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 responses remain unclear. Here, we evaluate GEOS-Chem chemical transport model and ten climate-chemistry models (CCMs) in Coupled Model Intercomparison Project Phase 6 (CMIP6) in capturing ozone-ENSO responses, quantify the roles of transport, chemistry, and biomass burning, and examine the future evolution of these responses. GEOS-Chem simulation over 2005–2020 well reproduces satellite-observed ozone-ENSO responses, including the instantaneous decrease (increase) in tropospheric column ozone (TCO) over tropical eastern (western) Pacific in El Niño, and the delayed responses in subtropics and mid-latitudes. The combined effects of transport, chemistry, and biomass burning emissions explain over 90 % of the simulated TCO variability in 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 (0.7) DU in the western (eastern) Pacific, which is offset by enhanced biomass burning emissions of 0.4 (0.1) DU. Only five of ten CMIP6 CCMs, with interactive tropospheric chemistry and accurate representation of anomalous circulation with ENSO, 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.