Abstract

Fluctuations in the Pacific Walker Circulation (PWC), a zonally oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions. Through a suite of coupled model simulations on tectonic timescales, we demonstrate that the PWC was ∼38° broader and ∼5% more intense during the Early Eocene relative to present. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient. However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remaining steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12°) and weakening (by ∼22%), which we show here is linked with the closure of the tropical seaways. Moreover, our results suggest that rising CO₂ favors a weaker PWC under the same land-sea configurations, a robust feature across the large spread of Cenozoic climates considered here, supporting a weakening of the PWC in a warmer future.

中文翻译：

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太平洋沃克环流跨越新生代的大转变

摘要

太平洋沃克环流 (PWC) 的波动是一种横跨热带太平洋的纬向翻转单元，可导致广泛的气候和生物地球化学扰动。PWC 在新生代是如何发展的，其温室气体和大陆位置发生了重大变化，目前尚不清楚。通过对构造时间尺度的一套耦合模型模拟，我们证明 PWC 在早始新世期间比现在宽约 38°，强度增加约 5%。随着气候从早始新世到晚中新世变冷，PWC 的宽度缩小，同时强度增加，这与增强的太平洋纬向温度梯度有关。然而，从早始新世到晚中新世，西部和东部分支的位置表现不同，由于西热带太平洋的地理相对稳定，西部边缘随着时间的推移保持稳定；随着南美洲大陆向西北移动，东部边缘随时间向西迁移。PWC 发生在晚中新世和晚上新世之间的过渡，表现为向东移动（西部和东部边缘向东迁移 >12°）和减弱（约 22%），我们在这里显示这与关闭热带航道。此外，我们的结果表明，不断上升的 CO 表现为东移（西边和东边都向东迁移 >12°）和减弱（约 22%），我们在这里显示这与热带海道的关闭有关。此外，我们的结果表明，不断上升的 CO 表现为东移（西边和东边都向东迁移 >12°）和减弱（约 22%），我们在这里显示这与热带海道的关闭有关。此外，我们的结果表明，不断上升的 CO₂有利于在相同的海陆配置下 PWC 较弱，这是这里考虑的大量新生代气候的强大特征，支持 PWC 在温暖的未来减弱。