Historical Change Of El Nino Properties Sheds Light On Future Changes Of Extreme El Nino

El Nino's intensity change under anthropogenic warming is of great importance to society, yet current climate models' projections remain largely uncertain. The current classification of El Nino does not distinguish the strong from the moderate El Nino events, making it difficult to project future change of El Nino's intensity. Here we classify 33 El Nino events from 1901 to 2017 by cluster analysis of the onset and amplification processes, and the resultant 4 types of El Nino distinguish the strong from the moderate events and the onset from successive events. The 3 categories of El Nino onset exhibit distinct development mechanisms. We find El Nino onset regime has changed from eastern Pacific origin to western Pacific origin with more frequent occurrence of extreme events since the 1970s. This regime change is hypothesized to arise from a background warming in the western Pacific and the associated increased zonal and vertical sea-surface temperature (SST) gradients in the equatorial central Pacific, which reveals a controlling factor that could lead to increased extreme El Nino events in the future. The Coupled Model Intercomparison Project phase 5 (CMIP5) models' projections demonstrate that both the frequency and intensity of the strong El Nino events will increase significantly if the projected central Pacific zonal SST gradients become enhanced. If the currently observed background changes continue under future anthropogenic forcing, more frequent strong El Nino events are anticipated. The models' uncertainty in the projected equatorial zonal SST gradients, however, remains a major roadblock for faithful prediction of El Nino's future changes.