The second phase of the Pliocene Model Intercomparison Project (PlioMIP2) has attracted many climate modelling groups in its continuing efforts to better understand the climate of the mid-Piacenzian warm period (mPWP) when atmospheric CO₂ was last closest to present-day levels. Like the first phase, PlioMIP1, it is an internationally coordinated initiative that allows for a systematic comparison of various models in a similar manner to the Paleoclimate Modelling Intercomparison Project (PMIP). Model intercomparison and model–data comparison now focus specifically on the interglacial at marine isotope stage KM5c (3.205 Ma), and experimental design is not only based on new boundary conditions but includes various sensitivity experiments. In this study, we present results from long-term model integrations using the MIROC4m (Model for Interdisciplinary Research on Climate) atmosphere–ocean coupled general circulation model, developed at the institutes CCSR, NIES and FRCGC in Japan. The core experiment, with CO₂ levels set to 400 ppm, shows a warming of 3.1 ^∘C compared to the pre-industrial period, with two-thirds of the warming being attributed to the increase in CO₂. Although this level of warming is less than that in the equivalent PlioMIP1 experiment, there is slightly better agreement with proxy sea surface temperature (SST) data at PRISM3 (PRISM – Pliocene Research Interpretation and Synoptic Mapping) locations, especially in the northern North Atlantic where there were large model–data discrepancies in PlioMIP1. Similar spatial changes in precipitation and sea ice are seen and the Arctic remains ice-free in the summer in the core experiments of both phases. Comparisons with both the proxy SST data and proxy surface air temperature data from paleobotanical sites indicate a weaker polar amplification in model results. Unlike PlioMIP1, the Atlantic Meridional Overturning Circulation (AMOC) is now stronger than that of the pre-industrial period, even though increasing CO₂ tends to weaken it. This stronger AMOC is a consequence of a closed Bering Strait in the PlioMIP2 paleogeography. Also, when present-day boundary conditions are replaced by those of the Pliocene, the dependency of the AMOC strength on CO₂ is significantly weakened. Sensitivity tests show that lower values of CO₂ give a global SST which is overall more consistent with the PRISM3 SST field presented in PlioMIP1, while SSTs at many of the PRISM4 sites are still too high to be reconciled with any of the model results. On the other hand, tropical Pacific SST in the core experiment agrees well with more recent proxy data, which suggested that PRISM3 SST there was overestimated. Future availability of climate reconstructions from proxy data will continue to help evaluate model results. The inclusion of dynamical vegetation and the effects of all possible extreme orbital configurations outside KM5c should be considered in future experiments using MIROC4m for the mPWP.

中文翻译：

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使用气候跨学科研究模型（MIROC4m）进行的上新世模型比对项目（PlioMIP2）模拟

上新世模式比对项目（PlioMIP2）的第二阶段吸引了许多气候建模团体，他们在不断努力以更好地理解大气中CO ₂浓度为Piacenzian中期暖期（mPWP）的气候。最后接近当前水平。像第一阶段PlioMIP1一样，它是一项国际协调的计划，可以以类似于古气候建模比对项目（PMIP）的方式对各种模型进行系统比较。模型的比较和模型的数据比较现在专门针对海洋同位素阶段KM5c（3.205？Ma）的冰间作用，实验设计不仅基于新的边界条件，还包括各种敏感性实验。在这项研究中，我们介绍了使用日本的CCSR，NIES和FRCGC研究所开发的MIROC4m（气候跨学科研究模型）大气-海洋耦合全环流模型的长期模型集成结果。核心实验，CO ₂设置为400âppm的水平，表明温度升高了3.1℃。^{一种？？？？}与工业化前的时期相比，碳增加，三分之二的变暖归因于CO ₂的增加。尽管这一变暖水平小于等效的PlioMIP1实验中的变暖水平，但与PRISM3（PRISM –上新世研究解释和天气映射）位置的代理海面温度（SST）数据有更好的一致性，特别是在在北大西洋北部，PlioMIP1中存在较大的模型数据差异。在两个阶段的核心实验中，可以看到降水和海冰的类似空间变化，并且在夏季，北极仍保持无冰状态。与来自古植物学站点的代理SST数据和代理地面空气温度数据的比较表明，模型结果中的极性放大作用较弱。与PlioMIP1不同的是，尽管二氧化碳排放量增加，但大西洋子午向翻转循环（AMOC）现在比工业化前时期更强₂趋于削弱它。较强的AMOC是PlioMIP2古地理中白令海峡封闭的结果。另外，当用上新世的边界条件代替当今的边界条件时，AMOC强度对CO ₂的依赖性显着减弱。敏感性测试表明，较低的CO ₂值给出一个总体上与PlioMIP1中提出的PRISM3 SST字段更一致的全局SST，而许多PRISM4站点的SST仍然太高而无法与任何模型结果相吻合。另一方面，核心实验中的热带太平洋海表温度与最近的代理数据非常吻合，这表明PRISM3海表温度被高估了。从替代数据获得的气候重建的未来可用性将继续帮助评估模型结果。在未来的实验中，应考虑将MIROC4m用于mPWP，包括动力学植被的包含以及KM5c以外所有可能的极端轨道配置的影响。