Transport in chemistry-climate models: Comparisons to observations and implications for projections of 21st century ozone

Reducing uncertainties in coupled chemistry-climate model projections of future changes in stratospheric ozone and climate depends critically on rigorous evaluation of model transport processes. We present results from the Transport Chapter of the SPARC Chemistry-Climate Model Validation (CCMVal) Report, which assesses the ability of the CCMVal2 models to reproduce observations of long-lived tracers that provide diagnostics of the large-scale overturning circulation, quasi-horizontal mixing, and subtropical and polar vortex mixing barriers. Newly published midlatitude profiles of the mean age of stratospheric air derived from CO2 and SF6 observations provide powerful new constraints on model transport when combined with previously available tropical mean age profiles. The age difference between the tropics and midlatitudes can be used to assess tropical upwelling independently of mixing across the edge of the tropics, and comparing model age profiles to the observed profiles in both the tropics and midlatitudes reduces the possibility of matching the observations through compensating errors in the circulation and mixing. We discuss the fidelity of CCM transport as well as projections of future changes in key transport diagnostics and implications for stratospheric ozone recovery.