Impact of ENSO on the UTLS chemical composition in the Asian Summer Monsoon Anticyclone

The changes in the convection and background dynamics over the Asian Summer Monsoon (ASM) are mainly responsible for the changes in the concentrations of the tropospheric pollutants within the Asian Summer Monsoon Anticyclone (ASMA). In this study, we have investigated the structural and chemical composition changes of the ASMA during different El Niño–Southern Oscillation (ENSO) conditions by making use of reanalysis products and Microwave Limb Sounder (MLS) satellite observations. Our analysis mainly concentrates on the July and August months of 2015 (El Niño) and 2022 (La Niña), which are the cold (Nino Index 3.4 exceeding −0.5 in magnitude throughout the year) and warm (Nino Index exceeding +0.5 in magnitude throughout the year) phase of the ENSO. The ASMA structure during different phases of the ENSO is found to be different when compared with the long-term (2005–2023) mean. The ASMA is found to have a larger zonal extent during La Niña (2022), extending from 20° to 140° E compared to El Niño (2015), which is from 20° to 100–105°E. A significant decrease (increase) in the concentration of the tropospheric species (CO and WV) is noticed within the ASMA in El Niño (La Niña). The decrease in the tropospheric species is clearly evident throughout the upper troposphere. The northeastern edge of the ASMA at 100 hPa and 150 hPa showed quite distinct behaviour with an enhancement (reduction) in the tropospheric species during La Niña (El Niño). Interestingly, there is an increase in O3 concentrations over the northeastern edge of ASMA during 2015, which is attributed to the stratospheric intrusion. Our results also revealed the enhancement of the tropospheric species over the western Pacific region during 2015 but not in 2022. Current findings highlight dynamic shifts linked to varying ENSO phases, leading to notable alterations in both the ASMA structure and Upper Troposphere and Lower Stratosphere (UTLS) chemical composition. •Structural changes in the Asian Summer Monsoon Anticyclone (ASMA) during the El-Nino and La-Nina phases of ENSO is reported.•Effect of background dynamics during different ENSO phases on the UTLS chemical composition (CO, WV and O3) is investigated•Significant differences in the UTLS chemical composition are observed within ASMA between EL-Nino and La-Nina phase of ENSO.•Strengthening and weaking of the ASMA resulted in the increase and decrease of CO, WV during La-Nina and El-Nino respectively.•Western Pacific ASMA mode with negative