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Global simulations with the Chemical Lagrangian Model of the Stratosphere(CLaMS) using artificial tracers of air mass origin are used to analyzetransport mechanisms from the Asian monsoon region into the lowerstratosphere. In a case study, the transport of air masses from the Asianmonsoon anticyclone originating in India/China by an eastward-migratinganticyclone which broke off from the main anticyclone on 20 September 2012and filaments separated at the northeastern flank of the anticyclone areanalyzed. Enhanced contributions of young air masses (younger than 5 months)are found within the separated anticyclone confined at the top by the thermaltropopause. Further, these air masses are confined by the anticycloniccirculation and, on the polar side, by the subtropical jet such that thevertical structure resembles a bubble within the upper troposphere.Subsequently, these air masses are transported eastwards along thesubtropical jet and enter the lower stratosphere by quasi-horizontaltransport in a region of double tropopauses most likely associated withRossby wave breaking events. As a result, thin filaments with enhancedsignatures of tropospheric trace gases were measured in the lowerstratosphere over Europe during the TACTS/ESMVal campaign in September 2012in very good agreement with CLaMS simulations. Our simulations demonstratethat source regions in Asia and in the Pacific Ocean have a significantimpact on the chemical composition of the lower stratosphere of the NorthernHemisphere. Young, moist air masses, in particular at the end of the monsoonseason in September/October 2012, flooded the extratropical lowerstratosphere in the Northern Hemisphere with contributions of up to ≈  30 % at 380 K (with the remaining fraction being aged air).In contrast, the contribution of young air masses to the Southern Hemisphereis much lower. At the end of October 2012, approximately 1.5 ppmvH<sub>2</sub>O is found in the lower Northern Hemisphere stratosphere (at380 K) from source regions both in Asia and in the tropical Pacific comparedto a mean water vapor content of  ≈  5 ppmv. In addition to this maintransport pathway from the Asian monsoon anticyclone to the east along thesubtropical jet and subsequent transport into the northern lowerstratosphere, a second horizontal transport pathway out of the anticyclone tothe west into the tropics (TTL) is found in agreement with MIPAS HCFC-22measurements

atmospheric chemistry and physics

Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012