Highly Significant Responses to Anthropogenic Forcings of the Midlatitude Jet in the Southern Hemisphere

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, a new 40-member ensemble of integrations, from 1920 to 2080, of the Community Earth System Model, version 5, is analyzed together with a companion 1800-yr-long preindustrial control integration of the same fully coupled model. For simplicity, only the latitudinal position and the strength of the zonal-mean eddy-driven jet are considered. Given the large year-to-year variability of these jet properties, this paper focuses on their decadal averages, which reflect the more slowly varying climate state. The analysis herein 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, a forced summertime poleward shift of the jet in the latter part of the twentieth century and a strengthening of the jet during the twenty-first century in all seasons of the year are found. Contrasting these with changes in the southern annular mode, this confirms earlier studies demonstrating that such a mode is unable to distinguish different structural changes in the midlatitude jet.