Transition from multi-year La Niña to strong El Niño rare but increased under global warming.

El Niño-Southern Oscillation (ENSO) exhibits a strong asymmetry between warm El Niño and cold La Niña in amplitude and temporal evolution. An El Niño often leads to a heat discharge in the equatorial Pacific conducive to its rapid termination and transition to a La Niña, whereas a La Niña persists and recharges the equatorial Pacific for consecutive years preconditioning development of a subsequent El Niño, as occurred in 2020-2023. Whether the multiyear-long heat recharge increases the likelihood of a transition to a strong El Niño remains unknown. Here, we show that such a transition is rare but more likely under transient greenhouse warming. In boreal spring and early summer after a multiyear La Niña, despite a substantial recharge in the western Pacific, thermocline remains anomalously shallow and sea surface temperature (SST) remains anomalously cold in the equatorial central Pacific. The cold conditions inhibit an ensuing eastward movement of atmosphere deep convection out of the warm western Pacific, delaying onset of ocean-atmosphere coupling, and hence growth of an El Niño. Under a high emission scenario, such a transition is still rare but more than twice as likely. The projected change is consistent with a projected weakening in climatological zonal SST gradient that promotes the eastward movement of atmosphere convection and a projected intensification in upper-ocean stratification of the equatorial Pacific that enhances the ocean-atmosphere coupling. Our result provides predictive insight of El Niño after multiyear La Niña, and advances our understanding of ENSO transition under greenhouse warming.