| In recent years,tropospheric ozone has been one of the most critical air pollutants in China.It is detrimental to human health and ecosystems.Ozone concentration is affected by precursor emissions and meteorological conditions.El Ni(?)o-Southern Oscillation(ENSO)is the most pronounced mode of internal variability of the Earth’s climate system.Quasi-biennial Oscillation(QBO)is the main mode of variability of zonal wind in the equatorial stratosphere.They have an important impact on atmospheric circulation and weather conditions around the world,which may further modulate the generation and temporal distribution of tropospheric ozone over China.In this work,the individual and synergistic modulation effects of ENSO and QBO on summertime tropospheric ozone over China has been quantitatively evaluated based on GEOS-Chem model simulations.The mechanisms of their influence are explained by meteorological factor analysis and process analysis.(1)The impacts of the two strong El Ni(?)o events(1997/98 and 2015/16)on summertime nearsurface ozone concentrations over China are examined.The results show that near-surface ozone concentrations increased by 3 ppb over southern China during the summer of the developing phase of the two El Ni(?)o events.It is linked to the weakened southerlies,which leads to the accumulation of ozone in southern China.Besides,the increase of ozone over northeast China in the summer of1997 is more than 6 ppb,which mainly due to the increase of chemical production caused by high temperature,sunny weather and low humidity.In contrast,in the summer of the decaying period,ozone concentrations decrease over many regions of China when the El Ni(?)o reverses into the cooling phase.The results show that strong El Ni(?)o plays an important role in modulating summer ozone concentrations over China.(2)We explore the impact of ENSO on summer tropospheric ozone in China from 1990 to 2019.The results show a positive correlation between near-surface ozone concentrations and the Ni(?)o3.4index in China,especially in southern China,with a statistically significant correlation coefficient of+0.55.Compared with the La Ni(?)a years,ozone concentrations increase exceeding 9 ppb(20% of the climatological mean)over southern China during the El Ni(?)a years.The modulation of ozone by ENSO in China extends to the middle and even upper troposphere.The convergence of ozone fluxes associated with weakening southerly winds is the primary cause of the increased ozone concentrations over southern China during El Ni(?)o years relative to La Ni(?)a years.About 74% of the change of nearsurface increase in near-surface ozone during the El Ni(?)o years is attributed to domestic emissions.This study highlights the importance of ENSO in regulating tropospheric ozone concentrations in China.(3)We further investigate the effect of QBO on the interannual variability of the summertime tropospheric ozone based on model simulations and satellite retrievals.The effect of single QBO on summertime ozone events in China is not significant.However,when the sea surface temperature over the eastern tropical Pacific is warmer than normal,QBO has a significant positive correlation with near-surface ozone concentrations over central China(+0.53),indicating that the modulation of QBO on the summer ozone concentration in China was influenced by the ENSO phase.Compared with the easterly phase of QBO,ozone concentrations over central China increase by 3 ppb during the westerly phase under the warm sea surface temperature anomaly.Meteorological elements and process analysis suggest that the increased ozone concentrations over central China are mainly attributed to the suppression of the vertical wind field during the westerly phase of the QBO,which results in anomalous downward transports of ozone.This work highlights a potentially important role of QBO on summertime ozone concentrations over China during warm phase of SST anomalies over the eastern tropical Pacific(ENSO warm phase). |
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