Arctic Oscillation Analysis and Forecasts

July 25, 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.

  • Summary

  • The Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are currently negative and are predicted to remain negative to neutral over the next two weeks.
  • The current negative AO/NAO is reflective of mostly positive pressure/geopotential height anomalies across the North Atlantic side of the Arctic basin, especially near Greenland.   In contrast, mostly negative pressure/geopotential height anomalies extend across the North Pacific side of the Arctic basin.
  • The forecasts of a negative AO/NAO tendency are reflective of persistent positive pressure/geopotential height anomalies near Greenland and negative pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic especially in the region of Northern Europe.
  • Predictions of more expansive negative geopotential height anomalies across Northern Europe should lead to cooler temperatures for Western Europe and even Western Asia.  Above normal geopotential height anomalies are predicted across Southern Europe likely retarding the southward flow of cooler air from Northern Europe.
  • North America has been dominated by positive geopotential height anomalies and above normal temperatures so far this month.  The weather models predict for geopotential heights to remain above normal for much of North America into early August.  One exception could be western North America for next week, as negative geopotential height anomalies as a piece of lower geopotential heights drops down from the Arctic into western North America, resulting in cooler temperatures.
  • East Asia is also dominated by positive geopotential height anomalies and warm temperatures and the weather models predict a continuation of this overall pattern.

Impacts

Overall it has been a warm summer so far across the continents of the Northern Hemisphere (NH) and that theme looks to mostly continue with the possible exceptions of northwestern Eurasia and possibly even parts of western North America.  The AO is currently negative, reflective of positive geopotential height anomalies on the North Atlantic side of the Arctic especially near Greenland.  Persistent high geopotential heights near Greenland also known as Greenland blocking will contribute to lowering geopotential heights across Northern Europe.  This will favor a negative tendency in the AO/NAO over the next two weeks. 

Elsewhere across the mid-latitudes, positive geopotential height anomalies will dominate including Southern Europe, East Asia and Eastern North America.  Expansive positive geopotential height anomalies will likely result in the persistence of above normal temperatures in these regions.  The most expansive region of negative geopotential height anomalies covers the North Pacific side of the Arctic.  Next week, a piece is predicted to break off from an area of low pressure/geopotential heights on the North Pacific side of the Arctic and drop down across Western Canada and the Pacific Northwest resulting in a drop in temperatures across the region.

Beyond next week little change is predicted in the overall atmospheric circulation.  The negative tendency in the AO/NAO should remain as Greenland blocking is predicted to persist.  This will favor negative pressure/geopotential height anomalies across northwest Europe.  However elsewhere across the NH, mid-latitudes geopotential height anomalies should remain predominantly positive.  Overall positive height anomalies will likely favor above normal temperatures to remain widespread across both continents, especially in North America.  Some possible exceptions include northwest Europe and Central Siberia where troughs could bring cooler temperatures to those regions.  Another trough in Western Canada could bring cooler temperatures to Western Canada and even parts of the Western United States (US) next week.

Recent and Very Near Term Conditions

The AO is currently negative but is predicted to trend positive towards neutral by the end of this week (Figure 1), as negative geopotential height anomalies consolidate across the North Pacific side of the Arctic and the northeastern Atlantic quadrant of the Arctic while geopotential height anomalies remain positive across the northwestern quadrant of the North Atlantic (Figure 2)

Figure 1. The predicted daily-mean near-surface AO from the 00Z 25 July 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.

The North Atlantic sector is dominated by positive geopotential height anomalies including eastern North America and much of Europe with the greatest positive departures across Greenland (Figure 2).  An exception to the high heights, is an area of low pressure/negative geopotential height anomalies stretching from Iceland to Great Britain (Figure 2). Negative heights across northwest Europe and a secondary area of low heights in the Tyrrhenian Sea combined with positive geopotetnial height anomalies elsewhere across Europe (Figure 2) are resulting in mixed temperature anomalies for Europe (Figure 3). The deepest negative geopotential heights across the hemisphere is currently spinning in the Laptev and East Siberian Seas with a trough/negative geopotential height anomalies extending southwestward to the Black Sea (Figure 2) resulting in below normal temperatures for much of western Asia (Figure 3). Further east, ridging/positive geopotential height anomalies stretch from central and Eastern Siberia southward to Central and East Asia (Figure 2) resulting in above normal temperatures for much of Central and Eastern Siberia and East Asia (Figure 3). One exception is a cutoff low/negative geopotential height anomalies center just east of Lake Baikal in southeastern Siberia (Figure 2) bringing below normal temperatures to that region (Figure 3).  Southwesterly flow out ahead of this area of low pressure has resulted in heavy rains across Northern China including Beijing.

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 25 July 2016 at 00Z. Note the low heights over Great Britain, Western Asia, Central Siberia and the northwestern North America with high heights over the Eastern Europe, East Asia and the US. 

Much of North America is dominated by ridging/positive geopotential height anomalies (Figure 2).  This is resulting in above normal temperatures across the US and Southern Canada (Figure 3).   However a fairly deep trough/area of negative geopotential height anomalies stretches across northwestern Canada and Alaska (Figure 2) resulting in below normal temperatures for northwestern Canada (Figure 3).  There is also a weak trough exiting the Northeastern US (Figure 2) that resulted in a brief period of below normal temperatures for New England (Figure 3).

Figure 3. Analyzed surface temperature anomalies (°C; shading) on 25 July 2016. Note the warm temperatures across much of North America and East Asia with cool temperatures in Western Europe, Western Asia, Northwestern Canada and the Northeastern US.

Across North America the Jet Stream has retreated north into Canada with storms traversing from Western Canada to Eastern Canada but with some of the moisture extending southward into the Eastern US as well (Figure 4).  The pattern is predicted to be relatively dry across Eurasia with some exceptions including the Balkans, with the arrival of the low pressure from the Adriatic Sea, Northern India and northeastern China and Japan, as the heavy precipitation that has plagued Northern China slides eastward (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/s; shading) from 26 – 30 July 2016. Note the wet conditions across Eastern Europe, northern India, northeastern China, Japan, Western Canada and eastern North America. The forecasts are from the 00Z 25 July 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to remain near neutral during the course of next week (Figure 1).  The near neutral AO is a result of mixed heights across the North Atlantic side of the Arctic with positive pressure/geopotential height anomalies dominating the northwestern North Atlantic including Greenland and negative pressure/geopotential height anomalies dominating the northeastern North Atlantic including Northern Europe (Figure 5a).

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 31 July – 4 August 2016. (b) Same as (a) except averaged from 5 – 9 August 2016. The forecasts are from the 25 July 2016 00z GFS ensemble.

Negative geopotential height anomalies are predicted to become more expansive across Northern Europe stretching from Great Britain to Scandinavia (Figure 5a).  With troughing extending into Northern and Central Europe, normal to below normal temperatures are predicted for Northern and Western Europe (Figure 6, though the cool temperatures may be overdone).  Further south, positive geopotential height anomalies are predicted to stretch from the eastern North Atlantic eastward towards Southern Europe and Western Asia (Figure 5a) resulting in normal to above normal temperatures for Southern Europe and Western Asia (Figure 6). Northeast and south of the Western Asia ridging areas of negative geopotential height anomalies in Central Siberia and east of the Caspian Seas (Figure 5a) will bring below normal temperatures stretching from Central Siberia to Iran (Figure 6). Further east, strong positive geopotential height anomalies dominate much of East Asia (Figure 5a) likely resulting in above normal temperatures for much of East Asia (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 31 July – 4 August 2016. Note the warm temperatures across much North America and East Asia with cool temperatures in Western Europe and Western Asia. The forecasts are from the 00Z 25 July 2016 GFS ensemble.

Positive geopotential height anomalies are predicted to dominate all of eastern North America this period while negative geopotential height anomalies stretch from Alaska to the Pacific Northwest (Figure 5a).  Southwesterly flow upstream of a trough in the Western US in combination with ridging/positive geopotential height anomalies in eastern North America (Figure 5a) will result in above normal temperatures for much of the Eastern US and Canada (Figure 6). Temperatures should average closer to normal across Western Canada and the US including Alaska (Figure 6, the warm temperatures may be overdone).

Figure 7.  Forecasted precipitation anomalies (mm/s; shading) from 31 July – 4 August 2016. Note the wet conditions across much of the Canada, the Eastern US and the Asian monsoon region. The forecasts are from the 00Z 25 July 2016 GFS ensemble.

With the Jet Stream remaining north into Canada, storms are predicted to traverse from west to east across Canada bringing with them relatively wet weather to Canada (Figure 7).   A dry pattern is predicted to persist across Eurasia except in Northern and Eastern Europe and the monsoon regions (Figure 7).

3-4 week

Overall little change is predicted in the NH atmospheric circulation this period.  The AO is predicted to remain near neutral or slightly negative into the second week of August (Figure 1) as negative pressure/geopotential height anomalies are predicted to persist in the northeastern North Atlantic while positive pressure/geopotential height anomalies remain in the northwestern North Atlantic including Greenland (Figure 5b). 

The general pattern of lower heights to the north and higher heights to the south is predicted to persist across Eurasia this period.  Negative geopotential height anomalies are predicted to persist across Northern Europe with positive geopotential height anomalies stretched across Southern Europe (Figure 5b). Therefore little change is expected in the temperature pattern with the best chance of normal to below normal temperatures across Northern Europe and normal to above normal temperatures across Southern Europe (Figure 8, again the extensive coverage of cool temperatures may be overdone).  The positive geopotential height anomalies in Southern Europe are predicted to extend into Western Asia (Figure 5b). This should result in in normal to above normal temperatures for western Asia (if the predicted circulation pattern in Figure 5b is correct then temperatures are likely too cool as shown in Figure 8). To the northeast and south of the ridge in Western Asia, troughs/negative geopotential height anomalies are predicted in Central Siberia and Central Asia (Figure 5b) resulting in below normal temperatures for Central Siberia and Central Asia (Figure 8). Positive geopotential height anomalies are predicted to persist across East Asia (Figure 5b) resulting in above normal temperatures (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 5 – 9 August 2016. Note the warm temperatures across North America and East Asia, with cool temperatures in Northern Europe and Western Asia.   The forecasts are from the 00Z 25 July 2016 GFS ensemble.

Models predict the heat dome to persist for North America as well with ongoing ridging/positive geopotential height anomalies predicted to persist across central North America (Figure 5b). This should continue to favor temperatures averaging seasonable to above normal for most of North America (Figure 8). Some weak troughing is predicted just offshore of the west coast of North America as well as some weak troughing in eastern Canada downstream of the ridge axis in the continental interior (Figure 5b).  If these troughs are amplified enough, temperatures may average cooler in these regions than shown (Figure 8).

Figure 9. Forecasted precipitation anomalies (mm/s; shading) from 5 – 9 August 2016. Note the wet conditions across much Canada and the Eastern US but dry for Eurasia except for Northern Europe and in the monsoon regions. The forecasts are from the 00Z 25 July 2016 GFS ensemble.

The ongoing theme of strong ridging across North America will maintain the Jet Stream north across Canada but then dipping into the Eastern US with embedded disturbances and potentially more wet conditions (Figure 9).  Meanwhile the dry pattern is predicted to persist across Eurasia except in Northern Europe, the monsoon regions and potentially Northern China (Figure 9).

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows overall warm/positive PCHs for the next two weeks (Figure 10) consistent with the predicted negative AO bias. Low Arctic sea ice and snow cover will continue to favor warm/positive PCHs.  Therefore I expect little change in the tropospheric PCHs in the coming weeks.

Figure 10. 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 25 July 2016 GFS ensemble.

In general the relationship between the AO and the weather across the mid-latitudes is much weaker in summer and can even be opposite of that in winter.  Though so far this summer, negative AO conditions have favored troughing and cooler temperatures in Northern Europe more so than in the Eastern US.   Also with atmospheric dynamics at their weakest point in the NH, persistence favors widespread warmth across the NH continents.