Most of the headline-grabbing cold weather outbreaks during the Northern Hemisphere winters of 2009-2010 and 2010-2011 can be explained by natural variability. However, a greater percentage of the Northern Hemisphere was dominated by anomalously warm weather outbreaks in these seasons and increasing global temperature trends over the past half-century may have been more responsible than natural variability.
For many residents of the Northern Hemisphere, the winters of 2009-2010 and 2010-2011 were frigidly cold and punctuated by massive snowstorms like Snowmageddon and the Snowpocalypse. Eurasia, Scandinavia, the United Kingdom, Mexico and the American Southwest and Southeast all had record outbreaks of cold weather, and the North Atlantic Oscillation (NAO) was largely to blame. The NAO measures the strength of a longitudinal, atmospheric pressure gradient that stretches across the North Atlantic from the Azores High to the Icelandic Low. When this gradient is weak - as it was during those two winters - blocking patterns develop and steer Arctic air masses into the Eastern United States and Northern Eurasia.
The NAO's pressure gradient became unprecedentedly weak during the 2009-2010 and 2010-2011 winter seasons, but the ensuing outbreaks of cold weather were warmer than expected, and they had a smaller geographical footprint than the 63-year average ending in 2011. In fact, the Northern Hemisphere was not anomalously cold during either winter. So how could this happen when the NAO was pushing harder for cold weather than ever before?
Well, the lower half of the Northern Hemisphere was experiencing extreme warm events that were much more severe, persistent and widespread than the cold snaps farther north. Guirguis et al. 2011 developed a severe cold index (SCI) from the past 63 winters and discovered that the 2009-2010 and 2010-2011 winters only ranked 21st and 34th, respectively. However, the severe heat indices (SHI) for the 2009-2010 and 2010-2011 winters ranked much higher at 12th and fourth, respectively. Additionally, the extreme heat events persisted for longer periods of time than the cold snaps up north and they spread over areas that were two and a half to three times bigger than expected. So why was it so much hotter than normal down there? Researchers investigated the role of natural climate variability by comparing the 2009-2010 and 2010-2011 winter SHIs to what is expected under average NAO, ENSO and PDO conditions. No arrangement of these teleconnection patterns explained why even half of the SHIs had occurred, but 50 percent of these SHIs were explained when the increasing global temperature trends of the past half century were included. Therefore, these extreme warm events may have symptoms of increasing temperatures rather than natural variability, and the NAO's typical chilling effect may have been reduced by this background heat.