Overview of Large-scale Tropospheric Transport in the Chemistry Climate Model Initiative (CCMI) Simulations

The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. Here we compare the large-scale tropospheric transport properties among different models in the Chemistry Climate Modeling Initiative (CCMI) with a focus on transport defined with respect to the Northern Hemisphere (NH) midlatitude surface. Among simulations of the recent past (1980-2010) we show that there are substantial differences in their global-scale tropospheric transport properties. For example, the mean transit time since southern hemisphere air last contacted the NH midlatitude surface differs by ~30-40% among simulations. We show that these differences are most likely associated with differences in parameterized convection over the oceans, such that the spread in transport among simulations constrained with analysis fields is as large as the spread among free-running simulations.