Influence of ENSO on Tropospheric Ozone Variability in East Asia

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 Nino 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 Nino 3.4 index in El Nino/La Nina developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Nino 3.4 index. Ozone concentrations in China and the Korean Peninsula are similar to 5 ppb higher during El Nino developing summer compared to La Nina in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Nino/La Nina 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 Nino compared to La Nina. 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 similar 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 Nino-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 Nino developing summer are generally higher than its La Nina counterpart throughout the troposphere. In contrast, ozone concentrations in El Nino decaying spring are lower than that in La Nina 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 Nino decaying spring than during La Nina 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.