Warm and Cold Winter Temperature Extremes in the Northern Hemisphere

In a paper that discusses winter temperature extremes (cold and warm), Guirguis et al. (2011) examine the severity of two recent winters, 2009/10 and 2010/11, in the Northern Hemisphere (NH) in the historical context of winters over the past 63 years. The daily extremes data (warm and cold) were obtained using near surface daily temperatures from the NCEP-NCAR global reanalysis of historical data (Kalnay et al. 1996). Extreme cold and warm events were identified from those data using the 5th and 95th percentile threshold. In addition, a regional index was calculated as the spatial average of local daily exceedance values so as to represent intensity and spatial extent of cold and warm extremes. So what did the authors find?

The authors report that while some parts of the NH clearly experienced very low temperatures, extreme warm events were much more prevalent in both magnitude and spatial extent in the NH during 2009/10 and 2010/11 winters and also during previous winters (December-February) between 1949-2010. Further, referring to the two most recent winters, the authors claim that "these winters' widespread and intense warm extremes together with a continuing hemispheric decline in cold snap activity was a pattern fully consistent with a continuation of the warming trend observed in recent decades." But is this really so?

Quite possibly, the present study has too narrow a focus on the winter season in the mid-latitude regions of the NH to pass this judgment. Only the winter (December-February) season of the NH was analyzed, for example, while no attempt was made to study the rest of the year. And, this study seems to ignore many tropical regions of south Asia and parts of central Africa where climate change impacts are often much different than what is observed so far up in the mid-latitude regions. For example, winter season over northern India and the Himalayan foothills region has become colder since the new millennium, while summer season shows very little change in mean temperature. Also, some of the south Asian countries (e.g., Vietnam, latitude range 15-20°N) have witnessed winter cold spells in recent years. Further, the above study does not analyze changes in mean temperature trends in the Southern Hemisphere, where recent winters ( e.g, 2007 and 2010) were reported to be significantly colder than normal. A website www.icecap for ICECAP (International Project for climate and environmental assessment) documents a large number of examples of recent colder winters in the SH and in particular in South America.

Quite possibly, the present study has too narrow a focus on the winter season in the mid-latitude regions of the NH to pass this judgment, The study also seems to ignore the fact that since the new millennium, there have been four winter seasons (2002/03, 2005/06, 2009/10 and 2010/11) which were significantly colder than normal in the NH. In particular, the severity of 2002/03 winter was experienced from Newfoundland in the Canadian Atlantic to the south Asian countries in Bangladesh and Vietnam, where several hundred people died of long exposure to cold spells. Are these frequent cold winters linked to Anthropogenic Global warming or part of natural variability? The above study does not provided an answer. Several recent studies (Lockwood et al., 2010; Wang et al., 2010; Woolings et al., 2010; Jung et al., 2011) suggest linkage of recent cold winters to extreme negative phase of the NAO (North Atlantic Oscillation) and to solar variability.

Additional References
Jung, T., Vitart, F., Ferranti, L. and Morcette, J.-J. 2011. Origin and predictability of the extreme negative winter of 2009/10. Geophysical Research Letters 38: L07701 doi:10.1029/2011GL046786.

Kalnay, E. et al. 1996. The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Metorological Society 77: 437-471.

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.

Woolings, T., Lockwood, M., Masato, G., Bell, C. and Gray, L. 2010. Enhanced signature of solar variability in Eurasian winter climate. Geophysical Research Letters 37: L20805 doi: 10.1029/2010GL044601.

Wang, C., Liu, H. and Lee, S.-K. 2010. The record-breaking cold temperatures during the winter of 2009/2010 in the Northern Hemisphere. Atmospheric Science Letters 11: 161-168.