Collapsed upwelling projected to weaken ENSO under sustained warming beyond the twenty-first century

The El Niño–Southern Oscillation (ENSO) in a warming climate has been studied extensively, but the response beyond 2100 has received little attention. Here, using long-term model simulations, we find that while ENSO variability exhibits diverse changes in the short term, there is a robust reduction in ENSO variability by 2300. Continued warming beyond 2100 pushes sea surface temperature above the convective threshold over the eastern Pacific, causing collapsed mean equatorial upwelling with intensified deep convection. We show that the weakened thermocline feedback due to the collapsed upwelling and increased thermal expansion coefficient, along with enhanced thermodynamic damping, are crucial to reducing ENSO amplitude under sustained warming. Our results suggest a threshold behaviour in the tropical Pacific, where a convective atmosphere over the eastern equatorial Pacific causes dramatic shifts in ENSO variability. This threshold is not crossed under low-emission scenarios.