Intensity-dependent changes in ENSO frequency and lifecycle characteristics under future climate change

Abstract The El Niño–Southern Oscillation (ENSO) is the dominant mode of interannual climate variability, yet its lifecycle evolution across intensity classes remains uncertain under future warming. Using a multi-model ensemble of 13 CMIP6 models, we investigate changes in the frequency and lifecycle characteristics of El Niño and La Niña events across intensity classes by comparing the future period (2030–2099) under SSP1−2.6, SSP2−4.5, and SSP5−8.5 with the historical period (1930–1999). El Niño and La Niña events are identified using the Oceanic Niño Index (ONI) and further analyzed using both a Niño 3 composite framework based on ensemble-mean longitude–time SST anomalies and an individual-event ONI-based framework applied to model time series. While the total frequency of El Niño and La Niña events remains broadly stable across future scenarios, a robust shift toward higher intensity events is projected, particularly under SSP5−8.5, where very strong El Niño events more than double, very strong La Niña events nearly double, and weak events are substantially reduced. Event duration and lifecycle characteristics exhibit strong intensity dependence. Under SSP5−8.5 in the Niño 3 composite framework, very strong El Niño events become substantially longer-lived due to an extended decay phase, whereas moderate La Niña events show a pronounced reduction in duration linked to shorter growth and decay phases. Mixed-layer heat budget analyses show that these changes are linked to modifications in the thermocline, zonal advective, and Ekman terms. Reduced cooling efficiency during the decay phase of very strong El Niño events contributes to their prolonged persistence under future warming, whereas very strong La Niña events exhibit stable heat budget characteristics, consistent with their small duration changes. Overall, ENSO is projected to shift toward higher-intensity events and increased persistence of extreme El Niño under future warming, with important implications for global climate extremes and predictability.