Evolution of Subtropical Pacific-Onset El Niño: How Its Onset Location Controls Its Decay Evolution

In the observations, El Niño events initiated by a subtropical Pacific mechanism (SP-onset El Niños) show larger uncertainty in their decay evolution patterns than those initiated by a tropical Pacific mechanism. A 2,200-year simulation of Community Earth System Model reproduces this observed feature and its SP-onset El Niños are analyzed to understand the cause of the large uncertainty. Results show that the onset location of SP-onset El Niño, which interacts with the eastern edge of the western Pacific warm pool, is a key factor controlling its decay evolution. When the onset is located east (west) of 155°E, the event has a strong tendency to reverse (maintain) its phase, leading to cyclic (multiyear) evolution. These two onset locations respectively activate Indo-Pacific and tropical-subtropical Pacific interactions to give rise to the different evolution patterns. The findings offer a potential way to predict the evolution of SP-onset El Niños using their onset locations.