Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia.

Ozone (O₃) in the upper troposphere and lower stratosphere (UTLS) is strongly regulated by upper-level circulation dynamics. Understanding the coupling between UTLS O₃ distribution and upper-level circulation dynamics is important not only to understand synoptic processes governing O₃ distribution and variability, but also to test the fidelity of chemistry transport models in simulating the stratosphere–troposphere exchange (STE) processes. This study presents the first systematic assessment of observationally constrained UTLS O₃ variability associated with upper-level circulation patterns over the Northeast Asia region. By applying the self-organized mapping (SOM) technique to 500, 250, and 100 hPa geopotential height (GPH) data, 12 circulation patterns are quantified and then used to characterize the UTLS O₃ distribution in the period 2000–2020 in both four-site (Beijing, Pohang, Tateno, and Sapporo) ozonesonde data and regional-scale satellite products. The underlying dynamic transport mechanism responsible for UTLS O₃ responses to different circulation patterns are further explored through correlation analysis between O₃ anomalies and transport indicators. The results indicate that although O₃ at almost all altitudes shows statistically significant sensitivity to circulation patterns, lower-stratospheric O₃ exhibits a far stronger sensitivity when compared with upper-tropospheric O₃. Circulation patterns featuring the East Asian Trough (EAT) show clear enhancement of O₃ southwest of the trough, and the enhancement zone moves with the eastward propagation of the EAT. Circulation patterns featuring eastward-shedding vortices of the Asia Summer Monsoon Anticyclone (ASMA) show the opposite signal, in which O₃ concentrations are decreased, especially at Sapporo, and the negative O₃ anomaly zone stretches from South Japan to Sakhalin Island. Each circulation pattern is characterized by distinct transport pathways, which play a determining role in the pattern-specific UTLS O₃ response. Positive O₃ anomalies are usually associated with post-trough downward and southward transport, whereas negative O₃ anomalies are commonly associated with fore-trough upward and northward transport. In the lower stratosphere, the correlation between O₃ anomalies and transport indicators is significantly stronger than that in the upper troposphere, and the strongest correlation occurs in the lower stratosphere of Beijing. [ABSTRACT FROM AUTHOR]