Glacial Changes in Indian Summer Monsoon δ¹⁸O Driven by Circulation and Moisture-Source Shifts

In this study, we investigate how the Indian summer monsoon, its water vapor sources, and isotopic signature of precipitation (δ^¹⁸O_precip) responded to the Last Glacial Maximum (LGM, ~21 ka BP) boundary conditions using an isotope-enabled general circulation model with water-vapor source tagging (iCESM1). The LGM presents a valuable case study for understanding the Indian monsoon responses to reduced CO₂, the presence of Laurentide ice sheets and ice-sheet topography, and orbital forcing.

The simulations show a pronounced weakening of Indian summer monsoon precipitation (~15%) during the LGM, in agreement with available proxy records. The drying reflects both thermodynamic and dynamic controls: lower temperatures reduce atmospheric water vapor content, while enhanced zonal temperature gradients between the relatively warm western Pacific and the cooler Indian subcontinent lead to anomalous subsidence over India, further suppressing rainfall.

Moisture source tagging indicates that the dominant source regions to monsoon rainfall-the South Indian Ocean, Arabian Sea, Central Indian Ocean, and continental recycling-remain the same between the pre-industrial control and the LGM, but their relative contributions are reduced under glacial conditions. The δ^¹⁸O_precip values over the Indian monsoon region are enriched by approximately 1‰ in the LGM simulation. A decomposition analysis shows that the enrichment is driven primarily by reduced contributions from distant, isotopically depleted water vapor sources and secondarily by weaker rainout during moisture transport from the Indian Ocean. These results suggest that glacial changes in Indian monsoon δ^¹⁸O_precip primarily reflect large-scale circulation and moisture-source shifts rather than local rainfall amount ("Amount Effect"), highlighting the importance of atmospheric dynamics when interpreting monsoon isotope records.