Abnormal reduction in Indian Monsoon rainfall during current warm period from the Holocene evolution perspective

Since the onset of post-industrial global warming, there remains significant controversy over whether the evolution of the Indian Monsoon has deviated from the natural baseline of ‘rainfall and heat synchronization’. This question requires a deep understanding of the long-term correlation between temperature and monsoon rainfall changes. We selected Yunlong Lake, a climate-sensitive subalpine lake located on the southeastern margin of the Tibetan Plateau, to study this issue. We semi-quantitatively revealed warm-season temperature changes using the relative abundance of warm-preference planktic diatoms, namely Achnanthidium catenatum and Aulacoseira granulata var. angustissima, found in our diatom seasonal survey. Meanwhile, by examining the spatial pattern of surface diatom assemblages along the water depth gradient, two diatom-water depth transfer functions were constructed by using the modern analogue technique (MAT) and weighted average partial least squares regression (WAPLS) method, which were used to quantitatively reconstruct past lake water level changes. Comparison with independent modern monitoring records, the warm-water diatom indicators and the two diatom transfer functions suggest to be sensitive and reliable for reconstructing long-term temperature and rainfall changes in the Indian Monsoon domain. Based on this, and relying on the complete Holocene sedimentary records from the profundal zone of the lake, we reconstructed the warm-season temperature changes and lake water level fluctuations during the Holocene. Our reconstruction indicates that the temperature and rainfall in the Indian Monsoon domain exhibit a synchronous, ‘three-stage’ stepwise change over the multi-millennial scale of the Holocene. These stages are roughly marked by the climate events of 8.2 ka and 4.2 ka, with the maximum warm-season temperature and lake water level occurring during the middle Holocene. We find that the maximum rainfall and heat during the middle Holocene in this region roughly coincide with the peak of warm-season (summer-autumn) insolation. This may be related to the significant thermal difference between land and ocean at that time, suggesting a synchronous change in rainfall and temperature in Indian Monsoon domain over long timescales.