Quantification of water vapour transport from the Asian monsoon to the stratosphere

Abstract

Abstract. Numerous studies have presented evidence that the Asian summer monsoon anticyclone substantially influences the distribution of trace gases – including water vapour – in the upper troposphere and lower stratosphere (e.g. Santee et al., 2017). Stratospheric water vapour in turn strongly affects surface climate (see e.g. Solomon et al., 2010). Here, we analyse the characteristics of water vapour transport from the upper troposphere in the Asian monsoon region to the stratosphere employing a multiannual simulation with the chemistry-transport model CLaMS (Chemical Lagrangian Model of the Stratosphere). This simulation is driven by meteorological data from ERA-Interim and features a water vapour tagging that allows us to assess the contributions of different upper tropospheric source regions to the stratospheric water vapour budget. Our results complement the analysis of air mass transport through the Asian monsoon anticyclone by Ploeger et al. (2017). The results show that the transport characteristics for water vapour are mainly determined by the bulk mass transport from the Asian monsoon region. Further, we find that, although the relative contribution from the Asian monsoon region to water vapour in the deep tropics is rather small (average peak contribution of 14 % at 450 K), the Asian monsoon region is very efficient in transporting water vapour to this region (when judged according to its comparatively small spatial extent). With respect to the Northern Hemisphere extratropics, the Asian monsoon region is much more impactful and efficient regarding water vapour transport than e.g. the North American monsoon region (averaged maximum contributions at 400 K of 29 % versus 6.4 %).

The efficiency of transport into the stratosphere via the Asian and North American summer monsoon circulations

“…The AOF from the NASM region to the LS-NH as reported by Nützel et al (2019) is much smaller (4.4 %) than the 21 % obtained in our study. The difference is due to different definitions of the LS-NH destination region (400 K level in Nützel et al, 2019; 340-380 K layer in our study), implying that transport from the NASM mainly affects the lowermost stratosphere, with relatively weak influences on the extratropical stratosphere above 400 K. Differences in AOF values quantifying transport to the TrP can also be explained by slightly different definitions of the TrP among these studies (see Table 2). However, while Nützel et al (2019) restricted their analysis only to the monsoon pathway (by considering only transport from the 370-380 K layer), our study illustrates the importance of the tropical pathway in transporting air from the 350-360 K layer in the monsoon regions to the TrP.…”

“…Our findings are also comparable to those of Vogel et al (2019), who also used CLaMS-EI but with full transport from the boundary layer to the stratosphere included. In Table 2, we summarize the methodologies (source, destination, time periods) and respective AOFs published by Vogel et al (2019) and Nützel et al (2019) in comparison with our results. The AOF from the NASM region to the LS-NH as reported by Nützel et al (2019) is much smaller (4.4 %) than the 21 % obtained in our study.…”

“…In Table 2, we summarize the methodologies (source, destination, time periods) and respective AOFs published by Vogel et al (2019) and Nützel et al (2019) in comparison with our results. The AOF from the NASM region to the LS-NH as reported by Nützel et al (2019) is much smaller (4.4 %) than the 21 % obtained in our study. The difference is due to different definitions of the LS-NH destination region (400 K level in Nützel et al, 2019; 340-380 K layer in our study), implying that transport from the NASM mainly affects the lowermost stratosphere, with relatively weak influences on the extratropical stratosphere above 400 K. Differences in AOF values quantifying transport to the TrP can also be explained by slightly different definitions of the TrP among these studies (see Table 2).…”

“…Since the tracers in the simulations are reset every July, we cannot infer how differences between CLaMS-EI and CLaMS-M2 evolve after June. Nützel et al (2019) recently quantified water vapour transport from the ASM and NASM regions to the stratosphere by using CLaMS-EI simulations in a similar configuration. In our approach as well as that of Nützel et al (2019), tracers are initialized in the upper troposphere above 350 K to reduce the impact of small-scale transport processes in the troposphere (e.g.…”

“…Dethof et al, 1999;Bannister et al, 2004;Fu et al, 2006;James et al, 2008;Randel et al, 2012). Note that large concentrations of monsoon tracers in the tropics do not always correlate with large water vapour mixing ratios in the tropical UTLS, as water vapour content at these altitudes depends not only on the origin of the air parcel but also on its temperature history (Fueglistaler and Haynes, 2005;Nützel et al, 2019).…”