Drivers of the δ¹⁸O Changes in Indian Summer Monsoon Precipitation between the Last Glacial Maximum and Pre-industrial Period

Abstract. In this study, we investigate the changes in water isotope ratios in the Indian summer monsoon precipitation (δ¹⁸O_precip) during the Last Glacial Maximum (LGM, ~21 ka Before Present) compared to the pre-industrial (PI) period, and the mechanisms driving these changes, using a general circulation model with water isotope and novel water vapor source-tagging capabilities.

During the LGM, the model simulates a substantial reduction (15 %) in monsoon precipitation over the Indian subcontinent, consistent with proxy records. This drying in LGM is associated with reduced atmospheric water vapor, a thermodynamic response to cooling, while the westerly circulation, a dynamics response, is strengthened over parts of the subcontinent. Additionally, zonal temperature gradients between a relatively less-cooled tropical Western Pacific Ocean and Indian subcontinent lead to anomalous subsidence over the Indian region, enhancing the drying. Water vapor source tagging shows that while the four dominant moisture sources for the monsoon (South Indian Ocean, Arabian Sea, Indian land recycling, and Central Indian Ocean) remained the same, their contributions were reduced during the LGM. The δ¹⁸O_precip values over the Indian monsoon region are enriched by approximately 1 ‰ in the LGM simulation, and we find that this enrichment was not driven by the local amount effect. A decomposition analysis shows that the enrichment was primarily caused by reduced contributions from distant, isotopically depleted water vapor sources and secondarily by reduced rainout during moisture transport from the Indian Ocean.

These findings have important implications for paleoclimate reconstructions, suggesting that δ¹⁸O records from the Indian region could be indicators of broad-scale atmospheric circulation rather than being direct proxies for local precipitation amount.