Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling

Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4°C, up to 5°C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.距今约3400万年前的新生代温室-冰室转变（Cenozoic Greenhouse-Icehouse transition）事件，其核心驱动因素被普遍认为是大气二氧化碳（CO₂）浓度的持续下降。此前，学界对“构造活动开启南大洋通道、进而引发具有热隔离效应的南极绕极流（Antarctic Circumpolar Current, ACC）”这一假说存在争议，原因在于现有海洋数值模型未能复现预期的向南极海岸的热输送过程。本研究采用搭载详细古水深（paleobathymetry）的高分辨率海洋数值模拟，证实构造活动确实在重塑南大洋环流格局与热输送过程中发挥了根本性作用，该结果与现有代用资料（proxy data）一致。当至少一条通道（塔斯马尼亚通道或德雷克通道）处于300米浅水深状态时，海洋涡旋（gyres）会将暖水输送至南极海域。当第二条通道的水深降至300米以下时，上述涡旋会逐渐减弱，进而导致南极表层海水出现显著降温——平均降温幅度为2~4℃，最高可达5℃，与此同时ACC仍维持较弱状态。本研究结果表明，构造变化对南大洋气候变化至关重要，在限定长期气候对二氧化碳的敏感性时，应予以充分重视。