Extreme Negative NAO Index Linked to One of the Coldest and Snowiest Winters of 2009-10

Jung et al. (2011) analyze the cause of extremely negative values of the NAO (North Atlantic Oscillation) index and its possible linkage to the winter of 2009-10, which was one of the coldest and snowiest over North America and Western Europe. The NAO is considered as one of the dominant modes of atmospheric variability over the Atlantic sector and impacts winter climate over North America and Europe. During the winter of 2009-10 the NAO index was in an extreme negative phase and may have caused it to be one of the coldest of the northern hemisphere in the new millennium.

The authors examine some of the large-scale phenomena like ENSO (El Niño-Southern Oscillation), the equatorial stratospheric QBO (Quasi-biennial "wind" Oscillation) and also northern hemispheric snow cover anomalies and their possible role in producing such an extreme negative value for NAO. An operational climate model for monthly forecasts developed at the ECMWF (European Centre for Medium-Range Weather Forecasts) was utilized to examine the possible role of the above mentioned features on NAO variability. The model simulations could not reproduce the extreme negative phase of NAO and this led the authors to conclude that the "the extreme negative phase of NAO during 2009-10 was a result of internal atmospheric dynamics" that was not predicted by the model. Most operational seasonal forecasting systems failed to predict this negative phase of NAO and consequently did not predict the severity of winter 2009-10 over Western Europe and North America.

There is now a growing realization among climate scientists and modelers that winters over Europe and North America have become colder and snowier in recent years. A recent paper (Petukhov and Semenov, 2010) examines a link between reduced sea ice in the Barents-Kara region of Arctic and cold winter extremes over northern continents of North Hemisphere. Another recent paper (Lockwood et al., 2010) examines the possible role of solar activity on cold winters in Europe.

Since the new millennium, there have been four winters (2002/03, 2005/06, 2009/10 and 2010/11) which are now adjudged as significantly colder and snowier over the northern hemisphere. Interestingly, the IPCC 2007 Climate Change Documents make NO reference to "colder winters in present or future climate"

As for why winters are becoming colder and snowier in a warmer climate, climate models do not provide an adequate answer.

Additional References
Lockwood, M., Harrison, R.G., Woolings, T. and Solanki, S.K. 2010. Are cold winters in Europe associated with solar activity? Environmental Research Letters 5: doi:10.1088/1748-9326/5/2024001.

Petukhov, V. and Semenov, V.A. 2010. A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. Journal of Geophysical Research 115: D21111 doi:10.1029/2009JD013568