eastern US in my opinion remains the most challenging.  Will ridging and heat with a stronghold in the Western US eventually overspread the Eastern US for weeks at a time this summer or will troughing and cooler temperatures keep the heat confined to the West, in part helped by wet soils?  I still believe the heat will eventually overspread the East as well but if ridging and heat does not become established in the East by early July the likelihood of heat in the East being more than transitory this summer will start to decrease significantly with each passing week.

I have also been discussing snow cover across the NH.  Snow cover this spring has been more resilient to melt than in previous recent springs. Snow cover and snow mass continue to be relatively high across the NH helped by in part by below normal temperatures in key regions, though the snow cover season across the NH (except at high elevations) is quickly coming to a close.  The snow cover remains more resilient in Eurasia relative to North America and Eurasian snow cover extent is still at decadal highs while North American snow cover extent is at decadal lows.  The relatively extensive snow cover may help to slow down the pace of melting Arctic sea ice this summer.

Near Term Conditions

1-5 day

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

Figure 1. (a) The predicted daily-mean near-surface AO from the 00Z 19 June 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 dominate much of Europe with the exception of troughing/negative geopotential height anomalies across the northern half of Scandinavia and the Black Sea region (Figure 2).  Predicted high heights over Europe favor above normal temperatures except for below normal temperatures across Scandinavia and Eastern Europe due to low heights and/or northerly flow (Figure 3).  Downstream of the European ridging, troughing/negative geopotential height anomalies (Figure 2) will persist below normal temperatures across Western Asia (Figure 3).  Much of Central Asia will be dominated by ridging/positive geopotential height anomalies (Figure 2).  With much of Central Asia predicted to be dominated by ridging/positive geopotential height anomalies, above normal temperatures are favored for much of Central Asia including Siberia (Figure 3). Some troughing/negative geopotential height anomalies are predicted across East Asia (Figure 2) resulting in seasonable temperatures for East Asia (Figure 3).  Southwesterly flow will result in above normal temperatures for Eastern Siberia (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 19 June 2017 at 00Z. Note the high heights over Europe, central Asia and western North America with low heights over Northern Europe, Western Asia and the Eastern US.

A deep trough/negative geopotential height anomalies south of the Aleutians is helping to pump up ridging/positive geopotential height anomalies across the Western US and Western Canada (Figure 2).  This is not only resulting in above normal temperatures for western North America (Figure 3) but also helping to support troughing/negative geopotential height anomalies downstream across Eastern Canada and the Eastern US (Figure 2).  Low heights and northerly flow are contributing to seasonable temperatures in Southeastern Canada and the Eastern US (Figure 3).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 20 – 24 June 2017. Note the warm temperatures across Siberia, Eastern Asia, the Western US and Canada with cool temperatures in Eastern Europe, Western Asia and seasonable temperatures in the Eastern US. The forecast is from the 00Z 19 June 2017 GFS ensemble.

With ridging overhead, below normal precipitation is predicted for much of Europe this week (Figure 4).  In contrast, troughing in the Eastern US this week is predicted to result in above normal precipitation for the Eastern US (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/day; shading) from 20 – 24 June 2017. Note the above normal rainfall over Southeast Asia and the Eastern US.  The forecasts are from the 00Z 19 June 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to trend negative back towards neutral but still remain positive next week as negative geopotential height anomalies continue to dominate much of the central Arctic (Figure 5a). And with weak geopotential height anomalies stretching from Greenland to Iceland, the NAO will likely trend negative back 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 25 – 29 June 2017. (b) Same as (a) except averaged from 30 June – 4 July 2017. The forecasts are from the 19 June 2017 00z GFS ensemble. 

The ridging/positive geopotential height anomalies stretching across Europe last period will persist across Central and Southern Europe (Figure 5a). With ridging/positive geopotential height anomalies and/or westerly flow dominating much of Europe, above normal temperatures are likely across much of Western and Central Europe (Figure 6). However, troughing/negative geopotential height anomalies will become more expansive across Northern Europe this period while persisting across Western Asia (Figure 6).  Due to northerly flow and ongoing troughing/negative geopotential height anomalies, normal to below normal temperatures are predicted across Northern and Eastern Europe and Western Asia (Figure 5a). Downstream of the Western Asia troughing, ridging/positive geopotential height anomalies will stretch across much of Central and East Asia (Figure 5a).  High heights and west to southwesterly flow will result in warm temperatures across much of Central and East Asia including much of Siberia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 25 – 29 June 2017. Note the warm temperatures across Western Europe, East Asia, and much of Canada and the US with cool temperatures in Eastern Europe and Western Asia. The forecasts are from the 00Z 19 June 2017 GFS ensemble.

The deep trough/negative geopotential height anomalies south of the Aleutians from the previous period is predicted to retrograde west this period towards the dateline (Figure 5a). This will help to anchor ridging/positive geopotential height anomalies downstream across western North America (Figure 5a).  Above normal geopotential heights in the Western US favor persistence of above normal temperatures while below normal geopotential heights in the Eastern US will favor normal to below normal temperatures across the Eastern US and Southeastern Canada (Figure 6).  Weak ridging/positive geopotential height anomalies are predicted in Northern Canada and Alaska (Figure 5a).  High heights, the early disappearance of snow cover and strong persistence of anomalies in these regions will support above normal temperatures (Figure 6).

Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 25 – 29 June 2017. Note the rainfall over Western Europe, Southeast Asia Western Canada and the Eastern US.  The forecasts are from the 00Z 19 June 2017 GFS ensemble.

Ridging dominating much of Europe will act to suppress precipitation across the continent, though increasing southwesterly flow may bring precipitation to Western Europe (Figure 7). Similarly, troughing and southwesterly flow in the Eastern US will support increased chances of rainfall in the Eastern US (Figure 7).

11-15 day

Continued negative pressure/geopotential height anomalies across the Arctic basin (Figure 6b) will likely bias the AO positive (Figure 1). With neutral to positive pressure/geopotential height anomalies across Greenland and Iceland, the NAO will likely be biased positive as well (Figure 1).

The ridging/positive geopotential height anomalies across Europe the past two periods is predicted to split into two pieces with one piece moving west into the eastern North Atlantic and the other piece sliding east into Eastern Europe with new troughing/below normal geopotential heights in Western Europe (Figure 5b). This pattern favors a return of below normal temperatures in Western Europe with the region for above normal temperatures pushing into Central Europe and possibly Eastern Europe.  Meanwhile the one constant of troughing/below normal geopotential heights (Figure 5b) and below normal temperatures across Western Asia looks to persist right into early July (Figure 8). Further upstream, ridging/positive geopotential height anomalies are predicted to continue for Central and Eastern Asia (Figure 5b) favoring mostly above normal temperatures for Central and East Asia including much of Siberia (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 30 June – 4 July 2017. Note the warm temperatures across East Asia, and much of Canada and the US with cool temperatures in Europe and Western Asia. The forecasts are from the 00Z 19 June 2017 GFS ensemble.

Predicted deep troughing/negative geopotential height anomalies predicted near the dateline will continue to drift west closer towards Asia.  This will maintain ridging/positive geopotential height anomalies across Alaska and western Canada but a new trough is predicted to come ashore in the Western US (Figure 5b).  The trough in the Western US will in part force rising geopotential height anomalies across the Eastern US (Figure 5b).  The only predicted remnant of negative geopotential height anomalies across North America is in Eastern Canada (Figure 5b).  This pattern favors widespread normal to above normal temperatures across North America (Figure 8) especially in Western Canada, Alaska and even the Western US.  The only possible predicted exceptions are Eastern Canada and the Northeastern US.

Figure 9.  Forecasted precipitation anomalies (mm/day; shading) from 30 June – 4 July 2017. Note the above normal rainfall over the monsoon regions of Asia and the Eastern US.  The forecasts are from the 00Z 19 June 2017 GFS ensemble.

Precipitation across Europe is favored downstream of the Western European trough and upstream of the Eastern European ridge (Figure 9).  With a turn to more ridging in the Eastern US lighter rainfall is predicted for 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 both in the stratosphere and in the troposphere (Figure 10).   The cold/negative PCHs are consistent with the predicted positive AO and low pressure predicted in the Arctic.  Cold/negative PCHs does favor ridging and warmer temperatures at lower latitudes across Europe and the US though at this time of year the relationship is not strong. If the PCHs warm and high latitude blocking returns, this could favor a return to a pattern that favors cooler temperatures, though this is not apparent from the PCH plot.

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 19 June 2017 GFS ensemble.

Surface Boundary Conditions

SSTs/El Niño/Southern Oscillation

Equatorial Pacific sea surface temperatures (SSTs) remain near normal in much of the tropical and subtropical Pacific (Figure 11).  If the SSTs further warm in the coming months then El Niño is possible next winter, though currently only a weak El Niño is predicted.  SSTs across the globe remain well above normal though below normal SSTs exist across the NH mid latitudes in both the North Pacific and the North Atlantic.

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

Currently no phase or only a very weak Madden Julian Oscillation (MJO) is favored (not shown).  Therefore it is unlikely that the MJO will have a significant influence on North American weather this period.