the other weak one in 2005/06 is a mixed type. Before 1993 there were also many strong and weak La Niñas. By and large, most of the strong ones, such as those in 1970/71, 1973/74, 1975/1976 and 1988/89, belong to the CP type. Almost all the weak ones belong to the EP or mixed types.

The eastward current anomaly referred to as the CECC has been proposed as being either a theoretical mechanism for El Niño generation (advection-reflection oscillator, Picault et al. 1996), as positive zonal-advection feedback (Jin et al. 2006 and subsequent citations), or a low-frequency component of ocean–atmosphere interaction that leads to state-dependent noise forcing (Puy et al. 2015, Levine et al. 2016). In order to verify these the evolution process of CECC is also illustrated in . A comparison between and shows a high consistency between the occurrences of CECC and El Niño. As the prosperous periods concerned, the CECC leads the SST anomaly by 2–3 months which accords well with the result given by Lagerloef et al. (2003). This consistency has verified that the existence of CECC is favourable for the occurrence of El Niño. In fact, it is a natural thing. The reason is that 