The El Niño-Southern Oscillation (ENSO) exhibits pronounced amplitude asymmetry, with El Niño events typically stronger than La Niña events. However, the response of this asymmetry to future greenhouse warming remains uncertain, with projections diverging substantially among studies—some indicating strengthening before 2100 CE and others weakening beyond 2100 CE. The mid-Pliocene (~3.3–3.0 Ma), with CO₂ levels of ~400 ppm and temperatures 2–3°C warmer than pre-industrial, provides a key paleo-analog. Here we use both proxy reconstruction and global climate model simulations to investigate the ENSO amplitude asymmetry during the mid-Pliocene. Our analysis of planktonic foraminiferal Mg/Ca and δ¹⁸O data from equatorial Pacific sediment cores shows lower ENSO skewness in the Pliocene than today. An ensemble of simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6), under mid-Pliocene (eoi400) and pre-industrial (piControl) conditions, consistently shows a significant weakening of ENSO skewness in the mid-Pliocene. This reduction is mainly manifested as a relative increase in La Niña amplitude, while the El Niño amplitude change is moderate. The weakened asymmetry arises from reduced nonlinearity of atmospheric convective response. In the warmer Pliocene mean state, background sea surface temperatures in the eastern Pacific continue to rise and exceed the convective threshold, allowing larger negative rainfall anomalies during La Niña events and thereby diminishing the convective feedback asymmetry between El Niño and La Niña. These results highlight ENSO asymmetry’s sensitivity to mean-state warming and help reduce uncertainty in future projections.
 

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