seasonal prediction of the Indian monsoon in the early 1930s (Walker, 1932), the modern concept of ENSO was established three decades later by the pioneering work of Bjerknes (1966, 1969) who visualized the linkage between the atmospheric Southern Oscillation and oceanic warming in the eastern Pacific (El Niño) via tight coupling among the sea surface temperature, surface winds, and precipitation. Bjerknes’s work laid a physical basis and a conceptual framework for ENSO prediction (Neelin et al., 1998; Burgers et al., 2005). The coupled instability mechanism described by Bjerknes (1969) works as follows. If a positive eastern equatorial SSTA exists, the temperature gradient between the eastern Pacific and the western Pacific is reduced, which then produces a weakening of the easterly trade winds, augmenting the warming in the eastern Pacific. The additional warming in the east further weakens the trade winds, constituting a coupled ocean–atmosphere positive feedback. A reversal of the argument explains the growth of a cold event. This positive feedback between the SSTA and the atmospheric wind anomaly leads to a continually growing anomaly, although it does not provide an explanation for what causes the transition from one extreme state to the other.
About the same time, independent but related theoretical findings coincidently emerged, i.e., the equatorial wave theory in an unbounded equatorial atmosphere (Matsuno, 1966) and in a finite ocean basin (Moore, 1968). These theoretical findings stimulated rapid development of the equatorial oceanography. In the 1970s and 1980s, Bjerknes’s conceptual model was transformed and expanded into theoretical and simple dynamical models, which advanced understanding of