The effect of mixing and residual transport on Mean Age of Air in CCMVal-2 and CCMI model simulations

Mean age of air (AoA) is a good measure of the overall capabilities of a general circulation model (GCM) to simulate stratospheric transport. It is influenced by mean transport along the residual circulation and by two-way mixing. A large spread in simulation of AoA by GCMs and chemistry-climate models (CCMs) has been found. The causes for those differences can be identified by untangling the effects of different processes on AoA in GCMs. Therefore, we analyze the effects of residual transport and two-way mixing on AoA using data from the climate chemistry model inter-comparison projects CCMVal-2 and CCMI-1. Transport along the residual circulation is measured by the residual circulation transit time (RCTT) and is calculated for all models. The difference between AoA and RCTT is then interpreted as the aging by mixing, including both mixing on resolved and subgrid scale. Further we define a mixing efficiency as the ratio of the two-way mixing mass flux across the subtropical barrier to the net residual mass flux. This mixing efficiency controls the ratio of tropical mean AoA to RCTT, and thus the relative increase in AoA by mixing. We find that the spread in AoA between models is caused to some extent by differences in residual circulation strength, but the main cause are differences in the mixing efficiency. Possible causes for differences in the mixing efficiency are discussed. Subgrid scale mixing (including numerical diffusion) likely contributes to the differences in the mixing efficiency, but the effects of different advection schemes and differences in the resolution of the models are hard to disentangle. We show evidence that the relative contribution of resolved versus parametrized wave forcing in the models influence the mixing efficiency. Moreover we look at AoA trends in the models and we investigate the processes that control AoA trends in order to better understand the current discrepancies between modelled and observed long-term changes in AoA.