SHARP-STC Phase II
While a dynamical stratosphere-troposphere coupling (STC) via the Northern and Southern Annular Modes (NAM/SAM) has been confirmed by the analyses of our own model simulations and in a number of other studies, the underlying mechanisms remain unclear. Therefore one focus of SHARP-STC in Phase II will be to identify which processes determine the individual development of NAM/SAM anomalies, from the initial tropospheric forcing of stratospheric weak or strong vortex events to the persistence of the downward propagated anomalies in the troposphere. SHARP-STC plans to extend its studies of STC on the impact of future stratospheric composition changes on the radiative forcing of the troposphere-surface system and the radiative heating of the troposphere. Potential future changes of the mass exchange between the stratosphere and the troposphere and consequences for tropospheric OH chemistry will be another novel subject of SHARP-STC. For these studies new transient simulations with different configurations of the state-of-the-art Chemistry-Climate-Model (CCM) EMAC will be carried out for the past and throughout the 21st century, and specific new diagnostic tools for the derivation of the radiative and chemical exchange processes will be applied. Moreover, based on the development of the coupled ocean-atmosphere CCM EMAC-MPIOM SHARP-STC will be able to pursue in Phase II its initial goal to investigate the role of additional climate feedback processes associated with the interactive ocean-atmosphere exchange on the STC. Finally, the question whether a full representation of the stratosphere is required in climate models to reproduce the observed tropospheric climate variability and reliably project its future evolution will be addressed by using output of CMIP-5 simulations of the Earth System Model of MPI-M (MPI-ESM).