Extreme Weather: Record-breaking Cold Winter of 2009/10 in the Northern Hemisphere

The two papers of Wang et al. (2010) and Cattiaus et al. (2010) discuss the extreme cold winter of 2009/10 in the Northern Hemisphere, when extreme low temperatures were recorded in Europe and North America. This record-breaking cold winter over Northern Hemisphere has been linked to the extreme (negative) phase of the NAO (North Atlantic Oscillation, a large-scale index based on surface pressure difference between Reykjavik-Iceland & Lisbon-Portugal) during the winter months. According to Wang et al., the NAO has been trending 'downward' since the early 1990s and the extreme low value of the NAO during December 2009-January 2010 produced a severe cold-air outbreak over North America, Europe, and parts of Asia. Wang et al. further add that if "this downward trend of NAO continues, more frequent cold outbreak and heavy snows are likely in the coming years."

Cattiaux et al. made a similar analysis of the NAO index and showed how the extreme persistence of daily NAO values led to the development of North Atlantic atmosphere blocking, which lead to the cold-air outbreak and severely cold winter over Europe. Cattiaux et al. further analyzed the severity of the winter of 2009/10 over Europe and concluded that it was comparable to the cold winter of 1963. They further state that the winter of 2010 "can be considered as an example of a cold extreme superimposed on a warming climate (of Europe)."

So what does the future hold? Wang et al. conclude that similar cold winters are likely to recur, while Cattiaux et al. suggest that future winters over Europe may not be as cold as experienced in 2009/10. Another paper (Kodra et al., 2011) suggests that "cold extremes are expected to decline in frequency by the end of 2100, but there will still be cold weather extremes which could be as intense or even more intense than present cold weather extremes." In view of these contrasting conclusions, the jury is still out!

Additional References
Kodra, E., Steinhaeuser, K. and Ganguly, A.R. 2011. Persisting cold extremes under 21st century warming scenarios. Geophysical Research Letters 38: doi:10.1029/2011GL047103.