Arctic Oscillation Analysis and Forecasts

May 18, 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 positive but is predicted to trend negative over the next two weeks.
  • The positive AO in the short term is reflective of predictions of negative pressure/geopotential height anomalies on the North Atlantic side of the Arctic basin and positive pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic.
  • However over the next two weeks, the models predict that positive pressure/geopotential height anomalies from the North Pacific side of the Arctic and from near the Urals will slide across the across into the North Atlantic side of the Arctic and force negative pressure/geopotential height anomalies further south in the North Atlantic.  This will initiate a negative AO trend.
  • The region that experiences the most consistent below normal temperatures during negative AO regimes is northern Eurasia.  Given that widespread below normal temperatures already cover much of western Eurasia, the models predict that the most widespread and persistent below normal temperatures will be western Eurasia, especially Europe over the next two weeks.
  • In contrast much of North America will be dominated by above normal temperatures. However one exception could be Southeastern Canada and the Northeastern United States (US).  Currently temperatures are below normal in the Eastern US and a negative AO trend favors potentially more below normal temperatures for Southeastern Canada and the Northeastern US.
  • I continue to favor overall positive polar cap geopotential height anomalies (PCHs) across the high latitudes; though synoptic systems will impose natural variability on the overall positive tendency.   The positive tropospheric PCHs related to the polar vortex (PV) breakdown are likely to persist in the absence of troposphere-stratosphere coupling. Furthermore the positive PCH tendency at high latitudes will be further reinforced by low sea ice and snow cover in the Arctic.  Positive PCHs favor an overall negative AO.

Impacts

The AO is currently positive, mostly forced by negative geopotential height anomalies across the North Atlantic side of the Arctic and positive geopotential height anomalies across the mid-latitudes of the North Atlantic.  Yet despite the positive AO, temperatures are below normal across much of Europe, with some exceptions including the United Kingdom, and the Eastern US.  Even though these regions are usually warm during positive AO regimes, lingering troughs/negative geopotential height anomalies from the recent negative AO period of last week is allowing below normal temperatures to persist in these regions.

Over the next two weeks the weather models are predicting that positive geopotential height anomalies on the North Pacific side of the Arctic and from the Urals region of northwest Asia will both converge on the North Atlantic side of the Arctic.  This will force a negative trend in the AO over the next two weeks, with the AO remaining positive this week but turning negative next week.

With the AO remaining negative and positive geopotential heights across the Arctic, should favor negative geopotential height anomalies across the mid-latitudes of the North Atlantic.  How much of the troughing/negative geopotential heights from the North Atlantic Ocean spread into Europe and the Eastern US will help determine temperature anomalies in these regions in the coming weeks.  The weather models currently favor Europe for the best chance of below normal temperatures.  However the models predict a vigorous area of below normal geopotential heights to develop in the eastern North Atlantic, which will help to build ridging/positive geopotential height anomalies in the western Mediterranean.  Near the ridging temperatures will likely be above normal while temperatures will be cooler in troughing on either side of the ridging.  How amplified the ridge becomes and the location of the ridge axis will likely determine the temperature anomaly pattern across Europe.

The forecast for North America, especially the high latitudes is for ridging/positive geopotential height anomalies to dominate, leading to an overall warm pattern. An area of deep low pressure/ geopotential heights is spinning just east of Labrador and a trough/negative geopotential heights extend southwestward from the low pressure across the Northeastern US, the Great Lakes and even into the Southwestern US.  Temperatures are below normal in and around the trough axis. However that whole trough system is predicted to eventually exit the East Coast and to be replaced by ridging.  Instead a trough/negative geopotential heights is predicted to develop in the eastern North Pacific and/or Western US.  Southwesterly flow out ahead of the trough will help the warm temperatures currently in the Western US to spread into the Eastern US.  However next week with the AO turning negative and with northern Canada dominated by positive geopotential height anomalies, increases the possibility of more troughing/negative geopotential height anomalies in the Northeastern US and could bring another round of below normal temperatures.

Recent and Very Near Term Conditions

The AO is currently positive (Figure 1), even though geopotential height anomalies are mixed across the Arctic they are mostly negative across the North Atlantic side of the Arctic (Figure 2). Positive geopotential height anomalies across the mid-latitudes of the North Atlantic (Figure 2) are further forcing the AO into positive territory.

Figure 1. The predicted daily-mean near-surface AO from the 00Z 18 May 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 greatest negative geopotential height anomalies are centered between Scandinavia and Svalbard (Figure 2).  A trough/negative geopotential height anomalies extend southward from the parent low across Central and Eastern Europe (Figure 2). Negative geopotential height anomalies are bringing below normal temperatures to most of Europe, especially Central and Eastern Europe and Western Asia (Figure 3). Further upstream, ridging/positive geopotential height anomalies centered near the Ural Mountains (Figure 2) are resulting in above normal temperatures for Northwest Asia and western Siberia (Figure 3). Further east still, another trough/negative geopotential height anomalies in Central Siberia (Figure 2) are bringing seasonably cool temperature to that region (Figure 3).  Finally ridging/positive geopotential height anomalies extending from Eastern Siberia through much of East Asia (Figure 2) are resulting in above normal temperatures stretching from Eastern Siberia southward across East Asia (Figure 3).

 

Figure 2. 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) on 18 May 2016 at 00Z. Note the low heights near Greeenland, Scandinavia and Labrador with high heights over the Urals, East Asia and north of Alaska. 

The main region of negative geopotential heights in the North American sector is a deep area low pressure/geopotential heights just east of Labrador (Figure 2).  Extending southwestward from the parent low is a trough axis of low pressure/geopotential heights  through the Northeastern US, the Great Lakes and into the Southwestern US (Figure 2).  The negative geopotential height anomalies (Figure 2) are resulting in below normal across much of the Eastern US and even parts of the Southwestern US (Figure 3).  Elsewhere much of Canada and the West Coast of the US are dominated by positive pressure/geopotential height anomalies (Figure 2) resulting in a mild southwesterly flow of air and warm temperatures (Figure 3).  An area of strong positive geopotential height anomalies lies just north of Alaska allowing for low pressure to spin to its south over Alaska (Figure 2).  Northerly flow between the two pressure systems has brought some cooler temperatures but the cooling is predicted to be transient with a return to well above normal temperatures (Figure 3).  

Figure 3. Forecasted surface temperature anomalies (°C; shading) from 19 – 23 May 2016. Note the warm temperatures across much of western North America and East Asia with cool temperatures in the Northeastern US, Europe, Europe and Western Asia. The forecasts are from the 00Z 18 May 2016 GFS ensemble.

Across the US, with a trough in the Eastern US and areas of low pressure predicted to traverse the country, an overall wet pattern is predicted to persist especially in the Rockies and the Southeastern US (Figure 4).  Southwesterly flow and a series of jet stream disturbances are predicted to bring wet weather to Western Europe and Southeast Asia as well (Figure 4). 

Figure 4. Forecasted precipitation anomalies (mm/s; shading) from 19 – 23 May 2016. Note the wet conditions across Western Europe, the Canadian and US Rockies but especially the Southeastern US and the Mid-Atlantic. The forecasts are from the 00Z 18 May 2016 GFS ensemble.

Near-Term

1-2 week

The AO is predicted to turn negative during the course of next week (Figure 1) as positive pressure/geopotential height anomalies expand across much the Arctic, especially near the North Pole but stretching into the North Atlantic side.  Further south across the eastern North Atlantic, west of Europe an area of low pressure/geopotential height anomalies are predicted to deepen and expand (Figure 5a).  High pressure across the north and low pressure across the mid-latitudes of the North Atlantic projects on to a negative AO.

Figure 5. (a) Forecasted average 500 mb geopotential heights (dam; contours) and geopotential height anomalies (m; shading) across the Northern Hemisphere from 24 – 28 May 2016. (b) Same as (a) except averaged from 29 May – 2 June 2016. The forecasts are from the 18 May 2016 00z GFS ensemble. 

The area of deep low pressure/geopotential height anomalies in the eastern North Atlantic is predicted to expand into Western Europe (Figure 5a). Below normal geopotential heights will help maintain below normal temperatures for much of Europe (Figure 6).  However ridging/positive geopotential height anomalies are predicted to build across the western Mediterranean and Southern Europe (Figure 5a), therefore above normal temperatures may be more widespread than predicted for the region (Figure 6).  Further downstream, an area of negative geopotential height anomalies is predicted for the Eastern Mediterranean and southwestern Asia (Figure 5a) resulting in below normal temperatures for Southeastern Europe and Southwestern Asia (Figure 6). Further north, above normal temperatures are predicted to persist for Western Siberia and expand into Northern Europe (Figure 6) as positive pressure/geopotential height anomalies persist near the Urals (Figure 5a).  Further downstream, troughing/negative geopotential height anomalies are predicted to stretch from Central Siberia into Central Asia (Figure 5a) resulting in below normal temperatures for Central Siberia and Central Asia (Figure 6).  Further east still, persistent ridging/positive geopotential height anomalies extending from Eastern Siberia through much of East Asia (Figure 2) are predicted to maintain above normal temperatures stretching from Eastern Siberia southward across East Asia (Figure 3).

Figure 6. Forecasted surface temperature anomalies (°C; shading) from 24 – 28 May 2016. Note the warm temperatures across much North America and East Asia with cool temperatures in Europe, Western Asia and Central Asia. The forecasts are from the 00Z 18 May 2016 GFS ensemble.

In the wake of the deep low pressure near Eastern Canada strong positive geopotential height anomalies build over Southeastern Canada and the Northeastern US (Figure 5a).  The rising geopotential heights across the Eastern US/Canada and southwesterly flow (Figure 5a) will usher in warmer air and above normal temperatures for the Eastern US and Canada (Figure 6). A trough/negative geopotential height anomalies are predicted for the Mid-Atlantic and the Southeastern US  (Figure 5a), which could bring cooler temperatures along the Eastern Seaboard than shown.  A trough is predicted to approach the West Coast of the US (Figure 5a) bringing with it possibly cooler temperatures for the West Coast but southwesterly flow ahead of the trough will maintain above normal temperatures for the Western US (Figure 6). Meanwhile positive geopotential height anomalies and southwesterly flow (Figure 5a) should result in normal to above normal temperatures for much Western Canada and Alaska (Figure 6).

Figure 7. Forecasted precipitation anomalies (mm/s; shading) from 24 – 28 May 2016. Note the wet conditions across much of the US but dry for Europe. The forecasts are from the 00Z 18 May 2016 GFS ensemble.

The strong high latitude blocking across Northern Canada and Alaska is predicted to suppress the Jet Stream south across the US (Figure 5a).  Disturbances in the Jet Stream across the US could potentially result in more wet conditions from the Pacific Northwest to the East Coast (Figure 7).   Meanwhile expanding high pressure is predicted to result in overall dry conditions for Europe (Figure 7).

3-4 week

The AO is predicted to remain negative into early June (Figure 1) as positive pressure/geopotential height anomalies are predicted to continue to dominate the Arctic with negative pressure/geopotential height anomalies in the mid-latitude North Atlantic (Figure 5b). 

A mostly stagnant flow is predicted for Eurasia.  The ridging/positive geopotential height anomalies across the western Mediterranean and Southern Europe are predicted to build and expand this period to cover much of Europe (Figure 5b).   This pattern favors a warming pattern across Europe and near seasonable temperatures are predicted for much of Europe (Figure 8).  Further downstream, the area of negative geopotential height anomalies in the Eastern Mediterranean and Southwestern Asia is predicted to persist (Figure 5a) maintaining below normal temperatures for Southeastern Europe and Southwestern Asia (Figure 6). Further north, the seemingly permanent ridging centered near the Urals is predicted to persist (Figure 5b) resulting in ongoing above normal temperatures for Western Siberia and Northern Europe (Figure 8).  Further downstream, the trough/negative geopotential height anomalies in Central Siberia sags south into Central Asia (Figure 5b) resulting in below normal temperatures for Central Asia (Figure 8). However temperatures are predicted to be above normal in Central Siberia (Figure 8) in response to rising geopotential heights (Figure 5b).  Further east still, ridging/positive geopotential height anomalies extending from Eastern Siberia through much of East Asia continue to persist (Figure 5b) maintaining above normal temperatures stretching from Eastern Siberia southward across East Asia (Figure 8).

Figure 8. Forecasted surface temperature anomalies (°C; shading) from 29 May – 2 June 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 18 May 2016 GFS ensemble.

Ridging/positive geopotential height anomalies are predicted to dominate almost all of North America with the biggest positive anomalies in the Canadian Plains (Figure 5b). This should continue to favor temperatures averaging seasonable to above normal for much of North America (Figure 8). One exception could be in the region of weak troughing/negative geopotential height anomalies along the immediate West Coast of the US (Figure 5b).  However with high latitude blocking and a predicted negative AO, troughing could be more amplified in Eastern Canada and/or US with cooler temperatures than predicted (Figure 8).

Figure 9. Forecasted precipitation anomalies (mm/s; shading) from 29 May – 2 June 2016. Note the wet conditions across much of the US but dry for Europe. The forecasts are from the 00Z 18 May 2016 GFS ensemble.

The continued high latitude blocking across northern North America is predicted to maintain a suppressed Jet Stream across the US (Figure 5b).  With ongoing disturbances crossing the US, the forecast is for potentially more wet conditions from the Pacific Northwest to the East Coast (Figure 9).   Meanwhile expansive ridging is predicted to result in overall dry conditions for Europe (Figure 9). 

Longer Term

30–day

The residual of warm tropospheric PCHs that resulted in the aftermath PV split/disruption looks to continue into early June (Figure 10). The latest PCH plot suggests that though the main descent of positive/warm PCHs peaked the end of April, warm/positive PCH continued into May and are still favored for the next couple of weeks.  Furthermore, low sea ice and the lack of snow cover across the Arctic also favor warm PCHs in the troposphere. Therefore the best forecast is probably one of persistence, which would favor overall warm PCHs and possibly even a negative AO through early June and possibly beyond. However 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. Longer-term AO predictions are more difficult in the summer and the weather related to variations in the AO more challenging to anticipate.

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 18 May 2016 GFS ensemble.