Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period during which atmospheric CO₂ levels were similar to recent historical values (~400 ppm). Several proxy reconstructions for the mid-Pliocene show highly reduced zonal sea surface temperature (SST) gradients in the tropical Pacific Ocean, indicating an El Niño-like mean state. However, past modelling studies do not show these highly reduced gradients. Efforts to understand mid-Pliocene climate dynamics have led to the Pliocene Model Intercomparison Project (PlioMIP). Results from the first phase (PlioMIP1) showed clear El Niño variability (albeit significantly reduced) and did not show the greatly reduced time-mean zonal SST gradient suggested by some of the proxies. In this work, we study El Niño-Southern Oscillation (ENSO) variability in the PlioMIP2 ensemble, which consists of additional global coupled climate models and updated boundary conditions compared to PlioMIP1. We quantify ENSO amplitude, period, spatial structure and flavour, as well as the tropical Pacific annual mean state in mid-Pliocene and pre-industrial simulations. Results show a reduced ENSO amplitude in the model-ensemble mean (−24 %) with respect to the pre-industrial, with 15 out of 17 individual models showing such a reduction. Furthermore, the spectral power of this variability considerably decreases in the 3–4 year band. The spatial structure of the dominant empirical orthogonal function shows no particular change in the patterns of tropical Pacific variability in the model-ensemble mean, compared to the pre-industrial. Although the time-mean zonal SST gradient in the equatorial Pacific decreases for 14 out of 17 models (0.2 °C reduction in the ensemble mean), there does not seem to be a correlation with the decrease in ENSO amplitude. The models showing the most ‘El Niño-like’ mean state changes show a similar ENSO amplitude as in the pre-industrial reference, while models showing more ‘La Niña-like’ mean state changes generally show a large reduction in ENSO variability. The PlioMIP2 results show a reasonable agreement both with time-mean proxies indicating a reduced zonal SST gradient, as well as reconstructions indicating a reduced, or similar, ENSO variability.

中文翻译：

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根据 PlioMIP2 集合，减少了上新世中期的厄尔尼诺现象

摘要。中上新世暖期 (3.264–3.025 Ma) 是大气 CO ₂水平与最近的历史值（~400 ppm）相似。上新世中期的几个代理重建显示热带太平洋的纬向海面温度 (SST) 梯度大大降低，表明存在类似厄尔尼诺现象的平均状态。然而，过去的建模研究并未显示这些高度降低的梯度。了解上新世中期气候动态的努力促成了上新世模型比对项目 (PlioMIP)。第一阶段 (PlioMIP1) 的结果显示出明显的厄尔尼诺变化（尽管显着降低），并且没有显示出一些代理建议的显着降低的时间平均纬向海温梯度。在这项工作中，我们研究了 PlioMIP2 集合中的厄尔尼诺-南方涛动 (ENSO) 变异性，与 PlioMIP1 相比，该集合包括额外的全球耦合气候模型和更新的边界条件。我们量化了 ENSO 振幅、周期、空间结构和风味，以及上新世中期和工业化前模拟中热带太平洋的年平均状态。结果显示，与工业化前相比，模型集合平均值 (-24 %) 的 ENSO 振幅降低，17 个单独模型中有 15 个显示出这种降低。此外，这种可变性的光谱功率在 3-4 年波段显着降低。与工业化前相比，主导经验正交函数的空间结构显示模式-集合均值的热带太平洋变率模式没有特别变化。尽管 17 个模型中的 14 个模型的赤道太平洋时间平均纬向 SST 梯度下降（集合平均值下降 0.2 °C），但似乎与 ENSO 振幅的下降没有相关性。显示最“类似厄尔尼诺”的平均状态变化的模型显示出与工业化前参考相似的 ENSO 幅度，而显示更多“类似拉尼娜”平均状态变化的模型通常显示 ENSO 变异性大幅降低。PlioMIP2 结果显示了合理的一致性，时间平均代理表明纬向 SST 梯度降低，重建表明 ENSO 变异性降低或类似。