Highly significant responses to anthropogenic forcings
نویسندگان
چکیده
It has been suggested that changes in the atmospheric circulation caused by anthropogenic forcings are highly uncertain, owing to the large natural variability intrinsic to the system. Here, to assess the statistical significance of such changes for the midlatitude, large-scale atmospheric circulation of the Southern Hemisphere, we analyze a new 40-member ensemble of integrations, from 1920 to 2080, of the Community Earth System Model, version 5, together with a companion 1,800-year-long pre-industrial control integration of the same fully-coupled model. For simplicity, we only consider the latitudinal position and the strength of the zonal mean eddy-driven jet. Given the large year-to-year variability of these jet properties, we focus on their decadal averages, which reflect the more slowly varying climate state. Our analysis reveals that the forced response in such decadal averages easily emerges from the natural variability, with only a few model integrations typically needed to establish statistical significance. In particular, we find a forced summertime poleward shift of the jet in the latter part of the 20th Century, and a strengthening of the jet during the 21st century in all seasons of the year. Contrasting these with changes in the Southern Annular Mode, we confirm earlier studies demonstrating that such a mode is unable to distinguish different structural changes in the midlatitude jet. 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
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