The Flushing of the Northern Lower Stratosphere and the Influence of the Monsoon: Results from Tacts/Esmval 2012

We present results from the German research aircraft HALO during the TACTS/ESMVal project (Transport and Composition in the UT/LMS and Earth System Model Validationj). Global measurements of various trace species were performed from the arctic to the early antarctic vortex in August to September 2012. Here, we focus on the northern hemispheric distribution of CO, N2O and ozone above 380 K. In this region we could for the first time identify mixing lines, which indicate mixing between stratospheric air masses of different origin. Introducing a new pair of correlation species (N2O-CO) we could identify air masses, which do not involve mixing directly at the tropopause. The use of CO-N2O correlations provides additional knowledge on the end members and therefore the reservoirs involved in the mixing processes within the stratosphere compared to the CO-O3-correlation. Trace gas distributions for the atmospheric region between the extratropical tropopause and potential temperatures up to Θ = 405 K indicate mixing of 'young' stratospheric air with aged processed air. Backward trajectories indicate that the “younger” stratospheric air masses originate in the Asian monsoon region and contribute to the chemical composition of air above Θ = 380 K, thereby circumventing the TTL region. Therefore we conclude that the monsoon significantly contributes to the flushing of the extratropical UTLS during summer and autumn.