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Simulation of tropical tropospheric ozone variation from 1982 to 2010: The meteorological impact of two types of ENSO event

The effects of two types of ENSO events on tropical ozone (O3) variations from 1982 to 2010, and the mechanisms underlying these effects, were analyzed using observations and model simulations. Tropospheric column O3 anomalies (TCOA) during canonical El Niño were different from El Niño Modoki. Absolute TCOA values are larger during canonical El Niño than during El Niño Modoki in most regions. La Niña events were not separated into the different types because of their similarity in terms of sea surface temperature patterns. TCOA in La Niña showed a reversed dipole from canonical El Niño. During canonical El Niño, anomalous downward motion together with suppressed convection weakened O3 outflow from the troposphere, causing an increase in tropospheric O3 over western Pacific. Over central and eastern Pacific, decreased O3 concentrations resulted primarily from a change in net chemical production of O3. The change in net O3 chemical production relates to increased levels of HOx under wetter condition. During El Niño Modoki, transport and chemical fluxes were similar but weaker than during canonical El Niño. During La Niña, O3 anomalies and transport fluxes were the opposite of those during the El Niño Modoki. Stratospheric O3 played a key role in the development of O3 anomaly above 250 hPa during ENSO events, contributing >30% to the O3 anomalies. The change in free tropospheric O3 affected the O3 anomaly from 850 hPa to 200 hPa (60% of O3 anomaly). The contribution of O3 from planetary boundary layer was concentrated at the surface, with a contribution of <15%.