The Asian monsoon as a gateway to the global stratosphere forboundary emissions as caused by energy supply systems

Increasing population and growing industries and the associated increased energy consumption in emerging Asian economies result in enhanced emissions of greenhouse gases, ozone-depleting substances, and pollution in this region. In summer, the Asian monsoon circulation is an important large-scale circulation system in the northern hemisphere, which connects the Asian source region with the global upper troposphere and lower stratosphere by strong convective upward transport of tropospheric source gases into the upper troposphere and horizontal transport and mixing. The circulation system consists of a large-scale anticyclone in the upper troposphere extending from Asia to the Middle East from early June until the end of September. In general, the Asian monsoon circulation is believed to provide an effective pathway for greenhouse gases, water vapour, and pollutants to the lower stratosphere of the Northern Hemisphere. However, the mechanisms for possible transport into the lower stratosphere are subjects of a longstanding debate. The impact of transport of trace gases influenced by the Asian monsoon anticyclone into the extratropical upper troposphere and lower stratosphere is important because changes in ozone and water vapour in this region have a significant impact on surface climate, even if the perturbation is relatively small, demonstrating the crucial role of changes in the chemical composition in this part of the Earth's atmosphere.

IEK-7 has already published some basic studies related to the topic of transport processes in the region of the Asian monsoon (Konopka et al., JGR, 2010; Ploeger et al., JGR, 2013; Vogel et al, ACP, 2014; Ploeger et al., ACP, 2015; Vogel et al, ACP, 2015).

Satellite measurements show a strong confinement of tropospheric source gases inside the Asian monsoon anticyclone during summer. Ploeger et al. (2015) determined a transport barrier that reflects the separation between air inside the core of the anticyclone and the background stratosphere in agreement with satellite measurements.

Further, in a paper by Vogel et al (2014), a new fast transport pathway is found from Asia into the lower stratosphere over Europe: it is demonstrated that the combination of rapid uplift by typhoons in the western Pacific and circulation of the Asian monsoon anticyclone have the capability to carry boundary emissions from Southeast Asia/West Pacific within approximately 5 weeks to the lower stratosphere in northern Europe (see Fig. 1).

In a recently published paper (Vogel et al, 2015), source regions in the Earth's boundary layer that contribute to the chemical composition of air masses within the  Asian monsoon anticyclone could be identified using the Chemical Lagrangian Model of the Stratosphere (CLaMS) developed at IEK-7. It could be shown using artificial tracers of air mass origin in CLaMS that the composition of the Asian monsoon anticyclone in the upper troposphere strongly depends on its intraseasonal variability and is therefore more complex than hitherto believed. The largest contributions to the composition of the air mass in the anticyclone are found from northern India and Southeast Asia. Minor contributions are from southern India and eastern China. The model simulations show that emissions from Asia have a significant impact on the chemical composition of the lower stratosphere of the Northern Hemisphere, in particular at the end of the monsoon season in September/October 2012.

Motivated by the current lack of detailed understanding of transport processes in the vicinity of the Asian monsoon into the lower stratosphere and the fact that no in situ measurements of various trace gases inside the Asian monsoon anticyclone exist, IEK-7 is strongly involved in organising and planning a measurement campaign in Asia. In the framework of European Project StratoClim, IEK-7 coordinates an aircraft measurement campaign planned for summer 2016 in Asia using the Russian high-altitude aircraft M55-Geophysica.  During the StratoClim campaign, chemical trace gases will be measured with high spatial resolution for the first time at high altitudes in the Asian Monsoon. Thus, an unique date set will be available end of 2016 to improve our understanding of transport  processes in the region of the Asian monsoon and their impact on the chemical composition of the upper troposphere and lower stratosphere and therefore for surface climate. The scientific work at IEK-7 will help to achieve a better basis to assess possible climate risks caused by future growth and possible changes in energy supply systems in Asia that could globally impact changes in the Earth's surface climate.

^{Fig. 1: Selected trajectories representing characteristic pathways with different vertical velocities as a function of potential temperature (here height coordinate: 380 K corresponds to ~14 km) and longitude for the time period between 17 August and 26 September 2012. The air mass positions are plotted every hour (coloured dots). Large distances between the single dots indicate very rapid uplift. The classification of the different vertical velocities is derived for the first 10 days of the trajectories (17-28 August 2012). For simplification, longitudes between 60°W and 20°E are shown twice. The coloured text gives information about the altitude origin and geographical origin of the different pathways. The figure illustrates different transport pathways of air masses with a different origin to the location of the in situ measurements over northern Europe using the German High Altitude and LOng Range Research Aircraft (HALO). The combination of very rapid uplift by a typhoon and eastward eddy shedding from the Asian monsoon anticyclone yields fast transport (~ 5 weeks) from Southeast Asia boundary layer sources to northern Europe (red) (adapted from Vogel et al., 2014).}