To project the Northern Hemisphere (NH) summer monsoon under global warming, it is necessary to fully understand the effects of major climatic forcings including increasing atmospheric CO 2 concentrations and warming-induced changes of orography and ice sheets. The mid-Pliocene Warm Period, a geological analogue with similar CO 2 concentrations and continent–ocean configuration to the modern day, provides an opportunity to clarify the relative contributions of CO 2 , orography, and ice sheets, as well as the underlying mechanisms. By applying a factor separation approach to Uoft-CCSM4 and COSMOS in PlioMIP2, this study quantifies the individual contributions on monsoon changes, with a focus on North Africa, South Asia, and East Asia. Our results show that orographic forcing is a non-negligible driver of enhanced summer monsoon precipitation in North Africa and South Asia, whereas large inter-model differences exist for the East Asian monsoon. The orographic influence operates through an underlying mechanism: altered boundary conditions lead to reduced surface albedo in the NH and an intensified Atlantic Meridional Overturning Circulation, which increase the NH surface air temperature, in turn driving a northward shift of the Intertropical Convergence Zone and NH monsoon enhancement, especially over North Africa and South Asia. These findings suggest that in a future warmer climate, orographic changes induced by sea level rise and ice sheet melting may alter the NH summer monsoon, highlighting the importance of incorporating evolving boundary conditions into global climate models. 摘要: 在全球变暖背景下, 准确预测北半球夏季风的演变特征具有重要科学意义.中上新世暖期具有与现代相近的CO₂浓度及海陆分布格局, 为揭示CO₂,地形与冰盖对季风的影响及其物理机制提供了关键的地质参照.本研究基于PlioMIP2中的Uoft‑CCSM4与COSMOS模式, 采用对称因子分解法, 量化分析各强迫因子对北非,南亚及东亚夏季风影响的相对贡献.结果表明, 地形强迫对北非与南亚季风降水的增强作用不容忽视, 而东亚存在模式间的显著差异.其物理机制表现为:地形变化降低北半球地表反照率并增强大西洋经向翻转环流, 促使北半球地表增温, 进而推动热带辐合带北移, 最终导致北半球夏季风增强.研究表明, 未来全球变暖背景下, 海平面上升与冰盖消融引发的地形改变可能显著影响北半球夏季风.
Li et al. (Fri,) studied this question.