Modoki” (Ashok et al. 2007) and “warm-pool El Niño” (Kug et al. 2009).

Many classical theories have been devised to explain the ENSO-cycle mechanism, such as positive Bjerknes feedback (Bjerknes 1969; Wyrtki 1975), delay oscillator theory (Suarez and Schopf 1988; Battisti and Hirst 1989) and recharge oscillator theory (Jin 1997; Jin and An 1999). As indicated by Xu et al. (2017), the EP type of El Niño is the conventional one, and its temporal evolution can be explained mostly by these classical theories, whereas there is still some debate about the mechanism responsible for the initiation of the CP El Niño. Among these mechanisms, there are three representative ones: (1) Ashok et al. (2009) argued that the same thermocline-upwelling feedback mechanism operates for the both types of El Niño, except that the region of the upwelling shifts from the eastern equatorial Pacific for the EP type to the central equatorial Pacific for the CP type. (2) Kug et al. (2009) argued it is the zonal ocean advection at the equator that contributes to the development of the CP type. (3) Yu et al. (2010) suggested it is the forcing from the subtropical and extra-tropical atmosphere that excites the CP El 