The Asian summer monsoon and its influence on the upper troposphere and lower stratosphere

Abstract

The Asian summer monsoon is one of the most important atmospheric circulation systems of the Earth, as the typical monsoon rainfalls are crucial for agriculture and economy in Asia. One key component of the Asian summer monsoon is the South Asian High (also referred to as the Tibetan anticyclone) - a large-scale anticyclone which covers South Asia and parts of the Middle East during boreal summer at upper tropospheric-lower stratospheric altitudes. As shown, for example, in satellite measurements, the Asian summer monsoon affects the chemical composition of the upper troposphere-lower stratosphere via the South Asian High. Thus, the Asian monsoon gains importance beyond the regional scale as its infl uence on the composition of the stratosphere has the potential to affect climate change. Consequently, in times of a changing climate and increasing emission in developing countries in Asia, detailed knowledge of the processes in the upper troposphere-lower stratosphere in the Asian monsoon region is important. In this thesis, reanalyses and observational data are employed to investigate the dynamical features of the South Asian High. Previously, based on 5-day mean data, the South Asian High centre was identified to be located preferentially above the Iranian and the Tibetan Plateau. It is shown, that this so-called bimodality is most likely an artefact of one particular reanalysis data set. Moreover, the connection of convection and the location of the South Asian High is investigated. Multiannual backward trajectories are analysed to investigate the transport processes from the planetary boundary layer to the South Asian High. It is found that east-west shifts of the South Asian High on interannual timescales are not notably related to the source region composition of the anticyclone's air masses. With respect to the debated transport from the Tibetan Plateau to the South Asian High, it is found that this transport pathway is favourable mainly in August, as the subtropical jet is located above the Tibetan Plateau and thus hampers transport of air from the Tibetan Plateau to the anticyclone in earlier stages of the monsoon season. Finally, chemistry-climate model simulations in various set-ups are evaluated with respect to their capability of reproducing chemical and dynamical features of the South Asian High on a climatological basis. Therefore, satellite observations and reanalyses data are employed. Based on these results, an outlook on the Asian summer monsoon and the South Asian High in the future is presented.