Rapid communication: Nonlinear sensitivity of El Niño-Southern Oscillation across climate states

Abstract. The El Niño-Southern Oscillation (ENSO) is the dominant mode of tropical climate variability. Understanding its sensitivity to climate states is of societal and ecosystem importance given the unabated global warming. Paleoclimate archives and climate models suggest that ENSO activity depends on mean state conditions. However, due to climate model biases, short observational record and proxy-data uncertainties, evaluating ENSO sensitivity remains challenging. Here we combine state-of-the-art model simulations of past climates and future warming to evaluate ENSO activity throughout a wide range of climate states. We find that the sensitivity of ENSO to the background climate is nonlinear and tied to the climatological position of the tropical Pacific convection centers, namely the Intertropical and South Pacific Convergence Zones. Simulations with atmospheric CO₂ lower than today display a poleward shift of the convection centers and weakened ENSO. Moderate equatorward shifts of the convection centers occur under CO₂-induced warming increasing ENSO activity, while strong equatorward shifts reduce ENSO variability in extreme CO₂ warming scenarios, resulting in a permanent El Niño-like mean state. Furthermore, we find that Eastern Pacific El Niños are more sensitive to the background state than Central El Niño events. Our results provide a comprehensive mechanism of how tropical Pacific mean state modulates ENSO activity.