Quantifying changes in central Vietnam rainfall amount since the Late Pleistocene

Paleorecords of the Southeast Asian autumn monsoon indicate that global sea-level change drove abrupt shifts in rainfall, yet the magnitude of these changes remains unquantified. Using δ⁴⁴Ca and Mg/Ca measurements from a speleothem from central Vietnam, we generate semi-quantitative rainfall records spanning 45–4 ka. When compared to modern rainfall, our results reveal a ∼50–60 % reduction in rainfall during the Last Glacial Maximum (LGM), associated with the sea-level lowstand. While climate models and a data assimilation product (PMIP4-CMIP6, iTRACE, DAMP-21ka) also show LGM drying (annual model mean = 8.5 % drier), the coarse spatial resolution of the models does not resolve the north-south trending Truong Son Mountains, an orographic barrier that blocks summer rainfall from reaching central Vietnam and enhances autumn rainfall. This causes a causes a summer rainfall wet bias in the models leading to an underestimation of annual rainfall change. However, simulations of autumn rainfall show better agreement with speleothem proxies (autumn model mean = 29 % drier), demonstrating that the models broadly capture changes in autumn monsoon rainfall amount. These findings underscore the sensitivity of the Southeast Asian autumn monsoon to sea-level change and highlight the value of using speleothem δ⁴⁴Ca and Mg/Ca to validate climate model simulations of rainfall amount.