Climate variability in the upper troposphere and stratosphere over Asia and its representation in modern reanalysis products

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The convection and circulation patterns associated with the Asian summer monsoon couple boundary layer pollution in South and East Asia to the upper troposphere and stratosphere, with substantial impacts on climate and chemistry. In this project we validate the performance of three reanalysis products (ERA-Interim, JRA-55, and MERRA-2) in this region by running the state-of-the-art chemistry transport model CLaMS (Chemical Lagrangian Model of the Stratosphere) from 1979 through 2017. We compare the resulting distributions of water vapor, ozone, methane, carbon dioxide, and mean age of air with satellite and in situ observations. In particular, we investigate diferences in transport within the Asian summer monsoon anticyclone, a key pathway for troposphere-stratosphere exchange during boreal summer and fall. We then use this framework to examine the efects of the Asian tropopause aerosol layer (ATAL) on cross-tropopause transport and the atmospheric radiation budget using aerosol analyses included in MERRA-2 together with CLaMS simulations, observations, and the Moderate Resolution Atmospheric Transmission (MODTRAN) radiation scheme. The ATAL is a recent phenomenon that has only emerged within the past 15 years. Our evaluation of the influence of the ATAL extends existing impact studies by examining not only the surfaceand top-of-atmosphere radiative forcing, but also the potential role of the ATAL in alteringthe thermodynamic structure of the tropopause layer and the composition of air entering the stratosphere.The key objectives of the proposals are:1. Validate different reanalysis products (ERA-Interim, JRA-55, MERRA-2) with respect to their abilities to resolve transport in the upper troposphere and stratosphere (UTS).2. Quantify uncertainties in the transport of the polluted air through the Asian monsoon anticyclone into the stratosphere.3. Quantify the impact of the Asian tropopause aerosol layer (ATAL) on the radiative budget over Asia.4. Estimate the potential effects of the ATAL on transport within the Asian monsoon anticyclone.