Arctic Oscillation and Polar Vortex Analysis and Forecasts

May 22, 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

  • Currently mixed 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). However, with mostly positive pressure/geopotential height anomalies across Greenland and Iceland the North Atlantic Oscillation (NAO) is negative.
  • As expected during a negative NAO, temperatures are below normal across western Eurasia including parts of Europe as a strong block/high pressure has developed in the eastern North Atlantic including Western and Central Europe with a cool, northerly flow downstream of the block across Eastern Europe and Western Asia.
  • However with positive pressure/geopotential height anomalies over Western Europe, temperatures have turned milder.
  • The ridge/positive geopotential height anomalies across Western Europe are predicted to persist for the next two weeks resulting in an overall mild pattern. The ridge will also favor troughing/negative geopotential height anomalies and cool temperatures downstream over Eastern Europe and Western Asia.
  • Positive geopotential height anomalies across Greenland that extend westward into Northern Canada are also contributing to negative pressure/geopotential height anomalies and below normal temperatures further south across Eastern Canada and the Eastern United States (US).
  • The positive geopotential height anomalies across Greenland and northeastern Canada are predicted to drift westward into Western Canada contributing to  troughing/a negative bias in geopotential height anomalies across the Eastern US over the next two weeks. Therefore, the pattern of cool temperatures across the Eastern US but warm temperatures in the Western US looks to continue into early June.
  • Across East Asia geopotential height and temperature anomalies are mixed but the predicted trend for the region is for overall above normal pressure/geopotential height anomalies favoring temperatures that average above normal over the next two weeks.

Impacts

The month of May has been characterized by strong high latitude blocking especially in the North Atlantic and is reflected in the monthly negative NAO.  The strong block in the North Atlantic contributed to relatively cool temperatures on both sides of the Atlantic in Europe, Western Asia and the Eastern US.  However that block has split into two pieces with one piece sliding over Western Europe and the other piece is sliding west and merging with locally high heights in Western North America.  With the ridge/positive geopotential height anomalies overhead in Western Europe, temperatures will trend warmer in the coming weeks over the Western half of Europe.  In contrast with the ridge/positive geopotential height anomalies remaining offset from the Eastern US, first to the east and now to the west, favor the continuation of troughing/negative geopotential height anomalies across Southeastern Canada and the Eastern US and therefore a cool bias in temperatures.

Persistence of anomalies is more important in summer than in winter.  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.  So even though both Europe and the Eastern US were relatively cool the first half of May the soils across the Eastern US are relatively wetter compared to that of Europe.  It turned much warmer across the Eastern US last week but it did not persist.  Now Western Europe is turning warmer and with dry soils already in place, the warmer temperatures may have more staying power.  And as we head into summer, Western Europe is at higher risk for longer stretches of dry weather and above normal temperature in the coming months.  In contrast the Eastern US is at lower risk for persistent dry, hot weather as long as the local soils remain relatively wet.

As I discussed in my previous blog, one constant over the past decade has been the collapse of NH snow cover extent in spring, especially late spring.  The rapid disappearance of snow cover across northern Eurasia and northern North America contributes to drying of the soil and warmer temperatures.  The resultant warmer temperatures also likely contribute to Arctic sea ice loss.  Snow cover this spring has been more resilient to melt than in previous recent springs.  More snow cover results in moister soils.  Moister soils result in cooler temperatures.  Snow cover and snow mass continue to be relatively high across the NH helped by in part by below normal temperatures in key regions.  However the snow cover has been more resilient in Eurasia relative to North America and snow cover across North America experienced a rapid decline over the past week.  And with more warm temperatures predicted across Northern Canada, the rapid decline in snow cover will likely continue.

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 positive near Greenland and Iceland (Figure 2), and therefore the NAO is negative.

Figure 1. (a) The predicted daily-mean near-surface AO from the 00Z 22 May 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 are widespread across Western and Central Europe forcing troughing/negative geopotential height anomalies downstream across Eastern Europe, Western Asia and all of northern Russia (Figure 2).  Predicted ridging centered over Western Europe favors above normal temperatures over Western and Central Europe (Figure 3).  However, northerly flow between the ridge in Western Europe and the trough over Eastern Europe and Western Asia, is predicted to persist cool temperatures across much of Eastern Europe and Western Asia that extends east across Western and Central Siberia (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 and downstream over East Asia including Eastern Siberia (Figure 3). Some troughing/negative geopotential height anomalies are predicted across Southeast Asia (Figure 2) resulting in normal to below normal temperatures for Southeast Asia (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 22 May 2017 at 00Z. Note the high heights over Greenland, Iceland, Europe and western North America with low heights over Eastern Europe, Western Asia and the Central US. 

Strong ridging/positive geopotential height anomalies centered in the northern and western North America are forcing troughing/negative geopotential height anomalies upstream across Southeastern Canada and the Eastern US (Figure 2).  Low heights and northerly flow are contributing to below normal temperatures in Southeastern Canada and the Eastern US (Figure 3).  With northern and western North America dominated by positive geopotential height anomalies above normal temperatures are predicted for Northern and Western Canada and the Western US (Figure 3).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 23 – 27 May 2017. Note the warm temperatures across Eastern Asia, the Western US and Canada with cool temperatures in Eastern Europe, Western Asia and the Eastern US. The forecast is from the 00Z 22 May 2017 GFS ensemble.

With ridging overhead, below normal precipitation is predicted for Western and Central 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 23 – 27 May 2017. Note the rainfall over Southeastern Europe and the Eastern US.  The forecasts are from the 00Z 22 May 2017 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to remain neutral to positive next week as neutral to negative geopotential height anomalies dominate much of the Arctic (Figure 5a). And with neutral to negative geopotential height anomalies stretching from Greenland to Iceland, the NAO will likely trend positive back into positive territory as well.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 28 May – 1 June 2017. (b) Same as (a) except averaged from 2 – 6 June 2017. The forecasts are from the 22 May 2017 00z GFS ensemble. 

The ridging/positive geopotential height anomalies centered over Western Europe last period will slowly drift east towards Central Europe while anchoring troughing/negative geopotential height anomalies across Eastern Europe and Western Asia (Figure 5a). Ridging/positive geopotential height anomalies will persist above normal temperatures across Western and Central Europe (Figure 6). In contrast, widespread below normal temperatures will persist across Eastern Europe and Western Asia (Figure 6) due to northerly flow and troughing (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 mild temperatures across much of Central and East Asia including Western and Eastern Siberia (Figure 6). Some remnant troughing/negative geopotential height anomalies in Central and southeastern Siberia (Figure 5a) will bring normal to below normal temperatures to the region (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 28 May – 1 June 2017. Note the warm temperatures across Western Europe, East Asia, and much of Canada and the Western US with cool temperatures in Eastern Europe, Western Asia and the Northeastern US. The forecasts are from the 00Z 22 May 2017 GFS ensemble.

Ridging/positive geopotential height anomalies will continue to amplify this period across Western North America favoring at least weak troughing/negative geopotential height anomalies in central and/or eastern North America (Figure 5a).  Above normal geopotential heights will favor above normal temperatures across Northern and Western Canada and the Western US (Figure 6).  Weak troughing/negative geopotential in the Central US will support seasonable temperstures in the Central and Eastern US (Figure 5a).  If the ridging across Western Canada is amplified next week, then the predicted troughing/negative geopotential height anomalies in the Eastern US could be more amplified than shown (Figure 5a) with more widespread below normal temperatures than shown (Figure 6).

Figure 7. Forecasted precipitation anomalies (mm/day; shading) from 28 May – 1 June 2017. Note the rainfall over Northern Europe, Spain, and the Eastern US.  The forecasts are from the 00Z 22 May 2017 GFS ensemble.

Troughing approaching Spain from the west will support above normal precipitation in the southwesterly flow across Western Europe (Figure 7). Similarly, the trough in the Eastern US will support continued above normal rainfall in the Eastern US (Figure 7).

11-15 day

Predicted rising pressure/geopotential height anomalies across the Arctic (Figure 6b) are predicted to force the AO back towards neutral (Figure 1). With neutral to positive pressure/geopotential height anomalies across Greenland and Iceland, the NAO will likely trend back to neutral as well (Figure 1).

The couplet of above ridging/positive geopotential height anomalies centered over Central Europe and troughing/below normal geopotential heights in Eastern Europe and Western Asia is predicted to persist this period (Figure 5b).  Therefore the stretch of above normal temperatures in Western and Central Europe with below normal temperatures across Eastern Europe and Western Asia looks to continue into the first week of June (Figure 8).  Further upstream, ridging/positive geopotential height anomalies are predicted for much of Central and Eastern Asia (Figure 5a) favoring mostly above normal temperatures for Central and East Asia especially Eastern Siberia (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 2 – 6 June 2017. Note the warm temperatures across East Asia, and much of Canada and the US (though probably overdone) with cool temperatures in Eastern Europe and Western Asia. The forecasts are from the 00Z 22 May 2017 GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to remain anchored in Western Canada while expanding further east to include Central Canada and the US (Figure 5b).  With positive geopotential height anomalies dominating much of North America, above normal temperatures are favored across the region but especially Western Canada and the Western US (Figure 8). The ridge centered over Western Canada will likely support troughing in Eastern Canada that extends south into the Eastern US (Figure 5b).  Seasonable to above normal temperatures are predicted for Southeastern Canada and the Eastern US (Figure 8) but even below normal temperatures are possible.

Figure 9.  Forecasted precipitation anomalies (mm/day; shading) from 2 – 6 June 2017. Note the above normal rainfall over Spain and the Eastern US.  The forecasts are from the 00Z 22 May 2017 GFS ensemble.

Troughing in Spain and Scandinavia could lead to above normal rainfall across these two regions (Figure 9).  Similarly troughing could bring more above normal rainfall to the Eastern US (Figure 9).

Longer Term

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows below normal PCHs in the stratosphere that extends through the troposphere over the next week (Figure 10).   The negative/cold PCHs are consistent with the positive trend in the AO and a weakening of high latitude blocking in the North Atlantic side of the Arctic.  These also coincide with warmer temperatures in Western Europe.  The PCH plot also suggests a return to positive/warm PCHs in early June, though the latest plot is weaker than earlier forecasts.  Warmer tropospheric PCHs would coincide with high latitude blocking and possibly cooler weather across Europe and the Eastern US.  Though in summer the influence of high latitude blocking is not as direct as during the winter and the impacts are less certain.

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 22 May 2017 GFS ensemble.

Surface Boundary Conditions

SSTs/El Niño/Southern Oscillation

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows below normal PCHs in the stratosphere that extends through the troposphere over the next week (Figure 10).   The negative/cold PCHs are consistent with the positive trend in the AO and a weakening of high latitude blocking in the North Atlantic side of the Arctic.  These also coincide with warmer temperatures in Western Europe.  The PCH plot also suggests a return to positive/warm PCHs in early June, though the latest plot is weaker than earlier forecasts.  Warmer tropospheric PCHs would coincide with high latitude blocking and possibly cooler weather across Europe and the Eastern US.  Though in summer the influence of high latitude blocking is not as direct as during the winter and the impacts are less certain.

Figure 11. The latest weekly-mean global SST anomalies (ending 21 May 2017). Data from NOAA OI High-Resolution dataset.

Currently no phase of the Madden Julian Oscillation (MJO) is favored.  However the MJO is predicted to emerge into phase two next week and then transition to phases three and four (Figure 12).  Phases two through four favor mostly cool and wet weather in the Eastern US.

Figure 12. Past and forecast values of the MJO index. Forecast values from the 00Z 21 May 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.