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An eight-member ensemble of ECHAM5-HAMMOZ simulations for a boreal summerseason is analysed to study the transport of aerosols in the uppertroposphere and lower stratosphere (UTLS) during the Asian summer monsoon(ASM). The simulations show persistent maxima in black carbon, organiccarbon, sulfate, and mineral dust aerosols within the anticyclone in theUTLS throughout the ASM (period from July to September), when convectiveactivity over the Indian subcontinent is highest, indicating that boundarylayer aerosol pollution is the source of this UTLS aerosol layer. Thesimulations identify deep convection and the associated heat-drivencirculation over the southern flanks of the Himalayas as the dominanttransport pathway of aerosols and water vapour into the tropical tropopauselayer (TTL). Comparison of model simulations with and without aerosolsindicates that anthropogenic aerosols are central to the formation of thistransport pathway. Aerosols act to increase cloud ice, water vapour, andtemperature in the model UTLS. Evidence of ASM transport of aerosols intothe stratosphere is also found, in agreement with aerosol extinctionmeasurements from the Halogen Occultation Experiment (HALOE) andStratospheric Aerosol and Gas Experiment (SAGE) II. As suggested by theobservations, aerosols are transported into the Southern Hemisphere aroundthe tropical tropopause by large-scale mixing processes. Aerosol-inducedcirculation changes also include a weakening of the main branch of theHadley circulation and a reduction of monsoon precipitation over India

Transport of aerosols into the UTLS and their impact on the Asian monsoon region as seen in a global model simulation