Lasting time of El Nino and circulation anomaly

Composite circulation is studied for long-lasting and short-lasting El Nino events. The result shows that the low-level wind and high-level velocity potential anomalies are quite different in the occurring, developing and vanishing of the two El Nino events. The result also reveals the key anomalous circulation for the occurring and vanishing of El Nino. Corresponding to the long-lasting El Nino event, cyclonic anomalous circulation over the eastern North Pacific weakens slowly and anticyclonic anomalous circulation over the western North Pacific strengthens slowly at 850 hPa. Therefore the tropical Pacific westerly anomaly keeps long. The anomalous positive velocity potential over the western South Pacific at 200 hPa, which is related to the strong Australian winter monsoon, helps a long duration of El Nino. Corresponding to the short-lasting El Nino, the anomalous positive velocity potential over the western Pacific at 200 hPa moves eastward quickly and results in the vanishing of El Nino event.