Transport from the Northern Hemisphere Midlatitude Surface: Comparisons Between the CCMI Models

The transport of chemicals is a major uncertainty in the modeling of tropospheric composition. Here we compare the transport of idealized tracers between different models in the Chemistry Climate Modeling Initiative (CCMI) using simulations constrained with observed sea surface temperatures. We show that there are substantial differences in their global-scale tropospheric transport properties due to large differences in parameterized convection between the simulations, especially over the oceans. Large transport differences appear both in the Arctic, where the concentrations of idealized 5-day and 50-day loss tracers differ by as much as 40%, and the Southern Hemisphere, where the mean transit time since air last contacted the Northern Hemisphere differs by ~30-40% between simulations. Furthermore, we show that, for some cases, the transport differences between constrained versions of models are larger than the differences between free-running simulations of the models, which have differing large-scale flow but much more similar convective mass fluxes. Our results indicate that more attention needs to be paid to convective parameterizations in order to understand large-scale tropospheric transport in models, particularly in simulations constrained with analyzed winds.