Trace Gas Budgets and Interhemispheric Differences of Composition and Transport in the UTLS

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The composition of the UTLS region (upper troposphere / lower stratosphere, i.e. the region between 8-20 km) plays a fundamental role for climate and surface temperatures. The trace gas budget of this region is determined by dynamical processes, which range from small scale turbulence, over breaking baroclinic and planetary waves to the stratospheric circulation. The resulting variability of the composition leads to significant uncertainties of climate simulations and estimates of future surface climate. A measurement based quantification of air mass origin, transport processes and time scales is therefore essential for a quantitative understanding of this region.Recent measurements mainly focused on the northern hemisphere and showed a clear seasonal cycle of the composition of the northern UTLS. During summer short transport time scales of weeks for transport particularly from the tropics and the monsoon region dominate the northern UTLS, whereas in winter long time scales and diabatic descent of aged stratospheric air masses prevail. The tropospheric contribution varies between 20-60% depending on season.The dominating atmospheric structures relevant for transport (e.g. the polar vortex and the Asian summer monsoon) introduce a strong hemispheric asymmetry and therefore seasonal asymmetric composition, age and transport structures. A measurement based air mass budget and quantification of the southern hemisphere UTLS is still missing. Within this project, this budget will be quantified on the basis of tracer measurements and the compared with the northern hemisphere including air mass fractions as well as transport time scales. This will include a study of the relevant processes which contribute to transport and mixing within the southern hemisphere, but also between both hemispheres.