1A: Asian Summer Monsoon: Transport from the Intensely Polluted Troposphere to the Stratosphere

The Asian Summer Monsoon (ASM) is a large weather system that impacts the lives of billions of people. While the meteorology of the ASM has been studied for more than a half century, its chemical impact and climate coupling through convective transport are research topics that have emerged from analysis of satellite data in the last decade (Randel and Park, 2006; Randel et al., 2010).  The large anticyclonic flow pattern in the upper troposphere and lower stratosphere (UTLS) associated with the ASM is a dominant circulation feature of the globe during boreal summer (Hoskins and Rodwell, 1995). This circulation is associated with a chemical fingerprint of enhanced pollution in upper levels, transported from near-surface levels. In addition, satellite and balloon measurements have revealed a previously unknown aerosol layer near the tropopause.  These observations indicate that ASM coupling of the polluted boundary layer to the largest UTLS dynamical system in the summer season through the deep monsoon convection has the potential to significantly impact composition and climate.

The behavior of ASM as a transport pathway is the focus of a recent model study using NCAR WACCM (Pan et al., 2016). The WACCM model simulations highlight a persistent plume of pollution in the ASM extending from the surface to the lower stratosphere, as shown in Fig. 1 for carbon monoxide (CO), a commonly used boundary layer pollution tracer. The WACCM simulation shows that the center of the uplift region (the so-called “chimney”) is near the southern edge of the Tibetan plateau. The model analysis shows that the uplifted boundary layer air is confined at upper levels by the anticyclonic circulation to form the patterns observed by satellites, and eventually dispersed into the stratosphere globally.  The model simulations show that the ASM system acts like a chimney over the most polluted region in the world: Northern India and Southern China, channeling pollution from the boundary layer into the stratosphere. The chemical and climate impact of this transport process is under further investigation.  

Figure 1: Latitude-pressure/altitude cross section of CO (ppbv) along ~90 E
Figure 1: Latitude-pressure/altitude cross section of CO (ppbv) along ~90 E (as shown on the map at the lower corner) for a selected day in August. Also shown are the tropopause height (black dots), selected isentropes (K; thin black lines), and locations of the easterly and westerly jets as indicated by selected zonal wind contours (m/s; blue dash and solid, respectively). The Tibetan plateau surface elevation is shown by the black terrain.

References

Hoskins, B. J., and M. J. Rodwell (1995), A model of the Asian summer monsoon, I, The global scale, J. Atmos. Sci., 52, 1329– 1340.

Randel, W. J., and M. Park (2006), Deep convective influence on the Asian summer monsoon anticyclone and associated tracer variability observed with Atmospheric Infrared Sounder (AIRS), J. Geophys. Res., 111, D12314, doi:10.1029/2005JD006490.

Randel, W. J., M. Park, L. Emmons, D. Kinnison, P. Bernath, K. A. Walker, C. Boone, and H. Pumphrey, Asian Monsoon Transport of Pollution to the Stratosphere, Science, 328, 611-613, 2010.

Pan, L. L., S. B. Honomichl, D. E. Kinnison, M. Abalos, W. J. Randel, J. W. Bergman, and J. Bian (2016), Transport of Chemical Tracers from the Boundary Layer to Stratosphere Associated with the Dynamics of the Asian Summer Monsoon, J. Geophys. Res. Atmos., 121, 14,159–14,174, doi:10.1002/2016JD025616.