Rapid communication: Nonlinear sensitivity of the 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. Palaeoclimate archives and climate models suggest that ENSO activity depends on mean state conditions. However, a common mechanism that can predict ENSO variability under a range of background conditions remains elusive. Here we combine climate model simulations of past climates and future warming, performed under the Paleoclimate Modelling Intercomparison Project (PMIP) and the Coupled Model Intercomparison Project (CMIP), 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 centres, namely the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). Simulations with atmospheric CO2 lower than today display a poleward shift in the convergence zones and weakened ENSO. Moderate equatorward shifts in the convergence zones occur under moderate 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. Our results provide a comprehensive mechanism of how tropical Pacific mean state modulates ENSO activity.