Role of intraseasonal oscillation in asymmetric impacts of El Nino and La Nina on the rainfall over southern China in boreal winter

The impacts of El Nino and La Nina on the rainfall over southern China in winter half year (November-April) are investigated diagnostically. It is found that positive rainfall anomalies with statistical significance appear over southern China in El Nino winter but no reversed significant rainfall anomalies in La Nina winter. The asymmetry of the rainfall anomalies are related with the asymmetry in anomalous circulation over western North Pacific (WNP) in lower troposphere, with a strong anomalous anticyclone in El Nino winter but a weak anomalous cyclone in La Nina winter. It is revealed that the asymmetric responses in anomalous circulation to El Nino and La Nina are affected by the intraseasonal oscillation (ISO). In El Nino winter the interannual variation is dominant and the ISO is weak, and the strong interannual variation leads to a strong response of the anomalous anticyclone over WNP in winter half year. However, in La Nina winter the ISO is dominant and the interannual variation is weak, and the weak interannual variation results in a weak response of the anomalous cyclone over WNP.