Springtime regimes of lower tropospheric ozone distribution over East and Southeast Asia over 21 years (2004–2024)

Abstract. Lower-tropospheric ozone (O3) pollution is a major environmental and societal concern in East and Southeast Asia (ESEA), where concentrations often exceed the thresholds recommended by the World Health Organization, causing hundreds of thousands of premature deaths each year. While lower-tropospheric O3 plays a key role in air quality, its systematic spatial variability and the influence of synoptic and climatological drivers on its distribution over long periods have not yet been characterised. In this study, we describe the most commonly occurring spatial patterns of lower-tropospheric O3 during spring (March and April), the season of its highest concentrations. These regional spatial patterns are called O3 regimes, which are derived from chemical reanalyses during over 21 years using a k-means clustering algorithm. This method allows us to identify four distinct regimes of O3 regional distribution, which are validated with respect to multispectral IASI+GOME2 satellite observations. Our results highlight the occurrence of a specific regime characterised by anticyclonic and high insolation conditions over ESEA, that favour the occurrence of biomass burning in Indochina and lead to positive O3 anomalies extending from Southeast Asia towards the Pacific. Its occurrence is significantly correlated with El Niño conditions. Conversely, a contrasting regime is associated with La Niña, which enhance convection and cloudiness, while producing a negative anomaly of tropospheric O3 over most of the region. These results highlight that lower-tropospheric O3 in spring exhibits different distribution patterns, which can be strongly driven by synoptic conditions and large-scale climate variability.