Timescale-dependent fingerprint of the Asian Summer Monsoon during the last Glacial and its impact on vegetation

The Asian Summer Monsoon is fundamental for the water supply of billions of people. It has undergone major changes over the Pleistocene in response to greenhouse gas and ice sheet forcing during glacial-interglacial transitions, orbital forcing from varying obliquity and precession, and millennial-scale shifts in the ocean circulation. Yet, the spatial fingerprint of these variations and their impact on local vegetation remain uncertain. Here, we present vegetation and climate reconstructions from a pollen record in Northeastern China covering the last 70kyr with unprecedented sub-centennial resolution. During the last Glacial, its position at the ecotone between cool mixed forest and steppe led to pronounced local vegetation changes, most likely driven by varying moisture availability. The vegetation changes occur synchronously with oxygen isotope variations in Chinese speleothems. However, the timescale-dependent contributions to the total variability differ between our precipitation reconstruction and the isotope record. A regional analysis of high-resolution proxy records covering the last Glacial supports comparatively stronger contributions from orbital-scale variability along the northern monsoon edge and from millennial-scale variability in India and Southern China. This suggests that orbital forcing and Atlantic Meridional Overturning Circulation (AMOC) variations possess distinct spatial fingerprints. Climate simulations indicate that the differences are driven by stronger North Pacific sea surface temperature changes in response to orbital forcing compared to AMOC shifts. The detected spatial heterogeneity of past monsoon variations can provide valuable insights into potential regional impacts of future monsoon changes.