Role of the eastern subtropical North Pacific Ocean on the El Niño’s transition processes

We examine the transition processes of El Niño occurring in 1 year after using observations, a multi-century model simulation and atmospheric general circulation model (AGCM) experiments. One type is characterized by a quick transition from an El Niño event into a La Niña event in the following winter, and the other type involves a slow decay to an almost neutral state or a continuous El Niño event. While both the Indian and the Atlantic Oceans contribute to the El Niño’s transition processes, we further find that sea surface temperature (SST) conditions in the eastern subtropical North Pacific and the associated surface wind anomalies play important roles in determining the different types of El Niño’s transition processes through atmosphere–ocean coupled processes. In particular, northeasterly wind anomalies in the central subtropical North Pacific during the early spring, which is in consequence of atmosphere–ocean coupled processes, contribute to decreasing ocean heat content (OHC) anomalies and strengthening easterly wind anomalies in the central tropical Pacific. These anomalies lead up to a transition into a La Niña event in the following winter. On the contrary, weakening of the northeasterly wind and warm SST anomalies in the eastern subtropical North Pacific during the early spring are conducive to a neutral state or a continuous El Niño event in the subsequent winter. Similar transition processes are also found in a multi-century model simulation. By conducting idealized AGCM experiments, we also show that the anomalous SST in the eastern subtropical North Pacific during El Niño peak season may induce surface wind anomalies in the central tropical Pacific during El Niño onset season. These results provide potential precursors for predicting the occurrence of a La Niña event, a neutral state and an El Niño event 1 year after the occurrence of an El Niño event.