Extreme Negative NAO Index Linked to One of the Coldest and Snowiest Winters of 2009-10
Jung, T., Vitart, F., Ferranti, L. and Morcrette, J.-J. 2011. Origin and predictability of the extreme negative NAO winter of 2009-10. Geophysical Research Letters 38: L07701 doi: 10.1029/2011GL046786.
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.
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