Arctic Oscillation Analysis and Forecasts

June 27, 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 positive and are predicted to remain positive over the next two weeks.
  • The positive AO/NAO are reflective of predictions of mostly negative pressure/geopotential height anomalies in the Central Arctic and across the North Atlantic side of the Arctic basin and positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic.
  • Yet despite the positive AO/NAO general troughiness or overall negative pressure/geopotential height anomalies are predicted for both eastern North America and especially Western Europe.
  • Negative geopotential height anomalies in the Eastern United States (US) and Western Europe will help to keep temperatures near seasonable levels for the end of June and early July.
  • Predicted positive geopotential heights in western North America and East Asia, will favor above normal temperatures into the foreseeable future.
  • Despite the tendency for high latitude blocking/positive geopotential height anomalies due to low Arctic sea ice and snow cover, consolidation of negative geopotential height anomalies in the Arctic basin looks to persist in the coming weeks.  This will favor a positive AO.  However AO linkages to mid-latitude weather are more tenuous in summer and do not preclude troughs/negative geopotential height anomalies in Europe and the Eastern US.

Impacts

The AO/NAO is currently positive, reflective of negative geopotential height anomalies both in the Central Arctic and across the North Atlantic side of the Arctic with positive geopotential height anomalies across the mid-latitudes of the North Atlantic.  But we are in the peak summer months where temperature gradients are weak across the Northern Hemisphere (NH) and therefore so are the dynamics and large-scale relationships.  Instead changes occur slowly and what you see is what you get.

Across the mid-latitudes ridging/positive geopotential height anomalies dominate the NH mid-latitudes with some exceptions.  The most significant trough/area of negative geopotential height anomalies stretches north to south across western Asia.  Elsewhere across Eurasia, weaker troughs occupy Central Europe and East Asia.  Across North America the only trough is a weak one south of Hudson Bay.   However just off the East Coast of the US there is a closed area of low geopotential heights. Across the NH land areas, above normal temperatures dominate except in the regions of negative geopotential height anomalies – Western Europe, western Asia, parts of East Asia, Western Ontario and Manitoba. Also northerly flow behind the coastal low is bringing below normal temperatures to the Mid-Atlantic States.

With peak summer and weak dynamics little movement is predicted in the main weather features embedded in the atmospheric circulation.  The most significant trough/area of negative geopotential height anomalies is predicted for northwest Europe.  Other weaker troughs are predicted for western Asia, East Asia and Eastern North America.  Above normal temperatures will be the rule in the coming weeks except in those regions with troughing/ negative geopotential height anomalies including Western Europe and the Eastern US where temperatures will be closer to seasonable or even seasonably cool.

Recent and Very Near Term Conditions

The AO is currently positive and is predicted to remain positive all of this week (Figure 1), due to mostly negative geopotential height anomalies both in the Central Arctic and across the North Atlantic side of the Arctic with positive geopotential height anomalies across the mid-latitudes of the North Atlantic (Figure 2).

Figure 1. The predicted daily-mean near-surface AO from the 00Z 27 June 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 most extensive area of negative geopotential height anomalies in the North Atlantic sector stretches from Hudson Bay to Iceland and across to Central Europe (Figure 2).  On either side of the Central Europe trough, positive geopotential height anomalies dominate both Western Europe and Eastern Europe (Figure 2). The negative heights are resulting in below normal temperatures for Central Europe while northerly flow is also bringing below normal temperatures to Western Europe (Figure 3). In contrast, positive heights are resulting in above normal temperatures for Eastern Europe (Figure 3).  Further upstream negative geopotential height anomalies stretch from the Urals southward to the Arabian Sea (Figure 2) resulting in below normal temperatures for much of western Asia (Figure 3). Further east, ridging/positive geopotential height anomalies stretch from Western Siberia southward to Central Asia (Figure 2) resulting in above normal temperatures for much of Siberia and Central Asia (Figure 3). Finally another trough/negative geopotential height anomalies cover much of East Asia including Eastern China (Figure 2) bringing seasonable temperatures to that region (Figure 3)

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 27 June 2016 at 00Z. Note the low heights over Iceland, western Asia and the Ontario with high heights over the Eastern Europe and much of North America. 

Meanwhile North America is dominated by ridging/positive geopotential height anomalies (Figure 2).  This is resulting in mostly above normal temperatures across the continent with a few exceptions (Figure 3).   One region is Manitoba and Western Ontario where a weak trough/negative geopotential height anomalies is swinging though (Figure 2) resulting in below normal temperatures south of Hudson Bay (Figure 3).  A trough offshore of the Eastern US (Figure 2) is resulting in northerly flow and seasonably cools temperatures for the Mid-Atlantic States (Figure 3) and an area of relatively lower geopotential height anomalies in the Southwestern US is resulting in seasonable temperatures (Figure 3).

Figure 3. Analyzed surface temperature anomalies (°C; shading) on 27 June 2016. Note the warm temperatures across much of North America and East Asia with cool temperatures in Western Europe, Western Asia and Greenland.

Across North America the Jet Stream is aligned northwest to southeast with storms traversing from Western Canada to the Eastern US bringing with them relatively wet weather (Figure 4).  The pattern is predicted to be relatively dry across Eurasia with the exception of the East Asian monsoon region (Figure 4).

Figure 4.  Forecasted precipitation anomalies (mm/s; shading) from 28 June – 2 July 2016. Note the wet conditions across Western Canada, the Eastern US and the East Asia monsoon region. The forecasts are from the 00Z 27 June 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to remain positive during the course of next week (Figure 1) as negative pressure/geopotential height anomalies dominate the North Atlantic side of the Arctic while positive pressure/geopotential height anomalies ring the mid-latitudes with the exception of western Asia (Figure 5a).

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 3 – 7 July 2016. (b) Same as (a) except averaged from 8 – 12 July 2016. The forecasts are from the 27 June 2016 00z GFS ensemble.

Negative geopotential height anomalies are predicted to consolidate in the northeastern North Atlantic between Iceland and Scandinavia with low heights encompassing northwest Europe (Figure 5a).  With troughing predicted for Western and Central Europe, normal to below normal temperatures are predicted for Western and Central Europe (Figure 6).  Further east, positive geopotential height anomalies are predicted to stretch from the Laptev Sea southwestward towards Southeastern Europe (Figure 5a) resulting in above normal temperatures from Western Siberia back towards the Black Sea (Figure 6). The area of negative geopotential height anomalies in western Asia from the previous period shrinks in areal coverage but remains south of the Laptev Sea ridging (Figure 5a) allowing below normal temperatures to persist across western Asia (Figure 6). Further east, positive geopotential height anomalies replace the negative geopotential height anomalies previously in East Asia (Figure 5a) resulting in above normal temperatures (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 3 – 7 July 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 27 June 2016 GFS ensemble.

Positive geopotential height anomalies are predicted to dominate southern and western North America (Figure 5a).  In contrast, troughing/negative geopotential height anomalies are predicted for Eastern Canada and the Northeastern US (Figure 5a). With positive heights still dominant, above normal temperatures are predicted for much of the US and Canada (Figure 6). One exception will likely be Southeastern Canada and the Northeastern US as northerly flow delivers air masses that originate in Northern Canada.

Figure 7.  Forecasted precipitation anomalies (mm/s; shading) from 3 – 7 July 2016. Note the wet conditions across much of the Canada, the Eastern US and the Asian monsoon region. The forecasts are from the 00Z 27 June 2016 GFS ensemble.

With the Jet Stream remaining aligned northwest to southeast, storms are predicted to traverse from Western Canada to the Eastern US bringing with them relatively wet weather (Figure 7).   A dry pattern is predicted to persist across Eurasia except in the monsoon regions (Figure 7).

3-4 week

The AO is predicted to remain positive through the second week of July (Figure 1) as negative pressure/geopotential height anomalies are predicted to dominate the North Atlantic side while positive pressure/geopotential height anomalies remain stretched across the mid-latitudes of the North Atlantic (Figure 5b).

Little change is predicted in the circulation across Eurasia.  Negative geopotential height anomalies are predicted to persist across northwest Europe (Figure 5b) with normal to below normal temperatures for Europe (Figure 8) though the expanse of cool temperatures is likely overdone as shown.  Positive geopotential height anomalies are predicted to remain in the Laptev Sea with negative geopotential height anomalies further south across western Asia (Figure 5b). This should result in in more normal to below normal temperatures for western Asia (Figure 8).  Further east, positive geopotential height anomalies persisting across East Asia (Figure 5b) will result in above normal temperatures (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 8 – 12 July 2016. Note the warm temperatures across North America and East Asia, with cool temperatures in Europe and Western Asia.   The forecasts are from the 00Z 27 June 2016 GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to dominate much of North America with the exception of the Southern US (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 along the west coast of North America as well as some remnant weak troughing in eastern North America (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 25 – 29 June 2016. Note the wet conditions across much Canada and the Eastern US but dry for Eurasia. The forecasts are from the 00Z 14 June 2016 GFS ensemble.

Forecasted precipitation anomalies (mm/s; shading) from 8 – 12 July 2016. Note the wet conditions across much Canada and the Eastern US but dry for Eurasia except in the monsoon regions. The forecasts are from the 00Z 27 June 2016 GFS ensemble.

Longer Term

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs)  suggests any warm/positive PCHs that persisted from the sudden stratospheric warming from the spring has abated (Figure 10). Now that negative geopotential height anomalies have entered the central Arctic it will be harder to displace.  Still low Arctic sea ice and ice 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 27 June 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.   Also the still very warm temperatures worldwide continue to favor widespread warmth across the NH continents.