Contribution of changes in atmospheric circulation patterns to extreme temperature trends

This study identifies statistically significant trends in mid-atmospheric circulation patterns that partially explain observed changes in extreme temperature occurrence over Eurasia and North America; although the underlying cause of circulation pattern trends remains uncertain, most extreme temperature trends are shown to be consistent with thermodynamic warming. Changes in atmospheric circulation — the position of the jet stream or intertropical convergence zone for example — may be linked to changes in the occurrence of temperature extremes, but quantitative evidence is scarce. Daniel Horton and colleagues identify statistically significant trends in mid-atmospheric circulation patterns over Eurasia and North America, with the trends partially explaining observed changes in extreme temperature. At present, it is unclear whether these trends are related to greenhouse gas emissions or natural variability, and better-understood thermodynamic changes control more of the overall trends in extremes. But in some regions and for some types of extreme temperature events, shifts in atmospheric circulation are an important actor. Surface weather conditions are closely governed by the large-scale circulation of the Earth’s atmosphere. Recent increases in the occurrence of some extreme weather phenomena1,2 have led to multiple mechanistic hypotheses linking changes in atmospheric circulation to increasing probability of extreme events3,4,5. However, observed evidence of long-term change in atmospheric circulation remains inconclusive6,7,8. Here we identify statistically significant trends in the occurrence of atmospheric circulation patterns, which partially explain observed trends in surface temperature extremes over seven mid-latitude regions of the Northern Hemisphere. Using self-organizing map cluster analysis9,10,11,12, we detect robust circulation pattern trends in a subset of these regions during both the satellite observation era (1979–2013) and the recent period of rapid Arctic sea-ice decline (1990–2013). Particularly substantial influences include the contribution of increasing trends in anticyclonic circulations to summer and autumn hot extremes over portions of Eurasia and North America, and the contribution of increasing trends in northerly flow to winter cold extremes over central Asia. Our results indicate that although a substantial portion of the observed change in extreme temperature occurrence has resulted from regional- and global-scale thermodynamic changes, the risk of extreme temperatures over some regions has also been altered by recent changes in the frequency, persistence and maximum duration of regional circulation patterns.