Impact of intensity variability of the Asian summer monsoon anticyclone on the chemical distribution in the upper troposphere and lower stratosphere

During the Asian summer monsoon(ASM)season,the process of stratosphere-troposphere exchange significantly affects the concentration and spatial distribution of chemical constituents in the upper troposphere and lower stratosphere(UTLS).However,the effect of the intensity of the Asian summer monsoon anticyclone(ASMA)on the horizontal distribution of chemical species within and around the ASMA,especially on the daily time scale,remains unclear.Here,the authors use the MERRA-2 reanalysis dataset and Aura Microwave Limb Sounder observations to study the impact of ASMA intensity on chemical distributions at 100 hPa during the ASM season.The intraseasonal variation of ASMA is classified into a strong period(SP)and weak period(WP),which refer to the periods when the intensity of ASMA remains strong and weak,respectively.The relatively low ozone(O_(3))region is found to be larger at 100 hPa during SPs,while its mixing ratio is lower than during WPs in summer.In June,analysis shows that the O_(3) horizontal distribution is mainly related to the intensity of AMSA,especially during SPs in June,while deep convections also impact the O_(3) horizontal distribution in July and August.These results indicate that the intraseasonal variation of the ASMA intensity coupled to deep convection can significantly affect the chemical distribution in the UTLS region during the ASM season.

Stratospheric entry point for upper-tropospheric air within the Asian summer monsoon anticyclone

The Asian summer monsoon(ASM) anticyclone isolates upper-tropospheric air within the interior of the anticyclone from the outside. Forward trajectory simulations in previous studies have shown that much of the air within the ASM anticyclone can be trapped for up to two or three weeks, not only laterally but also vertically. Here, we investigate the locations of exit points for upper-tropospheric air trapped within the ASM anticyclone, especially the preferred tropopause-crossing locations, using a 3-dimensional trajectory model. Forward trajectory calculations show two-thirds of the air crosses the tropopause at the southern part of the anticyclone via upward diabatic transport. Furthermore, some air crosses at northern and eastern parts via isentropic shedding, but air crosses rarely through the center of the anticyclone. However, calculations also show that many stratospheric parcels within the anticyclone are traceable from the upper-tropospheric anticyclone. This implies they cannot break through the tropopause directly overhead but instead enter the stratosphere via other entry points.

Atmospheric boundary layer sources for upper tropospheric air over the Asian summer monsoon region

Previous satellite measurements and model simulations have shown that the Asian summer monsoon（ASM） anticyclone is co-located with higher concentrations of pollutants, which are emitted in the continental atmospheric boundary layer（ABL）. Backward trajectory calculations show that the air at the 150-hPa level has the maximum frequency of ABL sources within 30 days over the most intensive convection regions and their downwind areas, which are not located within the ASM anticyclone,but rather at the southern flank or periphery of the ASM anticyclone. The upper tropospheric airs originated from the ABL sources include two parts： one from the ocean, which has the dominant impact to the south of 20°N, particularly over the South China Sea（SCS） and the west tropical Pacific Ocean; and another from the continent, which is dominant between 10°N and 30°N, particularly over the Bay of Bengal（BoB）, continental India, the Arabian Sea, and the Arabian Peninsula. It is the latter part that forms the higher pollutant concentration within the ASM anticyclone as shown by satellite measurements. Air in the ABL sources（both polluted and unpolluted） converges to the intensive convection region in the lower troposphere, and then traverses the middle troposphere through a wide group of upward pipes, and finally to the upper troposphere. These pipes in the middle troposphere are defined by the ASM intensive convections and cover the south of continental India,the BoB, the Tibetan Plateau, the Indochina Peninsula, the SCS, and the Philippine Sea.