This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged‐discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño‐La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric.

El Niño-Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical-subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions. Plain Language Summary El Niño-Southern Oscillation (ENSO) is one of the strongest climate variation phenomena in Earth's climate system, causing regional climate extremes and massive ecosystem impacts. An ENSO event can transition from one event to another in complex ways. An El Niño (La Niña) event can be preceded by a La Niña (El Niño) event to become a cyclic ENSO, by a neutral event to become an episodic ENSO, or by another El Niño (La Niña) event to become a multiyear ENSO. The complex nature of ENSO transition challenges our understanding of ENSO dynamics and its future responses to greenhouse warming. Here we show, using observational analyses, climate model simulations, and a novel framework focusing specifically on the onset processes of ENSO, that multiyear ENSO events related to the subtropical forcing are projected to decrease and cyclic ENSO events to increase as the climate warms. These changes in ENSO transition complexity are linked to the warming of the tropical Pacific mean state, which is a key factor controlling ENSO transitions through a series of tropical-subtropical interactions.

The Atlantic multidecadal variability (AMV) is a basinwide fluctuation on 50-80-year timescales in the surface ocean state of the North Atlantic, generally defined by a characteristic horseshoe shape in North Atlantic sea-surface temperature (SST) anomalies (Enfield et al., 2001;Goldenberg et al., 2001). This multidecadal variability has been studied intensively over the past few decades due to its observed control on decadal climates for regions around the Atlantic Ocean, such as Sahel (