Transport and composition of the southern hemisphere UTLS (SOUTHTRAC)

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Changes in the distribution of trace gases, like water vapor and ozone in the upper troposphere and lower stratosphere (UTLS) strongly impact radiative forcing and the Earth's climate and surface temperatures, and are of key importance for understanding climate change. Due to the high sensitivity of atmospheric radiative forcing to changes in greenhouse gases in the cold tropopause region even small variations e. g. in water vapour in the lower stratosphere are an important source of the decadal variability of the surface temperature. There is also large uncertainty in future changes of ozone and water vapour in the UTLS, limiting the ability to predict the radiative forcing due to stratospheric ozone recovery and related processes during the 21st century: Several studies  have shown that different chemistry climate models do not even agree on the sign of radiative forcing due to ozone recovery because of large uncertainties in modelled composition particularly ozone and water vapour changes in the vicinity of the tropopause. Due to different wave focing and strong differences in the polar vortices of both hemispheres, significant differences between transport and composition of the UTLS between the Southern and Northern Hemisphere are expected. Despite the importance of the global UTLS dedicated field deployments studying transport processes and structure of the southern UTLS have not yet been performed. Previous field campaigns focused on Antarctic ozone depletion and vortex processes or the tropics or tropospheric composition. In addition, the winter southern hemisphere affects the global stratospheric circulation since the Andes constitute a global hot spot for gravity wave activity. The propagation of these waves and their impact are not yet fully understood. Therefore, we propose a HALO campaign, which addresses the following open key aspects regarding the processes affecting the composition of the southern UTLS region: (1) Exchange processes at the southern hemisphere tropopause (2) Gravity waves in the southern hemisphere (3) Influence of biomass burning on the southern hemisphere UTLS (4) Impact of the Antarctic vortex on the UTLS