Variations in the amplitude of El Niño–Southern Oscillation in the past 250 million years

The El Niño–Southern Oscillation (ENSO), originating in the central and eastern equatorial Pacific, is a defining mode of interannual climate variability with profound impact on global climate and ecosystems. Although ongoing coordinated community efforts have offered insights into how ENSO will change in the future under anthropogenic warming, the geological history of ENSO remains intricate. In particular, there is a clear lack of systematic study on how ENSO has evolved in response to vast variations in land-sea distributions and climate mean states over geological timescales. To unravel this, we analyze a series of time-slice coupled climate simulations forced by changes of paleogeography, atmospheric CO₂ concentrations, and solar radiation in the past 250 million years (Myr). Our simulations for the first time demonstrate that ENSO is the leading mode of tropical Pacific sea surface temperature (SST) in the past 250 Myr. Further, the amplitude of ENSO is predominantly captured by the zonal advective feedback and thermocline feedback, both of which are primarily regulated by eastern equatorial Pacific climatological SST. These findings highlight the significance of climate mean states in interpretation of the amplitude of ENSO during the deep past, and provide enlightening implications for constraining future climate change.