The cyclical transition between El Nino and La Nina phases, interspersed with neutral ENSO states, is a fundamental, defining characteristic of the El Nino-Southern Oscillation phenomenon.
The claim describes the standard oscillatory definition and behavior of the El Nino-Southern Oscillation (ENSO) cycle, wherein the system moves between warm (El Nino), cool (La Nina), and neutral phases. As this is a well-established, definitional aspect of climate science confirmed by everyday meteorological observation and foundational climatology, no citation is required.