Composite Effects of ENSO and EASM on Summer Ozone Pollution in Two Regions of China

Abstract

AbstractCombining the 3‐D reanalysis with the surface observational O3 data, this study investigated the composite effects of the El Niño‐Southern Oscillation (ENSO) and East Asian summer monsoon (EASM) on summer O3 pollution in two regions of China, that is, the Beijing‐Tianjin‐Hebei (BTH) and the Yangtze River Delta (YRD). The results show that the El Niño has significantly enhanced the near surface O3 pollution in the BTH, but has little effect on surface O3 in the YRD. On the contrary, the La Niña has mainly reduced the surface O3 pollution in the YRD with little effect in the BTH. When the El Niño and a strong EASM occurred at the same time, the effect on O3 was more significant than that by the El Niño event alone due to the extra intensification of the vertical transport of O3 with about 10–30 μg/m3 from stratosphere and the worsening of the horizontal diffusion conditions (easterly wind anomalies) in the BTH. The La Niña with a weak EASM played a dominant role in the reduction of O3 pollution in the YRD by improving the meteorological conditions (especially in total cloud cover) and horizontal diffusion conditions (westerly wind anomalies). These composite effects were reflected in the trends of summer surface O3 in the BTH and the YRD, and became the crucial contributors to the abnormal changes of summer surface O3 in some years when the ENSO/EASM happened.