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

The El Niño-Southern Oscillation (ENSO) significantly affects the interannual variability of tropospheric ozone, but the quantitative contributions from individual processes and how the ozone-ENSO response will change in the future remain unclear. In this study, we apply the GEOS-Chem global chemical transport models to quantify the contribution of transport, chemistry, and biomass burning to ozone variability in different ENSO phases, evaluate the ability of different climate-chemistry models (CCMs) in the Coupled Model Intercomparison Project Phase 6 (CMIP6) in capturing the present-day ozone-ENSO response, and examine the future changes in such response. GEOS-Chem model simulation over 2005-2020 largely reproduces the ozone-ENSO response observed by the Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) instrument, including the instantaneous decrease (increase) in tropospheric ozone column (TCO) over tropical eastern (western) Pacific in the El Niño phase, and the delayed responses (3-9 months lagged behind the Nino 3.4 index) in South America and Africa. The combined effects of transport, chemistry, and biomass burning emissions explain 94%~98% of the variability of TCO in tropical Pacific during ENSO. Changes in transport patterns dominate the overall tropospheric ozone-ENSO response, by increasing TCO by 0.8 DU (53% of the total variability) in western Pacific region and decreasing TCO by 2.2 DU (92%) in the eastern Pacific region during the El Niño condition relative to the normal periods. Changes in atmospheric temperature, water vapor, and cloud cover reduce ozone in the lower and middle troposphere (500-800 hPa) in the eastern Pacific by 2.0 ppbv, comparable to the transport induced ozone decrease of 3.8 ppbv. Biomass burning emissions cause an averaged ozone increase of 0.8 DU in Indonesia during El Niño and 0.7 DU in Brazil during La Niña. We find that five out of ten CCMs in CMIP6 can reproduce the historical ozone-ENSO response in 1980-2014. Interactive tropospheric chemistry and accurate representation of vertical circulation in ENSO phases are vital for the CCMs to capture the ozone-ENSO response. These models with successful skills consistently indicate that the ozone-ENSO response will increase by approximately 20% by the end of the 21st century, driven by the strengthening anomalous circulation and high water vapor concentration in ENSO phases in a warming climate. These results are critical for understanding climate-chemistry interactions and for improving future ozone projection.