El Niño-Southern Oscillation and its impact on population exposure to ozone pollution and heatwave compound events in China

The El Niño Southern Oscillation (ENSO) has complex effects on human health through ozone pollution and heatwave compound events (OPHW). Although the climatic driving mechanisms of single extreme events in China have been preliminarily confirmed, the nexus of ENSO-OPHW-population exposure risk remains unclear. This study collected Niño 3.4 index, surface O3 and 209 Chinese cities' meteorological station data during the warm seasons (May–October) of 2013–2020, to reveal the effects of the ENSO on the three types of OPHW (ozone pollution-daytime heatwave: OPDH, ozone pollution-nighttime heatwave: OPNH, and ozone pollution-daytime and nighttime compound heatwave: OPCH) and the OPHW risks in major cities in China, via spatiotemporal statistics and geographical and temporal weighted regression (GTWR) modeling. Trend analyses indicate that ozone concentration, temperature, and the co-occurrence days of the three types of OPHW increase at a higher rate during El Niño periods than during La Niña periods. The driving effect of ENSO are found to amplify the OPHW is ranked as follows: ozone concentration (6.1 μg m⁻³) > temperature (1.8 °C) > co-occurrence days (0.2 days). Driven by the position of the subtropical high and anticyclonic changes over the continent, clear skies, high temperatures, low humidity, and low wind speeds significantly influence the three types of compound events. Moreover, during La Niña periods, the anomalous low-level anticyclone shows a significant southward shift compared to El Niño periods. During El Niño periods, the population exposure risk caused by OPHW is generally higher than during La Niña periods. OPDH, OPNH, and OPCH lead to the maximum relative changes of population exposure risk in the middle reaches of the Yangtze River (100 %), Sichuan-Chongqing (44.5 %), and Beijing-Tianjin-Hebei (11.2 %) urban agglomerations, respectively. These findings highlight the need for targeted research on the differential impacts of various compound events on regional population health and ecosystems. This study provides critical insights and methodological support for coordinated responses to extreme event changes and ozone pollution management.