The early Eocene (∼55 Ma) was the warmest period of the Cenozoic and was most likely characterized by extremely high atmospheric CO₂ concentrations. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 Earth system model, set up with paleogeographic reconstructions of this period from the DeepMIP project and with different levels of atmospheric CO₂. When compared with proxy-based reconstructions, the simulations reasonably capture both the reconstructed amplitude and pattern of early Eocene sea surface temperature. A comparison with simulations of modern conditions allows us to explore the changes in ocean circulation and the resulting ocean meridional heat transport. At a CO₂ level of 840 ppm, the early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60^∘ S), fed by deepwater formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the buildup of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with subpolar gyres, sustains the poleward advection of saline subtropical water to the convective regions in the Southern Ocean, thereby maintaining deepwater formation. This salt–advection feedback mechanism is akin to that responsible for the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3–0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20^∘ S, and contributes to keeping the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO₂ levels show that ocean circulation and heat transport are relatively insensitive to CO₂ doubling.

中文翻译：

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始新世初期剧烈的海洋倾覆及其对温暖的南部海洋的贡献

早期始新世（≥55° Ma）是新生代最温暖的时期，最有可能以极高的大气CO ₂浓度为特征。在这里，我们分析了IPSL-CM5A2地球系统模型对早期始新世的模拟，该模型由DeepMIP项目的这一时期的古地理重建和不同水平的大气CO _2建立。与基于代理的重建相比，模拟合理地捕获了始新世早期海面温度的重建幅度和模式。通过与现代条件模拟的比较，我们可以探索海洋环流的变化以及由此产生的海洋子午热传输。在CO _2下840 ppm，早期始新世模拟的特征是南半球强烈的深渊翻转环流（60°Sv等于60°S ^？（S），由南大洋的三个区域中的深水形成供给。封闭的Drake和Tasmanian通道有利于南大洋的对流，这为Weddell和Ross海中强亚极回旋流的形成提供了西部边界，并在对流中发展。与亚极地回旋相关的强倾覆环流维持了亚热带咸水向南极对流区的极向平流，从而维持了深水形成。这种盐平流反馈机制类似于造成当今北大西洋上空环流的机制。在55°Ma模拟中，强烈的深渊翻转环流主要导致极地海洋热量传输增加0.3°？0。^{一种？？？？}S，并有助于保持始新世的南大洋和南极洲温暖。不同大气CO ₂水平的模拟 表明，海洋环流和热传输对CO ₂倍增相对不敏感。