During the last two winters, some regions of the northern hemisphere experienced extreme cold not seen in recent decades though the seasons were also marked by more prominent, although less newsworthy extreme warm spells. Scripps Institution of Oceanography, UC San Diego, researchers examined daily wintertime temperature extremes since 1948. They found that the warm extremes were much more severe and widespread than the cold extremes during the Northern Hemisphere winters of 2009-10, which featured extreme snowfall on the East Coast dubbed "snowmaggedon," and 2010-11. Moreover, while the extreme cold was mostly attributable to a natural climate cycle, the extreme warmth was not. "We investigated the relationships between prominent natural climate modes and extreme temperatures, both warm and cold. Natural climate variability explained the cold extremes, the observed warmth was consistent with a long-term warming trend," said Kristen Guirguis, a Scripps postdoctoral researcher who is the lead author of the paper set to be published in the journal Geophysical Research Letters. The researchers created extreme temperature indices for the past 63 winters and placed the last two winters in this longer historical context. In terms of their cold extremes, the winters 2009-10 and 2010-11 ranked 21st and 34th, respectively, for the northern hemisphere as a whole. For warm extremes, these two winters ranked 12th and fourth, according to the record. Guirguis' team concluded that the extreme cold events by and large fell into norms that would be expected during the negative phase of the North Atlantic Oscillation (NAO), a prominent regional climate mode known to bring cold weather to northern Eurasia and Eastern North America. They reached the conclusion using a statistical model to explore the range of observed possibilities that would be expected in this phase of the oscillation. Study co-authors from left, Stephen Bennett, Kristen Guirguis, Rachel Schwartz and Alexander Gershunov The team compared records of extreme warm outbreaks over the two winters with the NAO as well as indices of El Nino - Southern Oscillation and its longer-term companion cycle, the Pacific Decadal Oscillation. This comparison, however, revealed that most of the extreme warmth was left unexplained. Including a linear warming trend better accounted for, but underestimated the recent warm extremes. "Over the last couple of years, natural variability seemed to produce the cold extremes while the warm extremes kept trending just as one would expect in a period of accelerating global warming," said Scripps climate researcher Alexander Gershunov, a report co-author. Gershunov noted, however, that the study shows that extreme cold events in the past two winters, though driven by a natural cycle, are still consistent with global warming trends. The oscillation would have made cold snaps even more severe if the global warming patterns superimposed upon it hadn't mitigated the cold. Extremes of cold and warmth affected much of the Northern Hemisphere, including locales such as this iced-over London canal, in the past two winters. Photo: JenniKate Wallace Scripps graduate student Rachel Schwartz and former Scripps Director of Business Development Stephen Bennett, a meteorologist, are also co-authors of the study. The research was funded in part by the Vetlesen Foundation via the Scripps Partnership for Hazards and Environmental Applied Research (SPHEAR).