High- and low-latitude forcing on precipitation variations over the central Qilian Mountains since 18.5 ka

The moisture of the northeastern Tibetan Plateau is jointly shaped by the East Asian Summer Monsoon (EASM) and the mid-latitude Westerlies, the two key atmospheric circulation systems originating from low- and high-latitudes, respectively. Modern observations demonstrate that the intensity change and meridional position of the Westerlies directly modulate the EASM precipitation, which sustains vast human populations and ecosystems across East Asia. However, the interaction between these two systems over geological time remains controversial, partly due to controversies in the reconstructed past precipitation intensity. Here, we present a high-resolution multi-proxy loess record (ZES) spanning the past 18.5 ka from the central Qilian Mountains, located in the monsoon marginal zone where both systems jointly operate. We first found that loess–paleosol carbonate δ 18 O of ZES record captures a summer signal and covaries inversely with δ 13 C, which tracks monsoon precipitation intensity. Together with comparisons between modern observations and geological records, this constrains EASM-dominated precipitation. We then establish a robust EASM index from multiple independent proxies including magnetic susceptibility, total carbonate content, dolomite content, carbonate δ 13 C, and colour reflectance. Our results reveal that EASM precipitation responded to combined high- and low-latitude forcing on both millennial timescales during the Last Deglaciation and orbital timescales manifested in the long-term trend since 18.5 ka, with the pre-Boreal oscillation offering complementary insights into early-Holocene centennial-scale interactions. The EASM precipitation intensity was suppressed during the Last Deglaciation, strengthened gradually through the early Holocene, peaked from ∼7.5 to 1.8 ka, and subsequently declined. Direct comparison of Westerlies and EASM signals retrieved from the same loess sequence reveals coupled interaction between the two systems since the mid-late Holocene, with westerly intensification accompanying monsoon strengthening. These findings indicate bidirectional linkage between high- and low-latitude systems and further suggest that, under continued global warming, the monsoon marginal zone of northwestern China may experience a progressive increase in precipitation.