in a narrow sense to indicate any SST anomalies in the equatorial Pacific, and their occurrence mechanisms are mainly concerned in the present study. To develop a better understanding of the ENSO, together with giving a review of existing classical theories, we trace back to the original problem and start the investigation from two fundamental physical facts. Other than the conventional empirical orthogonal function (EOF) method given by Lorenz (1956) for space-time data analyses, our new approach, named the extreme-point symmetric mode decomposition (ESMD) method (Wang and Li 2013, 2015), is adopted here.

According to the locations of the SST anomalies, El Niño and La Niña are classified as the eastern Pacific (EP) type (with significant anomalies in the eastern equatorial Pacific) and the central Pacific (CP) type (with significant anomalies in the central equatorial Pacific) (Kao and Yu 2009; Yeh et al. 2009; Capotondi et al. 2015). Historically, the EP type was recognized first, and was seen as the canonical type. Relative to the EP type, considerably less research has been devoted to the CP type. With regards to El Niño, the CP type has been referred to as the “date-line El Niño” (Larkin and Harrison 2005), “El Niño 