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Strengthening of Orbital- and Millennial-scale Asian Winter Monsoon Variability Accompanied Intensification of Northern Hemisphere Glaciation
author: source: Time:2024-04-29 font< big medium small >
In a study published in Nature Communications, researchers have documented that persistent millennial-scale Asian winter monsoon (AWM) intensity fluctuations were superimposed on 41-kyr and ~100-kyr orbital variability during both the warmer (higher-CO2) late Pliocene and colder (lower-CO2) early Pleistocene, in response to both external astronomical forcing and internal climate dynamics.
Led by Prof. AO Hong from the Institute of Earth Environment of the Chinese Academy of Sciences (CAS), the researchers constructed a palaeomagnetically dated centennial-resolution grain size record between 3.6 and 1.9 Ma from a previously undescribed Chinese loess-palaeosol/red clay section, which reflects both orbital- and millennial-scale AWM variability across the intensification of Northern Hemisphere glaciation (iNHG) at about 2.7 million years ago (Ma).
The iNHG marked a major global climate transition from a relatively stable warm Pliocene state to a cooler Quaternary regime characterized by larger-amplitude orbital-scale glacial-interglacial oscillations, which lasts until today. Northwesterly advection of the cold-dry AWM from high to low latitudes, which is induced by pressure gradients between the Siberian high-pressure cell over the cold mid- to high-latitude Asian continental interior and low-pressure cells over the relatively warm Indo‐Pacific Warm Pool and northwestern Pacific Ocean, links high- and low-latitude climatic processes and has profound impacts on Asian winter climate, agriculture, and people’s life. However, the features and underlying dynamics of orbital- and millennial-scale AWM variability remain poorly constrained across the iNHG due to a sparsity of well-dated high-resolution records.
To reconstruct both orbitally and millennially resolved AWM variability between 3.6 and 1.9 Ma and across the iNHG, the researchers collected in the field from a loess-palaeosol/red clay section (Fig. 1) on the central Chinese Loess Plateau 3,571 unoriented samples for grain size measurements and 251 oriented block samples for magnetostratigraphic analysis in the laboratory.
The resulting well-dated unprecedented centennial-resolution reconstruction suggests that the late Pliocene–early Pleistocene AWM was characterized by combined 41-kyr and ~100-kyr cycles, in response to ice volume and atmospheric CO2 forcing. Superimposed on orbital variability, millennial AWM intensity fluctuations persisted across the iNHG, in response to both external astronomical forcing and internal climate dynamics.
“Our high-resolution reconstruction demonstrates, to our surprise, that millennial-scale AWM variability already existed before the iNHG, two million years earlier than previously recognized, with markedly different climate-cryosphere boundary conditions compared to the much higher amplitude glacial cycles that mark the middle and late Pleistocene,” said Prof. AO, lead author of the study.
This study was funded by the Strategic Priority Research Program of CAS, the Second Tibetan Plateau Scientific Expedition and Research program, and the National Natural Science Foundation of China.

Fig. 1 Field photograph  of loess-palaeosol and red clay from the Chongxin section. (taken by AO Hong)
Contact: BAI Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn
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