Modeling Eocene Asian climate with geologically revised paleogeography

The Eocene was one of the warmest intervals of the Cenozoic, during which the Asian hydroclimate reorganized alongside major tectonic and paleogeographic changes. Yet, large uncertainties in the paleotopography of Eocene Asia—particularly across the Tibetan region—continue to obscure how continental dry–wet patterns evolved under greenhouse conditions. Here, we investigate the hydroclimatic evolution of the Eocene using a fully coupled climate simulation with revised paleogeographic boundary conditions, incorporating updated geological constraints on Asian paleogeography, including Tibetan paleotopography and the timing of the India–Asia collision. We reconstruct boundary conditions for the early (53 Ma) and late Eocene (38 Ma) featuring a characteristic “two-mountains–one-basin’’ configuration. The simulations reproduce a major transition from an extensive zonal arid belt in the early Eocene (53 Ma) to a more heterogeneous and generally wetter regime by the late Eocene (38 Ma), broadly consistent with available proxy evidence. By decomposing logarithmic changes in the aridity index into precipitation and evaporative-demand components, we show that late Eocene wetting arises from the combined effects of enhanced precipitation and reduced evaporative demand, with their relative importance varying regionally. Although East Asia becomes wetter overall, diagnostic analyses indicate that neither the early nor the late Eocene climate supports a coherent, modern-style East Asian monsoon. Together, these results highlight the coupled roles of modified topography, collision-related land–sea reorganization, and atmospheric greenhouse gas reduction in shaping the Eocene Asian hydroclimate, and provide new constraints on the tectonic and climatic processes underlying the emergence of present-day dry–wet patterns. 大约5000万年前, 地质证据指示印度板块已与欧亚大陆完成碰撞, 青藏随即进入地形重组期, 但当时并非整体高原, 而更可能是“两山夹一盆”的起伏格局.我们据此重建早始新世(53 Ma)与晚始新世(38 Ma)亚洲古地理, 并在“已完成碰撞”的新构造背景下, 通过模拟对比53 Ma与38 Ma时期, 评估亚洲由干转湿及季风何时能来.结果显示:早始新世亚洲存在广阔的纬向干旱带;到晚始新世, 随高原形成与温室气体下降, 气候整体转湿,区域差异增强.这一转型既来自降水增加, 也来自蒸发减弱, 且不同区域贡献不同, 与地质与古气候记录基本一致.值得注意的是, 即便青藏已有显著地形起伏, 模拟仍未出现现代式东亚季风, 提示碰撞与早期隆升不足以立刻触发季风.Take-home:季风不是“一撞就来”, 而是“高原成形 + 海陆格局 + 背景温室条件”共同到位后才出现.