Connections between different types of El Niño and Southern/Northern Oscillation

Abstract

It has long been acknowledged that there are two types of El Nino events, i.e., the eastern Pacific El Nino (EE) and the central Pacific El Nino (CE), according to the initial position of the anomalous warm water and its propagation direction. In this paper, the oceanic and atmospheric evolutions and the possible mechanisms of the two types of El Nino events were examined. It is found that all the El Nino events, CE or EE, could be attributed to the joint impacts of the eastward advection of warm water from the western Pacific warm pool (WPWP) and the local warming in the equatorial eastern Pacific. Before the occurrence of CE events, WPWP had long been in a state of being anomalous warm, so the strength of eastward advection of warm water was much stronger than that of EE, which played a major role in the formation of CE. While for the EE events, most contribution came from the local warming of the equatorial eastern Pacific. It is further identified that the immediate cause leading to the difference of the two types of El Nino events was the asynchronous variations of the Southern Oscillation (SO) and the Northern Oscillation (NO) as defined by Chen in 1984. When the transition from the positive phase of the NO (NO+) to NO− was prior to that from SO+ to SO−, there would be eastward propagation of westerly anomalies from the tropical western Pacific induced by NO and hence the growth of warm sea surface temperature anomalies in WPWP and its eastward propagation. This was followed by lagged SO-induced weakening of southeast trade winds and local warming in the equatorial eastern Pacific. These were conducive to the occurrence of the CE. On the contrary, the transition from SO+ to SO− leading the transition of NO would favor the occurrence of EE type events.