Understanding Re-intensified Multi-year El Niño Events

Abstract:

It is understood that the latest 2015-2016 extreme El Niño had distinct spatial characteristics as compared to the previous extreme El Niño events that occurred in 1997-1998 and 1982-1983. However, the latest 2015-2016 extreme El Niño event was also unique because of its temporal evolution as it was preceded by a weak El Niño and abruptly re-intensified afterward. To understand this unique temporal evolution of the re-intensified multi-year El Niño event during 2014-2016, we examine similar events in a 2,200-yr simulation of Community Earth System Model, version 1. By contrasting multi-year El Niño events with single-year ones, we find that the succession characteristics of multi-year El Niño events are primarily caused by a negative North Pacific Oscillation (NPO) in the 1^st-year boreal winter via subtropical Pacific coupling processes. Meanwhile, by contrasting re-intensified multi-year El Niño events with lingering ones, we find that the re-intensification characteristics are caused by a negative Indian Ocean Dipole (IOD) in the 1^st-year boreal fall via tropical inter-basin coupling processes. This study explains how these two factors invoke tropical‒extratropical interactions in the Pacific and inter-basin interactions between the Indian and Pacific Oceans to give rise to the re-intensified multi-year El Niño events. We also discuss how our findings can improve predictions of the evolution characteristics of El Niño events, particularly the re-intensified multi-year and transitional single-year events.