Overview of Large-Scale Transport in the Chemistry Climate Modeling Initiative (CCMI) Models (Invited Presentation)

The transport of chemicals is a major uncertainty in the modeling of tropospheric composition and stratosphere-troposphere exchange. Here we compare the large-scale transport properties among different models in the Chemistry Climate Modeling Initiative (CCMI). Among simulations of the recent past (1980-2010) we show that there are substantial differences in their global-scale transport properties. Idealized tracers subject to prescribed uniform loss, emitted both at the Earth’s surface and in the stratosphere, reveal large differences in stratosphere-to-troposphere transport. We also find large differences in terms of interhemispheric transport in the troposphere, where the mean transit time since southern hemisphere air last contacted the NH midlatitude differs by ~30-40% among simulations. Furthermore, we show that the spread in transport among simulations constrained with analysis fields is as large as the spread among free-running simulations, indicating that caution should be taken when interpreting simulations constrained with analyzed winds.

Among simulations of the 21st century we find that models project a consistent, albeit small (~5-10%), reduction in interhemispheric transport between the northern and southern hemispheres and enhanced transport from the troposphere into the stratosphere. The changes in interhemispheric transport are correlated with the amplitude of upper tropospheric tropical warming, such that models that warm more also feature larger decreases in (parameterized) convection and a stronger weakening of the boreal winter Hadley Circulation.