Non-uniform spatial difference in the South Asian summer monsoon during the mid-Piacenzian

The South Asian summer monsoon （SASM） during the mid-Piacenzian is analyzed through climate modelling with CAM4. The model results reveal a non-uniform spatial difference in the SASM during the mid-Piacenzian compared to the pre-industrial era, with the SASM being more intense north of -20°N but weaker south of -20°N. In particular, summer precipitation is higher in South Asia north of -20°N, accompanied by anomalous low-level southwesterlies from the Arabian Sea, whereas the precipitation is lower in South Asia south of-20°N, with anomalous low-level easterlies. These differences in the SASM are related to changes in sea level pressure （SLP） due to the different boundary conditions between the two periods. Further analysis isolates the climate effects of the different boundary conditions and indicates the combined difference in atmospheric carbon dioxide concentration and SST to be the most important factor in this difference in the SASM through the changes in SLR By comparison, the differences in vegetation and topography have limited effects. The availability of geological evidence is relative greater in northern India than in southern India, and comparison with this geological evidence shows the simulated monsoon climate to be qualitatively consistent with it, particularly for the wetter climate in northern India.

利用CAM4模拟研究了上新世暖期(3.264–3.025 Ma)相对工业革命前南亚夏季风的差异。模拟结果显示,上新世暖期相对工业革命前,南亚夏季风表现出不均一的空间差异。在~20°N以北,上新世暖期南亚夏季降水更多且西南风更强,而在~20°N以南降水减少并伴有异常的东风。这些差异与两个时期边界条件改变所引起的海表面气压的差异有关。进一步分析发现,两个时期大气CO_2含量和海表温度的差异是导致南亚夏季风差异的主要原因。通过对比发现,模拟得到的上新世暖期印度半岛北部的湿润气候与地质证据定性一致。