Tropospheric ozone responses to the El Niño–Southern Oscillation (ENSO): quantification of individual processes and future projections from multiple chemical models

Abstract. The El Niño–Southern Oscillation (ENSO) modulates tropospheric ozone variability, yet quantitative contributions from individual processes and future response remain unclear. Here, we evaluate the GEOS-Chem chemical transport model and 10 chemistry–climate models (CCMs) in the Coupled Model Intercomparison Project Phase 6 (CMIP6) in capturing ozone–ENSO response, quantify the roles of transport, chemistry, and biomass burning, and examine the future evolution of these responses. The GEOS-Chem simulation over 2005–2020 well reproduces the satellite-observed ozone–ENSO response, including the instantaneous decrease (increase) in tropospheric column ozone (TCO) over the tropical eastern (western) Pacific in El Niño, and the delayed response in subtropics and mid-latitudes. The combined effects of transport, chemistry, and biomass burning emissions explain over 90 % of the simulated TCO variability in the tropical Pacific during ENSO. Changes in transport patterns show the dominant role by explaining 53 % (+0.8 DU) and 92 % (−2.2 DU) of the variability in TCO, respectively, in the western and eastern Pacific during El Niño relative to normal periods. Chemical depletion reduces ozone by 0.2 and 0.7 DU, respectively, in the western and eastern Pacific, which is offset by enhanced biomass burning emissions of 0.4 and 0.1 DU. Only 5 out of the 10 CMIP6 CCMs, with interactive tropospheric chemistry and accurate representation of ENSO dynamics, reproduce the tropical ozone–ENSO response. These models consistently indicate that tropical ozone–ENSO response will increase by 15 %–40 % in 2100 under the SSP3-7.0 scenario, associated with strengthening anomalous circulation and increasing water vapor with global warming. These results are critical for understanding climate–chemistry interactions and for future ozone projection.