Westerly Variations in the Eastern Tibetan Plateau since the Last Interglacial Revealed by the Grain-Size Records of the Ganzi Loess

Abstract

The westerlies play an important role in driving climate change in the Tibetan Plateau (TP). However, little is known about the history of the westerlies in the TP owing to limited observations and a lack of robust reconstructions. The widely distributed eolian loess in the eastern TP is one of the ideal materials to retrieve the intensity history for the westerlies. A detailed grain-size and endmember model analysis (EMMA) on the Ganzi loess sequence located in the eastern TP revealed that the EMMA decomposed the loess grain-size components into four endmembers: EM1 (modal size 1.42 μm) is related to pedogenesis, and EM2 (modal size 7.10 μm) is transported by the westerlies. Silt modes (EM3 and EM4) are transported by the TP winter monsoon or near-surface airflows. The mass accumulation rate (MAR) of EM2 indicated the westerly variations. Combined with the MAR, we reconstructed the history of westerly intensity since the last interglacial period from the Ganzi loess sequence. We found that the intensity of the westerlies showed typical glacial/interglacial variations since the last interglacial period in the eastern TP. The westerly intensity was strong with large fluctuations during the glacial period, whereas it was weak and stable during the Holocene and the last interglacial. The temperature gradient between high and low latitudes caused by changes in insolation and ice volume in the northern hemisphere were the dominant forcing mechanisms for the westerly intensity variations.