ENSO (El Niño-Southern Oscillation) is characterized by interannual sea surface temperature (SST) variations in the eastern-to-central equatorial Pacific. In the composite ENSO event portrayed by Rasmusson and Carpenter (1982) SST anomalies develop along the coast of South America before propagating westward along the equator. However, it has become clear that there are events in which anomalies develop and remain near the International Dateline in the central equatorial Pacific. In fact, most of the El Niño events in the 21st Century (the 2002/03, 2004/05, and 2009/10 events) have had their largest SST anomaly in the western Pacific (Yu and Kim 2013). An example of an ENSO event in the east (1997) and an ENSO event in the west (2009) are shown in Figure 1. The fact that warming is observed sometimes in the east Pacific (EP), sometimes in the central Pacific (CP), and sometime simultaneously in both eastern and central Pacific (e.g., the 2006-07 event; Figure 1) has led to the suggestion that there are two types of events that represent physically distinct phenomena (Larkin and Harrison 2005; Yu and Kao 2007; Ashok et al. 2007; Guan and Nigam 2008; Kao and Yu 2009; Kug et al. 2009). There are also studies that further separate the two types of ENSO into more sub-types (Wang and Wang 2013). An alternative interpretation is that ENSO normally occurs in the central Pacific, with events sometimes displaced to the east and sometimes displaced to the west
