Arctic Oscillation and Polar Vortex Analysis and Forecasts

July 16, 2018

Special blog on winter 2016/2017 retrospective can be found here - http://www.aer.com/winter2017

Special blog on winter 2015/2016 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 we transitioned to a spring/summer schedule, which is once every two weeks. Snow accumulation forecasts will be replaced by precipitation forecasts. Also, there will be less emphasis on ice and snow boundary conditions and their influence on hemispheric weather.

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The AO/PV blog is partially supported by NSF grant AGS: 1657748.

Summary

  • The Arctic Oscillation (AO) is currently positive and is predicted to first oscillate in positive territory over the coming week and then trend negative for the last week of July.    
  • The current positive AO is reflective of mostly negative pressure/geopotential height anomalies across the Arctic and mostly positive pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is currently neutral with mixed pressure/geopotential height anomalies across Greenland.  The forecasts are for the NAO to also trend first positive and then negative.
  • Persistent ridging/positive geopotential height anomalies late spring and early summer spread across Northern Europe centered first across Scandinavia and then across the United Kingdom (UK) are predicted to retrograde once again back towards Scandinavia.  This pattern has resulted in widespread above normal temperatures across Europe with the greatest positive departures focused across Northern Europe including the United Kingdom (UK).  This overall pattern is likely to persist for the next couple of weeks.
  • The predicted dominant pattern across Asia over the next two weeks consists of troughing/negative geopotential height anomalies with below normal temperatures in Central Asia and ridging/positive geopotential height anomalies and above normal temperatures across Western and Eastern Asia.
  • A checkerboard pattern of geopotential height anomalies dominates North America currently with mixed temperatures anomalies.  However the model predicts that ridging/positive geopotential height anomalies and normal to above normal temperatures to consolidate across Canada and the Western United States (US) with troughing/negative geopotential height anomalies with normal to below normal temperatures in the Eastern US.
  • Arctic sea ice melt has accelerated over the past week but a predicted positive AO next week could slow sea ice melt.  A new record low sea ice minimum still looks unlikely in September.

Impacts

I have discussed fairly often this summer about the increasing trend of summer ridging/blocking in and around Greenland over the past two decades or so.  Given the robustness of this trend I anticipated for much of the summer that ridging that setup across Northern Europe to migrate across the North Atlantic towards Greenland as the summer progressed.  However there is clearly no sign in the weather model forecasts of this scenario.  It is possible that above normal sea ice in eastern Hudson Bay and fairly extensive below normal sea surface temperatures in and around Greenland are not favorable for ridging in the region.

I have also discussed the importance of persistence during the summer months and certainly this summer the persistence of the ridging/blocking across Northern Europe has been impressive.  Earlier it was centered over Scandinavia and then more recently it has been centered closer to the British Isles and the latest forecasts suggest that it will once again retreat back towards Scandinavia.  There are no signs of the Northern European ridging/blocking breaking down in the weather model forecasts and even in the longer range CFS forecasts but the ridging/blocking is already unusually persistent and I would expect the blocking to break down sometime in August.  Though I have to admit that all of my previous predictions of the demise of the blocking have been premature to say the least.  Regardless it wil be tough to reverse current temperature anomalies through the end of the summer, espcailly across Northern Europe and the western half of North America (see Figure i).

Figure i.  Surface temperature anomalies (using a 1981-2010 climatology) for the Northern Hemisphere for June 1 - July 12, 2018 (right). Data is from NCEP/NCAR reanalysis NCEP-NCAR data.

In North America there are signs of a more robust pattern change.  Late June and early July where characterized by strong ridging and record heat in the Eastern US.  There are no signs of a return to strong ridging and unseasonable heat in the weather model forecasts.  Instead the weather models are predicting troughing and seasonably cool temperatures in the Eastern US.  The pattern may be transitory followed by a warm-up but there no signs of a return to record heat.

Across Asia the forecast is for pretty much more of the same with ridging and heat in East Asia and troughing and cooler temperatures in the western half of Asia.

Near Term Conditions

1-5 day

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

Figure 1. (a) The predicted daily-mean AO at 1000 hPa from the 00Z 16 July 2018 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 stretches across Northern Europe with mostly troughing/negative geopotential height anomalies across Southern Europe (Figure 2).  Widespread ridging across the Northern Europe favors normal to above normal  temperatures for much of Northern Europe including the UK, while a conga line of troughs favor normal to below normal temperatures for Southern Europe (Figure 3). Meanwhile across Asia, troughing/negative geopotential height anomalies in Central Asia sandwiched in between ridging/positive geopotential height anomalies in Western and Eastern Asia(Figure 2) favors normal to above normal temperatures across much of West and East Asia with normal to below normal temperatures in Central Asia including much of Siberia except Eastern Siberia (Figure 3).

Figure 2. Observed 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) for 00Z 16 July 2018.

Ridging/positive geopotential height anomalies dominate much of Canada and the US (Figure 2) favoring widespread normal to above normal temperatures (Figure 3).  However shortwave troughing/negative geopotential height anomalies currently in Central Canada is predicted to dive southeastward carving out a new trough in the Eastern US later this week (Figure 2).  This renewed troughing will usher in normal to below normal temperatures later this week for the Eastern US (Figure 3).  Predicted troughing/negative geopotential height anomalies in Alaska  (Figure 2) will bring seasonable temperatures to Alaska (Figure 3).


Figure 3. Forecasted surface temperature anomalies (°C; shading) from 17  – 21 July 2018. The forecast is from the 00Z 16 July 2018 GFS ensemble.

Overall below normal precipitation is predicted for much of Europe and Northern Asia (Figure 4).  Exceptions are Spain and Southeastern Europe where troughing is predicted to bring above normal rainfall (Figure 4).  Precipitation is predicted to be above normal in the Southeastern US where new troughing will promote heavier precipitation (Figure 4).

Figure 4. Forecasted precipitation anomalies (mm/day; shading) from 17 – 21 July 2018. The forecast is from the 00Z 16 July 2018 GFS ensemble.

Mid-Term

6-10 day

The AO is predicted to be remain mostly positive next week (Figure 1) as negative geopotential height anomalies dominate the Arctic with mostly positive geopotential height anomalies circling the mid-latitudes (Figure 5a). And with negative geopotential height anomalies remaining across Greenland and positive geopotential height anomalies across the mid-latitudes of the North Atlantic, the NAO will likely be positive as well next week.