Han Zixuan, Werner Niklas, Wang Zhenqian, Li Xiangyu, Yao Zhengquan, Zhang Qiong
Key Laboratory of Marine Hazards Forecasting, Ministry of Natural Resources, Hohai University, Nanjing, China.
Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden.
Nat Commun. 2025 Aug 4;16(1):7172. doi: 10.1038/s41467-025-62186-y.
The Miocene epoch, marked by significant tectonic and climatic shifts, presents a unique period to study the evolution of South Asian summer monsoon (SASM) dynamics. Previous studies have shown conflicting evidence: wind proxies from the western Arabian Sea suggest a weaker Somali Jet during the Middle Miocene compared to the Late Miocene, while rain-related records indicate increased SASM rainfall. This apparent decoupling of monsoonal winds and rainfall has challenged our understanding of SASM variability. Here, using the fully coupled EC-Earth3 model, we identify a key driver of this decoupling: changes in African topography rather than other external forcings such as CO change. Our simulations reveal that changes in Miocene African topography weakened the cross-equatorial Somali Jet and reduced upwelling in the western Arabian Sea, while simultaneously enhancing monsoonal rainfall by inducing atmospheric circulation anomalies over the Arabian Sea. The weakened Somali Jet fostered a positive Indian Ocean Dipole-like warming pattern, further amplifying the monsoonal rainfall through ocean-atmosphere feedbacks. In contrast, CO forcing enhances both Somali Jet and rainfall simultaneously, showing no decoupling effect. These findings reconcile the discrepancies between wind and rainfall proxies and highlight the critical role of African topography in shaping the multi-stage evolution of the SASM system.
中新世时期以显著的构造和气候变化为特征，是研究南亚夏季风（SASM）动力学演变的独特时期。以往的研究呈现出相互矛盾的证据：来自阿拉伯海西部的风指标表明，与晚中新世相比，中中新世的索马里急流较弱，而与降雨相关的记录则显示SASM降雨量增加。这种季风风和降雨明显的脱钩对我们对SASM变率的理解提出了挑战。在此，我们使用完全耦合的EC-Earth3模型，确定了这种脱钩的一个关键驱动因素：非洲地形的变化，而非诸如二氧化碳变化等其他外部强迫。我们的模拟结果显示，中新世非洲地形的变化减弱了跨赤道的索马里急流，减少了阿拉伯海西部的上升流，同时通过在阿拉伯海诱发大气环流异常增强了季风降雨。减弱的索马里急流促成了一种类似印度洋偶极的正暖化模式，通过海气反馈进一步放大了季风降雨。相比之下，二氧化碳强迫同时增强了索马里急流和降雨，未显示出脱钩效应。这些发现调和了风指标和降雨指标之间的差异，并突出了非洲地形在塑造SASM系统多阶段演变中的关键作用。
