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 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. Based on the simulations on four selected SI events in 2017, it is estimated that SI over the TP can penetrate deeply into the near-surface atmosphere in SCB, with event-specific maxima of up to 38.7 % in O3 increments, providing an extra contribution of 11.1 %–16.0 % to regional O3 pollution. The evolution of South Asian High in the upper troposphere with the peripheral subsidence in the warm seasons and the subtropical westerly jet over the TP in the cold seasons facilitates tropopause folding, driving stratospheric O3 into the free troposphere. We further identified the dominant processes for SI-derived O3 entering the near-surface layer over the basin with (1) a direct intrusion pathway of downward transport over the central and eastern SCB regions in the warm seasons, and (2) an indirect pathway of downslope transport along the TP-SCB slope into the western SCB region in the cold season, revealing the distinct seasonal effects of TP thermal and mechanical forcing on stratospheric O3 transport into the SCB region. These findings highlight the critical role of the TP in modulating stratosphere-to-troposphere O3 transport and its implications for regional air quality changes.