Paleoclimate constrains future El Niño/Southern Oscillation increase

Abstract IPCC projections indicate that El Niño/Southern Oscillation (ENSO) variability will increase throughout the 21st century under global warming. However, the magnitude of the increase greatly varies among models. Here we use paleoclimate model simulations of the 4th phase of the Paleoclimate Model Intercomparison Project (PMIP4) to constrain the projected ENSO increase and propose a common mechanism to explain ENSO changes throughout climate states. We show that the projected ENSO increase is unprecedented in warm climates of the past ~3.3 million years and that ENSO operates in a bi-stable regime dictated by the position of the tropical Pacific Ocean convection centers, namely the Intertropical and South Pacific Convergence Zones. Constraining ENSO variability with the positions of the tropical convection centers indicates a 12% (3 to 19%) ENSO increase in the 21st century. Our results elucidate how tropical Pacific mean state has modulated ENSO activity and alerts for the unprecedented nature of the 21st century global warming.