During the last two winters, some regions of the northern hemisphere experienced extreme cold not seen in recent decades. But at the same time, the winters of 2009–10 and 2010–11 were also marked by more prominent, although less newsworthy, extreme warm spells.

New research examines daily wintertime temperature extremes since 1948 The study finds that the warm extremes were much more severe and widespread than the cold extremes during the northern hemisphere winters of 2009–10 (which featured an extreme snowfall episode 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, the study concludes.

"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," says Kristen Guirguis, a postdoctoral researcher at Scripps Institution of Oceanography, UC San Diego and lead author of the study, which is set to be published in the journal Geophysical Research Letters, a publication of the American Geophysical Union.

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 2009–10 and 2010–11 winters ranked 21st and 34th, respectively, for the northern hemisphere as a whole. For warm extremes, these two winters ranked much higher (12th and fourth), according to the record.

Guirguis' team concludes 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.

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 in the model's assumptions better accounted for the recent warm extremes, but still underestimated them.

"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," says Scripps climate researcher Alexander Gershunov, a report co-author.

Gershunov notes, 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.

The research was funded in part by the Vetlesen Foundation via the Scripps Partnership for Hazards and Environmental Applied Research (SPHEAR). The University Corporation for Atmospheric Research under the Postdocs Applying Climate Expertise (PACE) fellowship also supported the work.

Source: AGU