The El Niño/Southern Oscillation (ENSO), the dominant driver of year-to-year climate variability in the equatorial Pacific Ocean, impacts climate pattern across the globe. However, the response of the ENSO system to past and potential future temperature increases is not fully understood. Here we investigate ENSO variability in the warmer climate of the mid-Pliocene (~3.0–3.3 Ma), when surface temperatures were ~2–3 °C above modern values, in a large ensemble of climate models—the Pliocene Model Intercomparison Project. We show that the ensemble consistently suggests a weakening of ENSO variability, with a mean reduction of 25% (±16%). We further show that shifts in the equatorial Pacific mean state cannot fully explain these changes. Instead, ENSO was suppressed by a series of off-equatorial processes triggered by a northward displacement of the Pacific intertropical convergence zone: weakened convective feedback and intensified Southern Hemisphere circulation, which inhibit various processes that initiate ENSO. The connection between the climatological intertropical convergence zone position and ENSO we find in the past is expected to operate in our warming world with important ramifications for ENSO variability.

厄尔尼诺/南方涛动 (ENSO) 是赤道太平洋逐年气候变率的主要驱动因素，影响着全球的气候模式。然而，ENSO 系统对过去和未来潜在的温度升高的响应尚不完全清楚。在这里，我们在一个大型气候模型集合（上新世模型比对项目）中研究了上新世中期（~3.0-3.3 Ma）温暖气候中 ENSO 的变化，当时地表温度比现代值高约 2-3°C。我们表明，该集合始终表明 ENSO 变异性减弱，平均减少 25% (±16%)。我们进一步表明，赤道太平洋平均状态的变化不能完全解释这些变化。反而，ENSO受到太平洋热带辐合带向北位移引发的一系列非赤道过程的抑制：对流反馈减弱和南半球环流加强，从而抑制了引发ENSO的各种过程。我们过去发现的热带气候辐合带位置与 ENSO 之间的联系预计将在我们变暖的世界中发挥作用，并对 ENSO 变率产生重要影响。