Variability in the UTLS chemical composition during different modes of the Asian Summer Monsoon Anti-cyclone

Abstract The Asian Summer Monsoon Anticyclone (ASMA) is an important circulation in the upper troposphere and lower stratosphere (UTLS) region during the boreal summer. The center of the ASMA is not a stationary feature but oscillates between longitudes 50°-92.5°E showing different modes. The ASMA center oscillating over 50°-67.5°E, 80°-92.5°E and 67.5°-80°E are termed as IP (Iranian Plateau), TP (Tibetan Plateau) and Elongated mode, respectively. This work explores the spatial variability and the influence of the background dynamical conditions on the vertical chemical distribution of carbonmonoxide (CO), water vapor (WV) and ozone (O3) in the UTLS region during the different ASMA modes using MLS satellite and NCEP/NCAR reanalysis observations. MLS observations for the period 2010–2018 showed maximum CO, WV and minimum O3 concentrations at 100 hPa in the ASMA center over the respective IP, TP and elongated modes. Enhancement in the tropospheric tracers (CO and WV) throughout the upper troposphere during TP and Elongated mode is observed indicating the transport of boundary layer air to the UTLS region from across these regions. In contrast, the enhancement in the tropopause region during the IP mode is mainly due to the convective lifting over the Tibtean plateau longitudes and strong horizontal advection towards the IP longitudes. Our study also revealed that the vertical transport of the tropospheric tracers to the stratosphere is effective over Tibetan Plateau irrespective of the ASMA mode. The warmer tropopause temperature due to an increase in O3 concentrations over the TP supported the vertical transport of WV to lower stratosphere over this region irrespective of the ASMA mode.