Millennial-scale hydroclimate variations in the South Asian Monsoon region modulated by Indian Ocean Dipole variability

The Indian summer monsoon (ISM) precipitation plays a central role in regulating marine biogeochemical cycling in the Indian Ocean and has a strong impact on the society in the South Asian. Although the weakening of ISM rainfall during Heinrich Stadials (HSs) has been linked to Northern Hemisphere and Indian Ocean cooling, the role of additional forcing mechanisms remains hotly debated. Combing multiproxy millennial-scale records of ISM precipitation and runoff, and Indian Ocean temperature patterns with climate models, we demonstrate that ISM rainfall weakening during HS1 (~17.5–15.5 ka) resulted from weakened Indian Ocean Dipole (IOD)-like sea surface temperature (SST) patterns. Conversely, increased ISM precipitation during the Bølling-Allerød (B/A; 14.7–12.9 ka) coincided with strengthened IOD-like SST configurations. Sensitivity experiments show that a weakened IOD-like SST pattern, specifically involving anomalous warming over the eastern Indian Ocean during HS1, may have triggerred anticyclonic circulation in the atmosphere over the northern Indian Ocean. This atmospheric response appears to induce limited moisture transport from the Indian Ocean to the Indian subcontinent, decreasing ISM rainfall and runoff. Conversely, cyclonic atmospheric circulation over the northern Indian Ocean likely induced extreme ISM rainfall, where the resulting higher riverine nutrient supply stimulated enhanced primary productivity during the B/A. This study highlights the significant role of the IOD-like variability in driving Indian Ocean precipitation changes and underscores the implications for biogeochemical responses in the northern Indian Ocean under future climatic warming scenarios.