Differential impacts of the 2015–2020 El Niño/El Niño Modoki on seasonal ozone levels across China

Abstract

Utilizing daily data from nearly 1500 ground stations within a nationwide air-quality monitoring network spanning 2015 to 2020, this study identifies three primary spatiotemporal modes of ozone (O₃) concentration anomalies and explores the sensitivity of surface O₃ to both Eastern Pacific El Niño and Central Pacific El Niño (El Niño Modoki) across distinct Chinese regions and seasons. Results reveal limited sensitivity during spring and winter, wherein negative (positive) O₃ anomalies predominate across China during the positive (negative) phase of either ENSO type, with spring magnitudes approximately doubling those observed in winter. In contrast, during summer and autumn, O₃ anomalies exhibit significant variations depending on El Niño type and geographical location. Positive anomalies are notably pronounced in summer and autumn, particularly during El Niño Modoki in eastern China, while negative anomalies concentrate in western China during autumn, with a scattered distribution in summer. On average, magnitudes of O₃ anomalies during El Niño Modoki surpass those during El Niño thoroughly, with the most pronounced differences in magnitude during summer for positive anomalies. The differences in O₃ anomalies between El Niño and El Niño Modoki are attributed to variations in anomalous atmospheric circulations, wherein El Niño Modoki stands out with stronger anomalous 850-hPa highs and lows, along with stronger associated anomalous circulations, as well as larger extents and magnitudes of anomalous precipitation and solar radiation. These variations induce changes in low-level wind directions that influence regional O₃ transport, and alterations in solar radiation and precipitation, impacting both O₃ generation and removal processes.