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[1] Future climate predictions by global circulation models in the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st century. Here it is shown that differences in the projected magnitude of the trend in the Southern Hemisphere are well correlated with biases in the latitude of the jet in the simulation of 20th century climate. Furthermore, the latitude of the jet in the models' 20th century climatology is correlated with biases in the internal variability of the jet stream, as quantified by the time scale of the annular mode. Thus an equatorward bias in the position of the jet is associated with both enhanced persistence of the annular mode, and an increased poleward shift of the jet. These relationships appear to be robust throughout the year except in the austral summer, when differences in forcing, particularly stratospheric ozone, make it impossible to compare the response of one model with another. These results suggest that the fidelity of a model's simulation of the 20th century climate may be related to its fitness for climate prediction. The cause of this relationship is discussed, as well as the implications for climate change projections. Citation: Kidston, J., and E. P. Gerber (2010), Intermodel variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in 20th century climatology, Geophys. Res. Lett., 37, L09708,

This paper discusses the possible response of the large-scale atmospheric structure to a warmer climate. Using integrations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) in conjunction with physical arguments, we try to identify what changes are likely to be robust and what the underlying mechanisms might be. We focus on the large-scale zonallyaveraged circulation, in particular on height of the tropopause, the strength and position of the surface westerlies and the strength and extent of the Hadley Cell. We present analytic arguments and numerical calculations that suggest that under global warming the height of the tropopause will increase in both the transient response and final equilibrium state, and an increase is clearly found in all the comprehensive models in CMIP5. Upper stratospheric cooling is also found in the comprehensive models, and this too can be explained by a radiative argument. Regarding the circulation, most models show a slight expansion and weakening of the Hadley Cell, depending on season and hemisphere. The expansion is small and largely confined to winter but with some expansion in Southern Hemisphere summer. The weakening occurs principally in Northern Hemisphere but the intermodel scatter is large. There is also a general polewards shift in surface westerlies, but the changes are small and again are little larger than the inter-model variability in the change. This shift is positively correlated with the Hadley Cell expansion to a degree that depends somewhat on the metric chosen for the latter. There is a robust strengthening in the Southern Hemisphere surface winds across seasons. In the Northern Hemisphere there is a slight strengthening in the westerlies in most models in winter but a consistent weakening of the westerlies in summer. We present various physical arguments concerning these circulation changes but none that are both demonstrably correct and that account for the model results. We conclude that the above-mentioned large-scale thermodynamic/radiative changes in the large-scale atmospheric structure are generally robust, in the sense of being both well understood and consistently reproduced by comprehensive models. In that sense the dynamical changes are less robust given the current state of knowledge and simulation, although one cannot conclude that they are, in principle, unknowable or less predictable.

[1] An asymmetry in the persistence of the eddy-driven jet is demonstrated, whereby the equatorward-shifted (low-phase) jet is more persistent than the poleward-shifted (high-phase) jet. The asymmetry is investigated by stirring the nondivergent vorticity equation on the sphere and is shown to arise due to the sphericity of the earth, which inhibits poleward wave breaking when the jet is at high latitudes. This spherical effect becomes increasingly important as the mean jet is positioned at higher latitudes. The persistence of the annular mode decreases as the mean jet moves closer to the pole due to the decreased persistence of the high-phase state, while the low-phase state exhibits similar persistence regardless of the jet position. These results suggest that with the expected poleward shift of the jet due to increasing greenhouse gases, the annular mode's total persistence will decrease due to a decrease in the persistence of the high-phase.