Evaluating reanalysis representations of climatological trace gas distributions in the Asian monsoon tropopause layer

Abstract. Trace gas distributions in the upper troposphere and lower stratosphere (UTLS) have important radiative and chemical impacts on climate. Although researchers have traditionally shunned direct outputs from reanalysis products at these altitudes, a looming gap in satellite observations may soon render these products essential. Here we use data from the Aura Microwave Limb Sounder (MLS) and five meteorological and composition-focused reanalyses to address two questions: Can current reanalyses reproduce essential features of UTLS composition above the Asian summer monsoon (ASM)? If so, do they reproduce these distributions from internal physics or depend on data assimilation? All evaluated reanalyses capture regional water vapor anomalies despite moist biases in the zonal mean. Reanalysis water vapor budgets reveal the expected balance between advective hydration and ‘cold trap’ dehydration near the cold point; however, data assimilation effects are also influential. The scientific utility of reanalysis water vapor fields at these altitudes could be enhanced by suppressing assimilation effects to facilitate the dominant ‘advection–condensation’ balance, as is now done by ECMWF. The two reanalyses that provide CO show good agreement with observed convective enhancement, highlighting the value of including CO-like transport tracers in reanalyses. All five reanalyses also reproduce the seasonal ‘ozone valley’ above the monsoon, at least qualitatively, but the only reanalysis to provide a complete ozone budget relies heavily on data assimilation to do so. The composition reanalyses, with more sophisticated chemistry, provide a better match to ozone observations, but it remains unclear whether they can do so without Aura MLS.