Westerlies effect in Holocene paleoclimate records from the central Qinghai-Tibet Plateau

Our knowledge of the influence of the Westerlies on the hydroclimate of High Mountain Asia during the Holocene, especially during the mid-Holocene, remains poorly understood, limiting our understanding of the controlling mechanisms of past climate change in this region. Assessing the impact of the westerly winds has proven to be a challenge due to a lack of direct indicators. In this study, we summarize the influence of the Westerlies reflected in Holocene paleoclimate records from the central Qinghai-Tibet Plateau (CQTP). This is the transitional zone of atmospheric circulation where the climate is drier under the influence of westerly winds than under monsoonal influences. We identify two types of “Westerlies effect”, emerging at 13–11 ka (I) and 7–2 ka (II), and then assess potential circulation mechanisms by examining circulation indices and developing a Westerlies effect index (WEI) in combination with modern observations of precipitation stable isotopes. The WEI integrates signals of two independent environmental indicators, leaf wax δD from Linggo Co and median grain size from Chibuzhang Co. We observe two types of Westerlies effects: Type I prevailed in the Late Glacial, and Type II occurred under the cool conditions of the mid- to late Holocene (7.0–4.5 ka and 3.5–2.0 ka on the CQTP). During the times of Type II Westerlies effects, circulation affecting regional atmospheric precipitation appears to have shifted from being dominated by the South Asian Summer Monsoon (SASM) to a monsoon-westerly transition state with relatively lower total effective moisture. However, compared with the driest periods (e.g. the Younger Dryas), the westerly jets still brought considerable precipitation, especially in non-monsoon seasons, and even helped to maintain moderate lake water levels. The asynchronous relationship between lake levels (water balance) and atmospheric circulation patterns since ~2.0 ka indicates that air mass source (marine and continental) and trajectory can influence rates of water recycling and/or re-evaporation. This study sheds light on the hydrological processes and environmental dynamics associated with the interplay between the westerly winds and the monsoons on the QTP and indicates that at times during the Holocene the Westerlies played a more prominently role in delivering moisture to the region (especially the transitional zone) than was previously thought.