El Niño events usually mature during boreal winter, but the month in which sustained warming begins varies substantially from event to event. This onset-time diversity provides a temporal perspective on El Niño–Southern Oscillation (ENSO) diversity that is related to, but not reducible to, the conventional eastern- and central-Pacific classifications. Here we examine how the Pacific decadal background state is associated with the development of spring-onset (SP) and summer-onset (SU) El Niño events. Events are identified from monthly sea surface temperature anomalies using established criteria, and the long observational catalogue is used as descriptive context for the physical analysis.The tropical Pacific background state exhibits notable differences across decadal epochs, characterized by variations in equatorial wind patterns, thermocline configuration, convection distribution, and equatorial Pacific thermal structure. These background-state differences show seasonal evolution, with the contrast most pronounced during certain periods. Onset-aligned composites reveal that SP and SU events develop through distinct dynamical pathways, differing in the strength and spatial distribution of subsurface and surface signals. Changes in the background state alter the relative accessibility of these growth pathways, with implications for the efficiency of various feedback mechanisms during event development. Process-based diagnostics are employed to directly test the hypothesized reorganization of ENSO growth pathways, supporting a framework in which decadal background changes reorganize seasonally dependent development mechanisms rather than uniformly affecting all El Niño events.
 

El Niño , Pacific Decadal Oscillation, onset time,mean state