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

Combining the 3-D reanalysis with the surface observational O-3 data, this study investigated the composite effects of the El Nino-Southern Oscillation (ENSO) and East Asian summer monsoon (EASM) on summer O-3 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 Nino has significantly enhanced the near surface O-3 pollution in the BTH, but has little effect on surface O-3 in the YRD. On the contrary, the La Nina has mainly reduced the surface O-3 pollution in the YRD with little effect in the BTH. When the El Nino and a strong EASM occurred at the same time, the effect on O-3 was more significant than that by the El Nino event alone due to the extra intensification of the vertical transport of O-3 with about 10-30 mu g/m(3) from stratosphere and the worsening of the horizontal diffusion conditions (easterly wind anomalies) in the BTH. The La Nina with a weak EASM played a dominant role in the reduction of O-3 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 O-3 in the BTH and the YRD, and became the crucial contributors to the abnormal changes of summer surface O-3 in some years when the ENSO/EASM happened.