Variability and trends of upper-tropospheric aerosols over the Asian summer monsoon region: an AeroCom multi-model study

Abstract. Aerosols in the upper troposphere play an important role in Earth's radiative balance and atmospheric composition. Satellite observations show recurring enhancements of aerosol extinction coefficient (AEC) in the upper troposphere and near the tropopause over the Asian summer monsoon (ASM) anticyclone (ASMA) region during July–August. However, substantial uncertainties remain regarding the roles of ASM dynamics, climate variability, and surface emissions in shaping upper tropospheric aerosols, as well as global model performance in this region. We present results from an AeroCom-coordinated multi-model study addressing these issues with nine global models covering the period 2000–2018. Large inter-model spread is found in non-volcanic AEC over the ASMA region, with coefficients of variation of 64 %–86 %. Diagnostics using standardized tracers show that approximately half of this spread arises from differences in transport and wet removal processes, with discrepancies in wet scavenging contributing roughly eight times more to the inter-model variance than transport. The multi-model ensemble simulates a significant increase in non-volcanic AEC in ASMA over the two-decade period at ∼ 1.2 % yr−1, primarily driven by rising anthropogenic emissions in Asia. In contrast, interannual fluctuations are modulated by climate variability, represented by Multivariate ENSO Index. Comparison with satellite-retrieved AEC also reveals persistent model deficiencies, especially in representing volcanic aerosols. These findings highlight the importance of improving the aerosol wet scavenging schemes and provide a benchmark for future coordinated aerosol modeling and evaluation.