Is the Large-Scale Circulation Response to Increasing Greenhouse Gases Related to Climate Sensitivity? Lessons from CMIP5 Models

It is commonly assumed that the large-scale circulation response to increasing atmospheric greenhouse gas concentrations closely follows global-mean surface temperature warming: i.e., 1) in models with a higher climate sensitivity, the tropics will widen more and the mid-latitude jets will shift further poleward, and that 2) during periods of greater global-mean surface temperature rise, the tropics will widen more and the mid-latitude jets will shift further poleward. In this study, we re-examine these ideas using two sets of idealized model experiments from the CMIP5 archive: 1) an abrupt quadrupling of atmospheric CO₂, and 2) a 1% per year increase in atmospheric CO₂. In the Southern Hemisphere, higher climate sensitivity models generally expand the tropics more and shift the mid-latitude jet further poleward, particularly during summer and fall months. However, in the Northern Hemisphere, a poleward shift in the mid-latitude jet is poorly correlated with climate sensitivity, and the expansion of the tropics is only robustly correlated with climate sensitivity during the winter season. In fact, models with higher climate sensitivity actually contract the tropics during Northern Hemisphere summer. The relationships between climate sensitivity and the circulation changes are most apparent under high CO₂ conditions and are much less apparent in the transient response. Furthermore, not all aspects of the transient circulation response evolve monotonically with the global-mean surface temperature. Importantly, some features of the large-scale circulation appear to respond more rapidly than the global-mean surface temperature, implying that these features might be particularly relevant for detecting anthropogenic climate change.