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Abundance of tropospheric ozone is determined by the interplay among emissions, chemistry, and climate dependent transport. It is important to identify the essential factors that modulate ozone in China and Korea because of increasing ozone trends in this area. In this study, we investigated the correlations of tropospheric ozone in East Asia with El Niño Southern Oscillations (ENSO). The model simulated ozone from 2000 to 2020 with Community Atmosphere Model version 6 with chemistry (CAM6‐chem), ECMWF Atmospheric Composition Reanalysis data, and ozone profiles measured in South Korea, Japan, and Hong Kong. These data were utilized to understand the ozone–ENSO relationship in this region. We found that tropospheric ozone in East Asia is significantly correlated with the wintertime Niño 3.4 index in El Niño/La Niña developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Niño 3.4 index. Ozone concentrations in China and the Korean Peninsula are ∼5 ppb higher during El Niño developing summer compared to La Niña in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Niño/La Niña decaying spring, the opposite occurs in the same region. In the decaying spring, stronger variability was observed in the subtropical regions of the lower to mid troposphere, indicating higher ozone concentrations during El Niño compared to La Niña. This variability is associated with the fluctuations in the Walker circulation, large‐scale sinking/rising motions, and the accompanying biomass burning events. The model reproduced the ozone variability associated with ENSO in the ozonesonde observations. Plain Language Summary: Ozone in the troposphere (surface to ∼16 km) is a greenhouse gas and an air pollutant harmful to public health and agriculture. Asian countries such as China and South Korea have increasing ozone trends in the last decades. Therefore, it is imperative to understand critical factors affecting ozone variability in this region. Ozone abundance is determined by emissions, chemistry, and transport related to weather and climate. El Niño–Southern Oscillation (ENSO), an air–sea coupled phenomenon in the tropical ocean can have a global impact on the atmospheric environment. In this study, we found that tropospheric ozone in Asia varies significantly with ENSO. Ozone concentrations in China and the Korean Peninsula in El Niño developing summer are generally higher than its La Niña counterpart throughout the troposphere. In contrast, ozone concentrations in El Niño decaying spring are lower than that in La Niña in the mid to upper troposphere extratropical region, indicating important roles of stratospheric ozone intrusion. The opposite occurs in the subtropical region: higher ozone concentrations during El Niño decaying spring than during La Niña are found in association with large‐scale longitudinal circulation varying with ENSO. Long‐term ozone profile observations support findings in the global chemistry climate model results. Key Points: A new connection between El Niño Southern Oscillations (ENSO) and tropospheric ozone in East Asia is introduced using CAM‐chem, reanalysis data and ozonesonde observationsENSO‐driven ozone variations are affected by stratospheric intrusion, continental and maritime transport, and biomass burning emissionsThe impacts of ENSO on tropospheric ozone in East Asia vary depending on location, altitude, and season (developing summer, decaying spring) [ABSTRACT FROM AUTHOR]

The ozone concentration in the atmosphere has been recovering with the reduction in atmospheric ozone-depleting substances (ODS). However, ODS remain in the atmosphere for long periods, slowing recovery. Furthermore, greenhouse gas-induced climate change complicates ozone recovery. East Asia is a significant contributor to global climate change due to the increase in industrialization and the presence of complex climate conditions. We investigated ozone variations in East Asia using total column ozone data based on satellite and ground observations and compared the results and trends derived from a multi-linear regression (MLR) model. We found that the MLR model has relatively poor explanatory power for recent extraterrestrial and dynamical proxies, but the uncertainty can be reduced using monthly data and atmospheric proxies. The ozone trend in East Asia had the greatest increase in the vicinity of the Korean Peninsula and Manchuria from 1997 to 2017 (~1% per decade). Similarly, the trend derived from Brewer spectrophotometer data was 1.02 ± 1.45% per decade in Pohang and 1.27 ± 0.85% per decade in Seoul. When the analysis period was extended to 2020, the impact of atmospheric variability was greater, suggesting that recent climate change can increasingly contribute to total ozone variability. [ABSTRACT FROM AUTHOR]