Abstract:

The westerlies over mid-latitude Asia play a crucial role in the global climate system by regulating the transport of energy, water vapor, and dust. However, the long-term evolution and driving mechanisms of the upper-level westerlies since the Mid-Pleistocene remain poorly understood due to the lack of reliable proxies. Here, we investigate three representative loess sections under different circulation regimes, including the DK section in Tajikistan within the westerly-dominated region, the XF section on the Chinese Loess Plateau within the monsoon-dominated region, and the NYZ19 section in northeastern China within the westerly-monsoon transitional zone.
Using high-resolution grain-size records combined with single-sample unmixing (SSU) and Ensemble Empirical Mode Decomposition (EEMD) analyses, we extracted fine-grained components transported by the upper-level westerlies and reconstructed the spatiotemporal evolution of the upper-level westerlies over mid-latitude Asia since 800ka and explored their forcing mechanisms. The results show that pronounced glacial-interglacial cyclicity, with stronger westerlies during glacials and weaker westerlies during interglacials, although their amplitude and stability vary spatially. EEMD analyses show that all studied sections record precession, obliquity, and eccentricity cycles, although the dominant periodicities differ among sections.
Specifically, the fine-grained component transported by the upper-level westerlies in the DK section is dominated by 100 ka glacial periodicity, primarily reflecting the control of global ice volume on the background intensity of the westerlies. In contrast, the corresponding fine-grained components in the XF and NYZ19 sections exhibit a more pronounced 19-23 ka precessional cycle, suggesting that their variations were primarily controlled by the response of glacial activity over the upwind Tibetan Plateau to changes in summer insolation. Mechanistically, precession regulates glacier and periglacial activity across the Tibetan Plateau and its surrounding regions, thereby influencing the production and supply of fine-grained materials. Obliquity affects the meridional temperature gradient and the meridional position of the westerlies, thereby influencing the transport and depositional expression of fine-grained materials in downwind areas. The climate formed by different combinations of orbital parameters leads to spatially heterogeneous responses of the westerlies, which reflects shifts in the westerlies under precession and obliquity forcing. Meanwhile, the Tibetan Plateau profoundly affected the fine-grained material transport processes of the westerlies since the Middle Pleistocene through three pathways: enhancing sediment supply, facilitating vertical uplift of materials, and improving the transport efficiency of the westerlies.
Overall, orbital forcing, global ice-volume changes, and regional topography jointly shaped the evolution of the upper-level westerlies since the Mid-Pleistocene.