Arctic Oscillation Analysis and Forecasts

August 22, 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) is currently neutral but it is predicted to first trend positive week one and then trend negative week two.  In contrast, the North Atlantic Oscillation (NAO) is predicted to remain mostly negative over the next two weeks.
  • The current neutral AO is reflective of mostly mixed pressure/geopotential height anomalies both across the Arctic basin and across the mid-latitudes of the Northern Hemisphere (NH).
  • The forecasts of a positive and then negative AO trend are being driven by similar trends in the NAO and are reflective of weakening of negative pressure/geopotential height anomalies in the mid-latitudes of the North Atlantic week one followed by strengthening positive pressure/geopotential height anomalies in the Arctic week two.
  • With the NAO mostly negative over the two weeks, favors below normal temperatures across parts of Northern Eurasia, Currently temperatures are cool across Europe though temperatures will spike this week as well below normal temperatures plunge into Siberia.  However next week as the NAO turns more negative, the axis of below normal temperatures should return to western Eurasia.
  • Cooler temperatures are making their way across eastern North America for the first half of this week.  However for much of the following two weeks above normal temperatures are likely to dominate much of North America.    However if the NAO turns sharply more negative as predicted, temperature are likely to follow and turn cooler across eastern North America.
  • Current temperatures in East Asia are above normal however the cold air in Siberia is predicted to make its way into East Asia by the end of next week.  However as the NAO turns more negative once again next week, the axis of cooler temperatures should shift once more to western Eurasia.

Impacts

With both the AO and NAO trending positive this week so are temperature trends likely to trend upward across Europe and the Eastern US.  The positive NAO trend is reflective of a positive trend in geopotential height anomalies across the mid-latitudes of the North Atlantic including Europe and eastern North America.

But there are signs that the season is finally transitioning from summer to fall as temperatures are predicted to plunge across Siberia.  A very deep area of low geopotential heights near the North Pole will pinwheel its way towards Siberia this week.  Unseasonably cold temperatures and even snowfall will accompany the rapidly falling geopotential heights across Siberia.  The predicted turn to freezing temperatures and snowfall is the first sign this summer that winter is indeed coming.

The positive AO/NAO trend will be short lived as persistent blocking/positive geopotential height anomalies near Greenland are predicted to persist.  As long as high geopotential heights remain anchored near Greenland, the negative phases of the NAO and even AO are favored.  Therefore temperatures are predicted to cool once more across Europe next week.  Though cooler temperatures are not currently predicted for eastern North America, the longer and deeper that the NAO turns negative, cooler temperatures are favored across southeastern Canada and the Eastern United States (US) as well.  

East Asia is currently dominated by ridging/positive geopotential height anomalies with the exception of a cutoff low near northeast China/southeast Siberia.  With above normal geopotential height anomalies in place, warm temperatures are widespread across East Asia.  However the cold air spilling into Siberia will continue its trek southeastward and temperatures are likely to take a sharply colder turn by the end of the week.  The turn to cooler weather will likely be temporary as a falling NAO next week will once again pull below normal geopotential heights and cooler temperatures back to the other side of the continent near Europe.

Recent and Very Near Term Conditions

The AO is currently neutral (Figure 1), reflective of mixed geopotential height anomalies across the Arctic with negative anomalies near the North Pole but positive anomalies near Greenland (Figure 2).  Geopotential height anomalies are also mixed across the mid-latitudes but a deep trough in the Central North Atlantic (Figure 2) is helping to drive the NAO deeper into negative territory than the AO.

Figure 1. The predicted daily-mean near-surface AO from the 00Z 22 August 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.

Downstream of the deep trough/area of negative geopotential height anomalies in the central North Atlantic, ridging/area of positive geopotential height anomalies dominate Western Europe (Figure 2).  Further upstream a week trough is slowly propagating through Central and Eastern Europe (Figure 2).  However during the week, the area of positive geopotential height anomalies will become more expansive across Europe resulting in rising temperatures across Europe for the rest of the week (Figure 3). Further east of the Central European trough is a strong ridge/area of positive geopotential height anomalies over Western Asia (Figure 2) resulting in above normal temperatures for much of western Asia (Figure 3). Further east still, troughing/negative geopotential height anomalies stretch from Central Siberia southwestward towards Central Asia (Figure 2). Resulting northerly flow between the Western Asia ridge and Central Siberian trough will allow well below geopotential heights and accompanying cold temperatures to slide southward from the central Arctic into Siberia (Figure 3). Meanwhile East Asia is dominated by positive geopotential height anomalies with the exception of a cutoff low in southeast Siberia (Figure 2) resulting in widespread above normal temperatures across East Asia (Figure 3).

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 22 August 2016 at 00Z. Note the low heights over the Central Arctic, Central Siberia, and Canada with high heights over Western Europe, Western Asia, East Asia and Eastern Canada.

In a departure from most of the summer, negative geopotential height anomalies stretch across much of North America including Western and Central Canada and most of the US except along the immediate East Coast (Figure 2). A strong trough extending southward from a cutoff low in Hudson Bay is ushering below normal temperatures to the Eastern US.  However as the trough pushes offshore geoptential heights are predicted to rebound quickly and with the higher heights, southwesterly flow will usher above normal temperatures across the Eastern US for the remainder of the week (Figure 3).  A cutoff low in southwest Canada (Figure 2) will drop down into the Western US resulting in seasonably cool temperatures for the Western US for the early part of the week followed by warming temperatures (Figure 3).

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 23 – 27 August 2016. Note the warm temperatures across much of North America and East Asia with cool temperatures in Central Europe, Western and Central Siberia.  The forecasts are from the 00Z 22 August 2016 GFS ensemble.

The Jet Stream is predicted to remain mostly north into Canada, therefore storms are forecasted to traverse from west to east across Canada bringing with them relatively wet weather to Canada (Figure 4).  Also a trough in the Western US is predicted to draw monsoonal moisture northward into the Rockies (Figure 4).  Heavy precipitation is also predicted for the subtropical western North Atlantic (Figure 4) as a tropical cyclone activity becomes more active.  Deepening low pressure across Northern Eurasia will bring wet weather to Scandinavia and Siberia, some of which can be snow (Figure 4). Additionally low pressure in the southeastern Mediterranean will bring more wet weather to the Balkans (Figure 4). 

Figure 4.  Forecasted precipitation anomalies (mm/s; shading) from 23 – 27 August 2016. Note the wet conditions across Northern Europe, Siberia, Japan, the Rockies and the monsoon region of North America. The forecasts are from the 00Z 22 August 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to trend slowly negative during the course of next week (Figure 1).  The negative trend is a result of strengthening positive geopotential height anomalies near Greenland with lowering geopotential height anomalies once more in the mid-latitudes of the North Atlantic (Figure 5a).

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 28 August – 1 September 2016. (b) Same as (a) except averaged from 2 – 6 September 2016. The forecasts are from the 22 August 2016 00z GFS ensemble. 

Negative geopotential height anomalies are predicted to propagate in from the North Atlantic towards Northern Europe (Figure 5a).  Lowering heights will reverse the temperature rise of the previous week with temperatures averaging seasonably cool (Figure 6).  Meanwhile further south over Central and Southern Europe, positive geopotential height anomalies from the previous period will persist (Figure 5a), allowing warm temperatures to linger across much of Central and Southern Europe (Figure 6). Meanwhile the rest of Eurasia is predicted to be divided similarly to Europe, with negative geopotential height anomalies to the north across Siberia and positive geopotential height anomalies to the south across southwest and Central Asia (Figure 5a).  Likewise temperature anomalies will follow a west to east divide with normal to below normal temperatures predicted for northern Asia and normal to above normal temperatures predicted for southern Asia (Figure 6).  One exception to the cool north and warm south pattern could be East Asia.  Positive geopotential height anomalies and southwesterly flow are predicted to result in above normal temperatures for Eastern Siberia (Figure 6).  Meanwhile further south negative geopotential height anomalies and cooler temperatures that spread south out of Siberia last period are predicted to linger (Figure 6).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 28 August – 1 September 2016. Note the warm temperatures across much North America and southern Eurasia with cool temperatures in Northern Europe and much of Siberia. The forecasts are from the 00Z 22 August 2016 GFS ensemble.

Positive geopotential height anomalies are predicted to once again dominate almost all of North America this period with the exception of a trough/negative geopotential height anomalies along the West Coast of the US (Figure 5a).  High heights and a prevailing southwesterly flow ahead of the trough (Figure 5a) will result in above normal temperatures for much of the US and Canada (Figure 6). Temperatures could average closer to normal across the Canadian Maritimes and New England (Figure 6) under weak northwesterly flow upstream of a trough in the western North Atlantic (Figure 5a).

Figure 7.  Forecasted precipitation anomalies (mm/s; shading) from 28 August – 1 September 2016. Note the wet conditions across much of the Canada, the Eastern US, the monsoon region of North America and Japan. The forecasts are from the 00Z 22 August 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). Similar to temperatures, precipitation anomalies will be divided across a west to east divide with above normal precipitation to the north and below normal precipitation to the south (Figure 7). One exception will be Japan, where troughing will enhance precipitation thsi period (Figure 7).

3-4 week

Positive pressure/geopotential height anomalies are predicted to expand across the Arctic this period (Figure 5b). The rising heights will force the AO/NAO deeper into negative territory the first week of September (Figure 1).  A negative AO/NAO and positive height anomalies across Greenland favor troughs and cooler temperatures in Europe and possibly eastern North America.

The general pattern across western Eurasia is of above normal geopotential heights with the greatest positive anomalies in Eastern Asia (Figure 5b). One exception to the above normal geopotential heights across most of Eurasia because is troughing/negative geopotential height anomalies over Northern and Central Europe (Figure 5b).  Northwesterly flow and low heights should result in seasonable to seasonably cool temperatures predicted for much of Europe with the possible exception of Eastern Europe (Figure 8) where geopotential heights are predicted to be higher (Figure 5b) and the winds southwesterly.  Downstream of the ridging/positive geopotential height anomalies in Eastern Europe, northwesterly flow is predicted to usher in seasonable to seasonably cool temperatures for Western Asia (Figure 8).  Further east, strong ridging/positive geopotential height anomalies are predicted for Central and Eastern Siberia (Figure 5b). This should result in in above normal temperatures for Central and Eastern Siberia and much of East Asia (Figure 8). Weak troughing/negative geopotential height anomalies in Southeast and Southwest Asia (Figure 5b) should result in normal to below normal temperatures for Southwest and Southeast Asia (Figure 8).  

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 2 – 6 September 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 22 August 2016 GFS ensemble.

Models predict that the pattern of positive geopotential height anomalies dominating most of North America to persist (Figure 5b). This should continue to favor temperatures averaging seasonable to above normal for most of North America (Figure 8). One exception could be eastern North America where weak troughing is predicted (Figure 5b).  However the negative AO/Greenland blocking may force more amplified troughing and temperatures may average cooler in eastern North America, especially southeastern Canada and the Northeastern US, than shown (Figure 8).

Figure 9. Forecasted precipitation anomalies (mm/s; shading) from 2 – 6 September 2016. Note the wet conditions across much Canada and the Eastern US, the monsoon region of North America but dry for Eurasia except for Northern and Central Europe and Japan. The forecasts are from the 00Z 22 August 2016 GFS ensemble.

The ongoing theme of above normal geopotential heights 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).  The model predicts more wet conditions in the North American monsoon region (Figure 9).  Meanwhile a relatively normal precipitation pattern is predicted to persist across Eurasia except in Northern and Central Europe and Japan in the regions of the strongest troughs (Figure 9).

30–day

The latest plot of the tropospheric polar cap geopotential heights (PCHs) shows near normal PCHs currently but turning warmer/more positive for the following week and peaking the first few days of September (Figure 10), consistent with the predicted negative AO trend and Greenland blocking. 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 22 August 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.