87th AMS Annual Meeting

Thursday, 18 January 2007: 3:30 PM
The behavior of extreme cold-air outbreaks in a greenhouse-warmed world
214B (Henry B. Gonzalez Convention Center)
Stephen J. Vavrus, University of Wisconsin, Madison, WI; and J. E. Walsh, D. Portis, and W. L. Chapman
A natural overlap in studies of weather and climate is the response of extreme weather events to a climatic change. Until recently, however, little research was done on this topic, in part because realistic representation of extreme events demand higher fidelity from global climate models (GCMs) than simulations of mean conditions. Increasing computer power is now allowing higher-resolution GCMs to run longer integrations under various climate change scenarios to provide credible estimates of how weather extremes relate to mean climate. In this study we use output from seven state-of-the-art GCMs to analyze the behavior of extreme cold-air outbreaks (CAOs) under recent and future climatic conditions: late 20th century and projected 21st century regimes (SRES A1B greenhouse gas emission scenario).

In agreement with observations, the models generally simulate modern CAOs most frequently over western North America and Europe, regions downstream from preferred locations of atmospheric blocking. Future projections indicate that CAOsódefined with respect to late 20th century climateówill decline in frequency by 50 to 100% over most of the Northern Hemisphere during the 21st century. Certain regions, however, show relatively small changes and others actually experience more CAOs in the future, due to mean atmospheric circulation shifts and decadal variability that counter the thermodynamic tendency from greenhouse forcing. These areas generally experience greater near-surface wind flow from the north or of more continental origin during the 21st century and/or are especially prone to atmospheric blocking events. Simulated reductions in CAOs are smallest in western North America, the North Atlantic, and in central-southern regions of Asia. The muted Asian response is driven by a strong tendency for models to produce sea level pressure increases in the vicinity of the Mediterranean Sea, causing greater advection of continental air from northern and central Asia, while the tempered change over western North America is due to enhanced ridging along the West Coast and an increased frequency of upstream blocking events. The North Atlantic response is consistent with a slowdown of the thermohaline circulation, which either damps the warming regionally or results in a cooler mean climate in the vicinity of Greenland. The results of this study (1) underscore the complex and sometimes counterintuitive relationship between extreme weather and mean climate, and (2) suggest an important role for atmospheric blocking and mean circulation changes in bridging the timescales between short-term extreme events (cold waves) and long-term climatic regimes.

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