Three new equilibrium mid‐Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4) and Community Earth System Model versions 1.2 (CESM1.2) and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). These simulations reveal amplified MP climate change relative to the preindustrial going from CCSM4 to CESM2, seen in global and polar averages of surface warming, sea ice reduction in both the Arctic and the Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from enhanced Earth system sensitivity (ESS) to not only CO₂ change but also changes in boundary conditions primarily from vegetation and ice sheets. ESS is amplified by up to 70% in CCSM4 and up to 100% in CESM1.2 and CESM2 relative to the equilibrium climate sensitivity of respective models. Simulations disagree on several climate metrics. Different from CCSM4, both CESM1.2 and CESM2 show reduction of cloud cover, and weakened Walker circulation accompanied by an El Niño‐like mean state of the tropical Pacific in MP simulations relative to the preindustrial. This El Niño‐like mean state is consistent with paleo‐observational sea surface temperatures, suggesting an improvement upon CCSM4. The performances of MP simulations are assessed with a new compilation of observational MP sea surface temperature. The model‐data comparison suggests that CCSM4 is not sensitivity enough to the MP forcings, but CESM2 is likely too sensitive, especially in the tropics.

中文翻译：

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在中新世模拟中，气候响应和地球系统敏感性从CCSM4增加到CESM2

使用社区气候系统模型版本4（CCSM4）和社区地球系统模型版本1.2（CESM1.2）和2（CESM2）实施了三个新的平衡中新世（MP）模拟。遵循上新世模型比较项目第二阶段（PlioMIP2）的协议，在相同的边界和强迫条件下进行所有模拟。这些模拟揭示了从CCSM4到CESM2相对于工业化前的MP气候变化，从全球和极地平均的表面变暖，北极和南极海冰减少以及哈德利环流减弱的情况中可以看出。增强的全球平均变暖源于增强的地球系统不仅对CO _2的敏感性（ESS）变化，但边界条件也发生变化，主要来自植被和冰盖。相对于各个模型的平衡气候敏感性，在CCSM4中ESS放大了70％，在CESM1.2和CESM2中放大了100％。模拟在几个气候指标上存在分歧。与CCSM4不同，相对于工业化之前的MP，CESM1.2和CESM2均表现出云量减少，沃克环流减弱以及热带太平洋ElNiño样平均状态的现象。这种类似于厄尔尼诺现象的平均状态与古观测海表温度一致，表明对CCSM4的改善。通过对MP观测海表温度的新汇编来评估MP模拟的性能。模型数据比较表明，CCSM4对MP强制的敏感性不足，