Arctic Oscillation and Polar Vortex Analysis and Forecasts

September 21, 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 weakly positive and is predicted to first trend strongly positive and then negative over the next two weeks.    
  • The current positive AO is reflective of negative pressure/geopotential height anomalies across the Arctic and mixed pressure/geopotential height anomalies across the mid-latitudes. The North Atlantic Oscillation (NAO) is also currently positive with weak negative pressure/geopotential height anomalies across Greenland and Iceland and mixed pressure/geopotential height anomalies across the mid-latitudes of the North Atlantic.  The forecast is for the NAO to also first trend strongly positive and then negative over the next two weeks.
  • Currently troughing/negative geopotential height anomalies are confined to Northern Europe including the United Kingdom (UK) with ridging/positive geopotential height anomalies dominating much of the rest of Europe.  However the forecast is for troughing/negative geopotential height anomalies to drop south into Central Europe with normal to below normal temperatures across Northern and Central Europe for most of the next two weeks.
  • Much of Asia including the Middle East is predicted to be dominated by ridging/positive geopotential height anomalies and above normal temperatures for much of the next two weeks.  One exception will be East Asia where predicted weak troughing/negative geopotential height anomalies will result in normal to below normal temperatures. 
  • Early in the period troughing/negative geopotential height anomalies with normal to below normal temperatures are predicted to dominate much of Canada with ridging/positive geopotential height anomalies and above normal temperatures dominating the US. However next week the troughing and below normal temperatures are predicted to spill out of Canada across the Central US and then the Eastern US.
  • The Arctic sea ice extent minimum has likely occurred or is imminent.  The sea ice minimum is close to last year’s value and according to the National Snow and Ice Data Center (NSIDC) it is the sixth lowest on record.  The largest sea ice extent anomalies exist on the North Pacific side of the Arctic including the Chukchi and Beaufort Seas.  I discuss in the Impacts section how this could be influencing the weather outside of the Arctic during the fall.  Sea ice extent anomalies will continue to evolve over the coming months and I will be monitoring for possible clues for winter weather across the Northern Hemisphere (NH).
  • I will also be of course monitoring closely the advance of Siberian snow cover extent during the entire month of October.  It is still early and not much has fallen yet but if the recent past is any indication, October snow cover extent will once again be above normal this year.  I will be also watching snow cover across Canada, which has already gotten off to a fast start and this trend could continue based on model forecasts.

Impacts

So far widespread above normal temperatures across the Northern Hemisphere, but especially eastern North America and Europe, have characterized the month so far (though the models predict a fairly significant pattern change for both regions).  There have been two notable exceptions parts of Asia but in particular Western Canada.  The cold temperatures are supported by a highly anomalous ridge/positive geopotential height anomalies centered over the Bering Sea with troughing/negative geopotential height anomalies both upstream across Siberia and downstream across western Canada.  In Figure i, I show the decadal surface temperature trend for the three month average of October through December for the Northern Hemisphere (NH) continents.  The temperature trend over the past decade has been for widespread warming across the NH with two exceptions Siberia and Western Canada similar to what has been observed for the month of September so far.

Figure i.  Decadal trends in continental surface temperatures for October, November and December from 2008-2017.  The data is from the Climate Research Unit and UK Met Office CRUTEM4.

The widespread warming temperatures during the fall months is consistent with our expectations for climate change but the regional cooling temperatures in Siberia and Canada are not.  It is plausible that regional cooling is random or a consequence of natural variability of the atmosphere.  But the Bering and Chukchi Seas are the region with probably the greatest negative sea ice extent anomalies this late summer and early fall.  This is consistent with annual sea ice concentration trends showing a large area of negative trends in this region (Figure ii).  The large area of open water, where ice was previously present during the fall months, would result in a large flux of heat from the ocean into the overlying atmosphere.  This excess heating would promote dilation of geopotential heights over the Arctic region on the North Pacific side and promote ridging and blocking in the region.  And as we observed this September, ridging/blocking centered near the Bering and Chukchi Seas contribute to cold temperatures on either side of the North Pacific in Siberia and Western Canada.  This reasoning argues colder temperatures in Siberia and Western Canada observed this past September and the past decade in the fall months is not solely due to natural variability but at least partially a response to expansive ice loss on the North Pacific side of the Arctic.