Different evolution features for two types of El Ni?o and possible causes for these differences

摘要

>Observed sea surface temperature (SST) anomalies in the central eastern equatorial Pacific exhibit two kinds of phase evolution, that is, transition and no‐transition, for both the eastern Pacific (EP) and central Pacific (CP) El Ni?o events. The transition type of El Ni?o is characterized by a strong decay after its peak and followed by a rapid transition to a La Ni?a event in the subsequent winter, while the no‐transition type of both EP and CP El Ni?o is featured by a weaker decay after its mature phase and fails to develop a La Ni?a event in the decaying year. For the EP El Ni?o, the intensity of the anomalous easterly over the western equatorial Pacific in the transition type is stronger than that in the no‐transition type, which is likely determined by the coupling of the Indian Ocean dipole (IOD) during the developing phase and the Indian Ocean basin‐wide mode (IOBM) during the decaying phase. For the CP El Ni?o, larger differences of easterly wind anomalies between the transition type and no‐transition type are found over the central eastern equatorial Pacific during the decaying year, which is also likely related to the IOD and IOBM coupling process. In addition, the rapid decay of warm subsurface (80–160?m) temperature anomalies in the central eastern equatorial Pacific during the decaying phases is crucial for the phase transition for the two types of El Ni?o, together with the eastwards propagation of cold subsurface (100–200?m) temperature anomalies in the western equatorial Pacific. What is more, analyses of the mixed‐layer heat budget show that the phase evolution of the EP El Ni?o depends on dynamic forcing (zonal advection) due to the difference of anomalous mixed‐layer ocean currents, while the CP El Ni?o’s different phase evolution is mainly caused by thermodynamic process, that is, net surface heat flux anomalies.