Palaeoclimate simulations improve our understanding of the climate, inform us about the performance of climate models in a different climate scenario, and help to identify robust features of the climate system. Here, we analyse Arctic warming in an ensemble of 16 simulations of the mid-Pliocene Warm Period (mPWP), derived from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2). The PlioMIP2 ensemble simulates Arctic (60–90^∘ N) annual mean surface air temperature (SAT) increases of 3.7 to 11.6 ^∘C compared to the pre-industrial period, with a multi-model mean (MMM) increase of 7.2 ^∘C. The Arctic warming amplification ratio relative to global SAT anomalies in the ensemble ranges from 1.8 to 3.1 (MMM is 2.3). Sea ice extent anomalies range from −3.0 to $-\mathrm{10.4}\times {\mathrm{10}}^{\mathrm{6}}$ km², with a MMM anomaly of $-\mathrm{5.6}\times {\mathrm{10}}^{\mathrm{6}}$ km², which constitutes a decrease of 53 % compared to the pre-industrial period. The majority (11 out of 16) of models simulate summer sea-ice-free conditions ($\le \mathrm{1}\times {\mathrm{10}}^{\mathrm{6}}$ km²) in their mPWP simulation. The ensemble tends to underestimate SAT in the Arctic when compared to available reconstructions, although the degree of underestimation varies strongly between the simulations. The simulations with the highest Arctic SAT anomalies tend to match the proxy dataset in its current form better. The ensemble shows some agreement with reconstructions of sea ice, particularly with regard to seasonal sea ice. Large uncertainties limit the confidence that can be placed in the findings and the compatibility of the different proxy datasets. We show that while reducing uncertainties in the reconstructions could decrease the SAT data–model discord substantially, further improvements are likely to be found in enhanced boundary conditions or model physics. Lastly, we compare the Arctic warming in the mPWP to projections of future Arctic warming and find that the PlioMIP2 ensemble simulates greater Arctic amplification than CMIP5 future climate simulations and an increase instead of a decrease in Atlantic Meridional Overturning Circulation (AMOC) strength compared to pre-industrial period. The results highlight the importance of slow feedbacks in equilibrium climate simulations, and that caution must be taken when using simulations of the mPWP as an analogue for future climate change.

中文翻译：

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在上新世中期气候模拟中评估北极变暖

古气候模拟提高了我们对气候的了解，使我们了解了气候模型在不同气候情景下的性能，并有助于确定气候系统的强大特征。在这里，我们从上新世模式比较项目第二阶段（PlioMIP2）获得的上新世中期暖期（mPWP）的16个模拟集合中分析了北极变暖。所述PlioMIP2合奏会模拟北极（60-90 ^∘  N）的3.7年平均表面空气温度（SAT）增加至11.6  ^∘则为C到预工业期间，具有多模式平均（MMM）的7.2增加 ^∘ C.整体中北极变暖相对于全球SAT异常的放大率在1.8到3.1之间（MMM为2.3）。海冰范围异常范围从−3.0至$--\mathrm{10.4}\times {\mathrm{10}}^{\mathrm{6}}$ km ²，MMM异常为$--\mathrm{5.6}\times {\mathrm{10}}^{\mathrm{6}}$ km ²，与工业化前的时期相比减少了53％。大多数模型（16个中的11个）模拟了夏季无海冰的情况（$\le \mathrm{1个}\times {\mathrm{10}}^{\mathrm{6}}$ 公里²）在他们的mPWP模拟中。与可用的重建相比，该集合倾向于低估北极的SAT，尽管在模拟之间低估的程度差异很大。具有最高北极SAT异常的模拟倾向于更好地匹配当前形式的代理数据集。该合奏显示出与海冰的重建有一定的一致性，特别是在季节性海冰方面。较大的不确定性限制了可以置入结果中的置信度以及不同代理数据集的兼容性。我们表明，虽然减少重建过程中的不确定性可以大大降低SAT数据与模型之间的不和谐，但可能会在增强的边界条件或模型物理学中找到进一步的改进。最后，我们将mPWP中的北极变暖与未来北极变暖的预测进行了比较，发现与CMIP5未来气候模拟相比，PlioMIP2集合模拟的北极放大作用更大，与工业化之前相比，大西洋经向翻转循环（AMOC）强度增加而不是减少期。结果强调了平衡气候模拟中缓慢反馈的重要性，并且在将mPWP模拟用作未来气候变化的模拟时必须谨慎。