Impact of stratospheric intrusion on near-surface ozone over the Sichuan Basin in China driven by terrain forcing of Tibetan Plateau

Abstract. Stratospheric ozone (O₃) intrusion acts as a major natural source of tropospheric O₃ affecting atmospheric environment. Targeting a stratospheric intrusion (SI) hotspot, the Tibetan Plateau (TP) with the immediately adjoining O₃ pollution region of Sichuan Basin (SCB) in Southwest China, this study assesses the seasonal contribution of SI to the near-surface O₃ over SCB and reveals the multi-scale coupling mechanisms of atmospheric circulations with the seasonally discrepant terrain effects of the TP. Results show that SI over the TP penetrates deep to the near-surface atmosphere in SCB with a maximum increment of 38.7 % in the O₃ level, providing an extra contribution of 11.1–16.0 % to regional O₃ pollution. The evolution of South Asian High with the peripheral subsidence in the warm season and the subtropical jet strtucture in the cold season trigger tropopause folding, driving stratospheric O₃ injecting into the troposphere over the TP. Two primary pathways for SI-derived O₃ entering the near-surface layer over the basin are identified with downslope transport along the TP’s leeward slope into the western SCB region and downwind transport to the central and eastern SCB regions associated with the seasonally discrepant effects of TP thermal and mechanical forcing.