How UTLS processes can affect climate sensitivity in global models including interactive chemistry

Occurence of a negative ozone radiative feedback has been demonstrated in CO2-driven global warming simulations with coupled chemistry climate models (CCMs). In some CCMs, this ozone feedback induces a substantial change (reduction) of the climate sensitivity to CO2 increase, while in others the impact on climate sensitivity in negligible. Interaction between dynamics, transport, and radiation in the UTLS appears to be the key for understanding model differences. An important detail is the radiative feedback of decreasing ozone concentrations in the lowermost tropical stratosphere on tropopause temperature and stratospheric water vapor, which is apparently of different strength is different CCMs. However, the enhancement of tropical upwelling in response to surface warming, leading to increased transport of ozone-poor tropospheric air into the stratosphere, may also differ from model to model. We also address evidence from a CCM experiment indicating that non-CO2 forcings may induce significantly different process chains in the UTLS (compared to CO2-driven simulations), adding on the complexity of the role of UTLS in Earth system dynamics