Distinct response of Arabian sea upwelling and Indian summer monsoon rainfall on the millennial timescale

The upwelling-induced productivity in the Arabian Sea is often used to infer changes in the Indian summer monsoon (ISM). However, the evolution of ISM based on these productivity records is inconsistent with other moisture records during the last deglaciation (20-11ka), leading to uncertainty about ISM variations. Here, we use an isotope-enabled transient climate experiment (iTraCE) to investigate the reasons for this discrepancy, suggesting that the discrepancy arises from different driving processes between the ISM rainfall and Arabian Sea upwelling. The ISM rainfall decreases during Heinrich Stadial 1 (HS1) and increases during B & oslash;lling-Aller & oslash;d (BA), driven by moisture convergence. However, the Arabian Sea upwelling is more complex, with coastal and open sea upwelling responding differently to climatic changes. The coastal upwelling along the Oman coast intensifies from the Last Glacial Maximum (LGM) to BA primarily due to strengthening coastal winds driven by increased local land-sea thermal contrast. In contrast, open sea upwelling exhibits a more nuanced pattern. During HS1, it increases near the Oman coast but weakens offshore, while during BA warming, it weakens over a large part of the open sea region. These variations in open sea upwelling are attributed to changes in the meridional movement of monsoon low-level jet with a slight northward shift during HS1 and a significant shift during BA, affecting the extent of Ekman pumping contraction. Our research demonstrates that western Arabian Sea upwelling does not accurately reflect changes in ISM rainfall intensity during the last deglaciation due to these divergent driving processes. This study provides new insights into the upwelling-ISM rainfall relationship from a millennial-scale perspective, contributing to a more nuanced understanding of paleoclimatic changes in the region.