Potential factors modulating ENSO's influences on the East Asian trough in boreal winter

The eastern Pacific (EP) El Nino-Southern Oscillation (ENSO) is an important impact factor for the East Asian trough (EAT) variability during boreal winter. However, not all ENSO events can result in anomalous EAT events, which indicates that there could be some other factors modulating the relationship between ENSO and the EAT. The analysis of atmospheric circulations indicates that the intensity of the Asian Polar vortex (PV) plays an important role in the El Nino-EAT connection. The strengthened Asian PV favours the wave-train triggered by El Nino events propagates eastward from the North Pacific to East Asia and finally weakens the EAT. In contrast, during El Nino years with the weakened Asian PV, El Nino's influences are confined over western Siberia and the wave activity flux over Eurasia is weak, which consequently leads to a weak response of the EAT. For the La Nina-EAT connection, the North Pacific Oscillation (NPO) is a potential modulating factor. The La Nina and negative-phase NPO have a strengthened effect on the EAT, resulting in a strong response of the EAT to La Nina events. In contrast, the effects of La Nina and positive-phase NPO on the EAT are offset, leading to a weak response of the EAT to La Nina events. The above results are further confirmed by a numerical simulation of the pre-industrial Control experiment. Therefore, the simultaneous occurrence of the Asian PV and NPO should be a great concern when understanding and predicting the EAT based on ENSO information.