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The dominant influence of precession‐induced changes in summer insolation on orbital‐scale variability of the Asian summer monsoon (ASM) during the Holocene has been widely proposed; however, it remains unclear why the decline of the ASM started several thousand years after the peak summer insolation. Through comparisons of climate simulations and proxy records, our study reveals that the abrupt decline in the ASM coincided with an increase in spring insolation at the equator. The reduced spring insolation resulted in a cooler tropical Indian Ocean, which weakened and shifted northward the westerly jet due to decreased meridional thermal gradient. The South Asian high moved northward in conjunction with the westerly jet, causing anomalous upwards over northern South Asia, the Tibetan Plateau, as well as southwestern and northern China. The associated anomalous cyclone over the Tibetan Plateau enhanced the monsoonal moisture transport, subsequently intensifying the ASM circulation and precipitation. The ASM was enhanced by the decrease in spring insolation and was weakened by the opposite. The abrupt decline of the ASM was associated with an increase in spring insolation superimposed on a decrease in summer insolation. Consequently, orbital‐scale ASM variability is dominated by the precession not only via insolation changes in summer but also changes in spring. Plain Language Summary: It is widely acknowledged that orbital‐scale Asian summer monsoon (ASM) variability was controlled by precession‐dominated changes in Northern Hemisphere summer insolation. However, it is still not clear why the ASM did not weaken in response to the initial decrease in summer insolation (e.g., during the early to mid‐Holocene), as evidenced in numerous records. This study explores the underlying dynamics driving the decline of the ASM by comparing climate simulations with proxy records. Our findings reveal that an abrupt decline in the ASM commenced from minimum spring insolation at the equator, lagging the peak summer insolation by several thousand years. Reduced spring insolation had a prolonged impact on cooling of the tropical Indian Ocean in summer, leading to a decrease in meridional thermal gradient. Consequently, both westerly jet and South Asian high shifted northward, intensifying the ASM circulation and precipitation. The decreasing spring insolation slowed down the decline of the ASM, while the rising spring insolation accelerated it. Hence, an abrupt decline in the ASM was driven by increasing spring insolation superimposed on decreasing summer insolation. Key Points: An abrupt decline in the Asian summer monsoon (ASM) kept pace with the rise in equatorial spring insolationEquatorial spring insolation exerted a prolonged impact on the tropical Indian Ocean in the following summerThe ASM was modulated by the tropical Indian Ocean via shifting the Asian subtropical westerly jet [ABSTRACT FROM AUTHOR]

The precipitation changes on the northeastern Tibetan Plateau during the Holocene remain unclear due to discrepancies among different proxy records. We compared proxy records with the results from a transient simulation performed using the Kiel Climate Model forced by orbital variations, to analyse summer precipitation changes in this area during the Holocene (9.5-0 ka BP). The model results suggested increasing amounts of summer precipitation from 9.5 to ~6.2 ka BP and a persistent decline thereafter, which matched well with pollen records but was inconsistent with ostracod δ18O records. The Holocene climatic optimum lagged the Northern Hemisphere summer insolation maximum by ~3.5 ka, caused by the interplay between the East Asian summer monsoon (EASM) circulation and the mid-latitude westerlies. The ostracod δ18O values reflected the strength of the EASM circulation. A strong EASM circulation increased the transport of water vapour towards the northeastern Tibetan Plateau from the northwestern Pacific. Weakened mid-latitude westerlies increased the incursion of cold air masses into the northeastern Tibetan Plateau. During the early Holocene, relatively strong mid-latitude westerlies, resulting from an enhanced Arctic Oscillation, reduced summer precipitation on the northeastern Tibetan Plateau, in spite of a strong insolation-driven EASM circulation. The weakening EASM circulation and the strengthening westerlies together induced the decreasing trend of summer precipitation from the middle to late Holocene. In addition, summer precipitation variations were further modulated by sea-surface temperatures in the northwestern Pacific, through weakening the strength of the EASM. [ABSTRACT FROM AUTHOR]

The abrupt shift of the East Asian winter monsoon (EAWM) is closely linked to the Atlantic meridional overturning circulation (AMOC) triggered by freshwater hosing at millennium timescale. However, the mechanisms of the EAWM response to North Atlantic freshwater forcing are still not fully understood. This study focuses on the role of the Tibetan Plateau in millennium-scale EAWM variability, both in terms of the Siberian high and monsoonal northerlies, by performing two groups of sensitivity experiments at 8.2 ka BP forced by the same North Atlantic freshwater hosing, with different scenarios of the topography of the Tibetan Plateau. Simulation results reveal that the Tibetan Plateau played a crucial role in the EAWM response to North Atlantic freshwater forcing. Despite anomalous surface cooling over the Siberian and Mongolian regions caused by the slowdown of the AMOC, North Atlantic freshwater hosing could not effectively enhance the Siberian high with the lower Tibetan Plateau. Furthermore, with the lower Tibetan Plateau, the hosing-induced enhancement of monsoonal northerlies over East Asian coastal regions dramatically dwindled. The role of the Tibetan Plateau was mainly via amplifying the westward shift of the Asian subtropical westerly jet, which was triggered by AMOC-induced increase of meridional thermal gradient at 500 hPa over West Asia. The westerly jet's westward shift generated anomalous updrafts over the Northwest Pacific and Tibetan Plateau. The former updrafts created an anomalous cyclone over the Northwest Pacific, enhancing the monsoonal northerlies. The latter updrafts induced compensating downdrafts over Mongolia, and hence enhanced the Siberian high. Our results emphasize that the enhanced EAWM during the North Atlantic cold events was most likely linked to the upper-level westerly jet rather than the previous proposed local surface cooling. • The Tibetan Plateau played a critical role in millennium-scale EAWM variability. • Freshwater hosing enhanced the EAWM via the westward shift of the westerly jet. • The Tibetan Plateau amplified the westward movement of the westerly jet. [ABSTRACT FROM AUTHOR]

• Single grain K-feldspar pIR 50 IR 170 dating effectively dates samples over past 1 kyr. • Highest lake levels in Qaidam Basin over past 1 kyr during Little Ice Age. • Out-of-phase Central and East Asian precipitation drive asynchronous lake change. • EASM did not penetrate as far inland as Qaidam Basin during past 1 kyr. The moisture and precipitation evolution of mid-latitude Asia are dominated by the East Asian summer monsoon (EASM) and mid-latitude Westerlies atmospheric systems. How these two systems interacted along their transition zone in the northern Tibetan Plateau remains unclear, especially over centennially to millennial timescales. This study investigates moisture evolution along the EASM-Westerlies transition zone using 10 Late Holocene paleolake shorelines from Toson Lake in the northern Tibetan Plateau. Shorelines are dated using a newly developed single-grain post infrared-infrared stimulated luminescence (pIR 50 IR 170) dating protocol for 14,500 K-feldspar grains and used to reconstruct the evolution of Toson Lake over the past millennium. Results show that Toson Lake was stable during the Medieval Warm Period (MWP), expanded during the Little Ice Age (LIA) with a distinct highstand from AD1600–1700 punctuated by short-duration low period at ∼ CE 1650. After which, Toson Lake dramatically contracted into the Current Warm period (CWP). Lake level changes are consistent with Westerlies precipitation variability across Central Asia, in contrast to records from the EASM dominated northeastern Tibetan Plateau and northern China documenting an arid LIA and wet MWP. Reanalysis of modern climatic data are combined with our lake chronology to reveal that although EASM strength influences precipitation variability across northeastern Tibetan Plateau (NETP) and northern China, EASM precipitation has not directly penetrated deep inland over the past millennium. Instead, Westerlies moisture, modulated by the see-saw of the North Atlantic Oscillation, together with the moisture controlled by the anticyclonic circulation over Mongolia and negatively related to the EASM strength, dominated water vapour transport to the mid-latitudes of Central Asia. Out-of-phase moisture transport and interaction between the Westerlies and EASM drives asynchronous lake evolution between Central and East Asia over centennial-millennial timescales. [ABSTRACT FROM AUTHOR]

• According to loess thickness and basal ages, the TP can be divided into three zones (I, II and III) from west to east. • Western TP as Zone I is dust source area, eastern TP as Zone III is dust sink area, and Zone II at the central TP is transitional. • The current 1–3.5 m loess in Zone II will be stripped away as ecological disaster when next glacial time comes. The Tibetan Plateau (TP) hosts the largest alpine ecosystem in the world, and aeolian loess is an essential component of this ecosystem and landscape. However, little is known of loess distribution and fate, owing to the climate and landscape varying dramatically between Quaternary glacial and interglacial cycles. Recent research suggests that the interior Tibetan loess did not start to accumulate until ∼14 ka, when climate warming and wetting resulted in an increase in vegetation cover, enabling dust trapping. In this study, our extensive aeolian profile review and age analysis demonstrate that loess thickness and basal ages on the TP are very different regardless of terrain and landscape. According to loess distribution, thickness, and basal ages, we divided the TP into three zones: Zone I (western TP) and Zone III (eastern TP) are the dust source and sink areas, respectively, and Zone II (central TP) is transitional. The central TP became a dust source zone during the last glacial stage but turned into a dust sink during the last deglaciation and Holocene. The modeling results confirm that each zone has its own role in dust fixation and deflation and that these three zones are systematically interlinked by dust. Glacial-interglacial precipitation and vegetation cover changes controlled the switch in Zone II (central TP). We demonstrate that dust on the TP must have experienced enhanced excavation from west to east during the past glacial periods. During the next glacial period, the eastern TP loess may continue to accumulate, but the central TP loess will be eroded after vegetation degradation and removal, leading to a serious ecological disaster. This study provides insights into Tibetan loess accumulation patterns on a large spatial scale, improving the understanding of the past of Tibet loess and enabling predictions regarding future Tibet loess. [ABSTRACT FROM AUTHOR]