Understanding the Asian Summer Monsoon Anticyclone Dynamics, Pollutants, and Climate Implications: An Overview

The Asian Summer Monsoon Anticyclone (ASMA) is a prominent high-pressure system in the upper troposphere and lower stratosphere (UTLS) region, traps pollutants, allowing them to accumulate and influence the background atmosphere through radiative forcing. Any change in the ASMA will be reflected in the chemical composition of the trapped aerosols and trace gases within it. Therefore, understanding ASMA variability itself is a key objective. Reanalysis datasets reveal significant differences in capturing ASMA's dynamics, particularly monsoon-related circulations. This underscores the need for caution when using them for such studies. Unusual behaviour in ASMA has been observed during El Niño conditions, as well as during active and break monsoon phases. Similar to ASMA variability, the aerosols and trace gases also exhibit fluctuations, with noticeable asymmetry in their distribution within the anticyclone. Increasing trends in aerosol and carbon monoxide (CO) have been observed, while no significant trend in ozone. However, a surprisingly decreasing trend in water vapour has been noted. Deep convection over the Bay of Bengal and Northeast India, along with strong updrafts, identified as key drivers of vertical transport. Long-term trends reveal statistically significant increasing pollutant levels in ASMA, attributed to enhanced convection under global warming scenarios. Finally, the aerosol radiative forcing (ARF) is estimated to range from -0.13 W/m² to -0.19 W/m², with impacts comparable to regional GHG forcing and small volcanic eruptions. This study underscores the importance of understanding ASMA's variability, pollutant transport mechanisms, and radiative impacts to better assess its contribution to climate and atmospheric chemistry.