We compare results obtained from modeling the mid-Pliocene warm period using the Community Earth System Models (COSMOS, version: COSMOS-landveg r2413, 2009) with the two different modeling methodologies and sets of boundary conditions prescribed for the two phases of the Pliocene Model Intercomparison Project (PlioMIP), tagged PlioMIP1 and PlioMIP2. Here, we bridge the gap between our contributions to PlioMIP1 (Stepanek and Lohmann, 2012) and PlioMIP2 (Stepanek et al., 2020). We highlight some of the effects that differences in the chosen mid-Pliocene model setup (PlioMIP2 vs. PlioMIP1) have on the climate state as derived with COSMOS, as this information will be valuable in the framework of the model–model and model–data comparison within PlioMIP2. We evaluate the model sensitivity to improved mid-Pliocene boundary conditions using PlioMIP's core mid-Pliocene experiments for PlioMIP1 and PlioMIP2 and present further simulations in which we test model sensitivity to variations in paleogeography, orbit, and the concentration of CO₂. Firstly, we highlight major changes in boundary conditions from PlioMIP1 to PlioMIP2 and also the challenges recorded from the initial effort. The results derived from our simulations show that COSMOS simulates a mid-Pliocene climate state that is 0.29 ^∘C colder in PlioMIP2 if compared to PlioMIP1 (17.82 ^∘C in PlioMIP1, 17.53 ^∘C in PlioMIP2; values based on simulated surface skin temperature). On the one hand, high-latitude warming, which is supported by proxy evidence of the mid-Pliocene, is underestimated in simulations of both PlioMIP1 and PlioMIP2. On the other hand, spatial variations in surface air temperature (SAT), sea surface temperature (SST), and the distribution of sea ice suggest improvement of simulated SAT and SST in PlioMIP2 if employing the updated paleogeography. Our PlioMIP2 mid-Pliocene simulation produces warmer SSTs in the Arctic and North Atlantic Ocean than those derived from the respective PlioMIP1 climate state. The difference in prescribed CO₂ accounts for 0.5 ^∘C of temperature difference in the Arctic, leading to an ice-free summer in the PlioMIP1 simulation, and a quasi ice-free summer in PlioMIP2. Beyond the official set of PlioMIP2 simulations, we present further simulations and analyses that sample the phase space of potential alternative orbital forcings that have acted during the Pliocene and may have impacted geological records. Employing orbital forcing, which differs from that proposed for PlioMIP2 (i.e., corresponding to pre-industrial conditions) but falls into the mid-Pliocene time period targeted in PlioMIP, leads to pronounced annual and seasonal temperature variations. Our result identifies the changes in mid-Pliocene paleogeography from PRISM3 to PRISM4 as the major driver of the mid-Pliocene warmth within PlioMIP and not the minor differences in forcings.

中文翻译：

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中上新世气候对轨道强迫变化和PlioMIP边界条件的敏感性

我们将使用社区地球系统模型（COSMOS，版本：COSMOS-landveg r2413，2009）对上新世中期暖期进行建模的结果与针对上新世模型的两个阶段规定的两种不同建模方法和边界条件集进行了比较比对项目（PlioMIP），标记为PlioMIP1和PlioMIP2。在这里，我们弥合了我们对PlioMIP1 （Stepanek和Lohmann，2012） 和PlioMIP2 （Stepanek等，2020）的贡献之间的差距。。我们重点介绍了所选的上新世中期模型设置（PlioMIP2与PlioMIP1）的差异对使用COSMOS得出的气候状态的影响，因为该信息在模型-模型和模型-数据的框架中将是有价值的在PlioMIP2中进行比较。我们使用PlioMIP针对PlioMIP1和PlioMIP2进行的上新世中期核心实验，评估了模型对上新世中期边界条件的敏感性，并提供了进一步的模拟，其中我们测试了模型对古地理，轨道和CO ₂浓度变化的敏感性。首先，我们重点介绍了从PlioMIP1到PlioMIP2的边界条件的重大变化，以及从最初工作中记录的挑战。从我们的模拟得出的结果表明，COSMOS模拟年年上新世的气候状态，则为0.29  ^∘ Ç如果相比PlioMIP1（17.82在PlioMIP2寒冷 ^∘ Ç在PlioMIP1，17.53  ^∘ Ç在PlioMIP2中；值基于模拟的表面皮肤温度）。一方面，在上新世中期的代理证据的支持下，高纬度变暖在PlioMIP1和PlioMIP2的模拟中均被低估了。另一方面，如果使用更新的古地理学，地表空气温度（SAT），海面温度（SST）和海冰分布的空间变化表明PlioMIP2中模拟SAT和SST的改进。我们的PlioMIP2中上新世模拟在北极和北大西洋产生的SST比从各自PlioMIP1气候状态得出的SST温暖。在规定的差CO ₂ 0.5帐户 ^∘ Ç北极的温度差异，导致在PlioMIP1仿真中出现无冰夏季，而在PlioMIP2中出现准无冰夏季。除了官方的PlioMIP2模拟集外，我们还提供了进一步的模拟和分析，以采样在上新世期间起作用并可能影响地质记录的潜在替代轨道强迫的相空间。采用与PlioMIP2建议的轨道强迫不同（即对应于工业化前的条件）但落入PlioMIP目标的上新世中期的轨道强迫会导致明显的年度和季节性温度变化。我们的结果确定了上新世中期古地理从PRISM3到PRISM4的变化是PlioMIP内上新世中期温暖的主要驱动力，而不是强迫的微小差异。