Jet transitions caused multiple abrupt droughts in the Asian summer monsoon margin during Holocene times

The Asian Summer Monsoon (ASM) margin is vulnerable to climate change and a distinct boundary of population and archeological sites exists along the margin. Although lots of studies have been undertaken along the margin, the Holocene history of climate variation is highly debated; for example, whether the climatic optimum occurred in the early Holocene or middle Holocene. In addition, moisture variations on millennial to centennial timescales remain unclear. In this study, we reconstruct moisture variation at the modern junction of the ASM and the Westerlies based on a well-dated (13.3-0.5 ka) loess section in northwest China. On an orbital timescale, moisture variations generally follow changes in insolation with the longest and most intense humid period from 11.3 ka to 8.7 ka, supporting the concept of an early Holocene climatic optimum in the ASM margin. On millennial to centennial timescales, four sequences of long-term gradual wetting and subsequent abrupt drought are noted, i.e., thousands of years were required to reach a favorable environment in the ASM margin, while in only a few hundred years of drought would occur subsequently. Terminations of humid periods correspond to the occurrences of cold events in the North Atlantic realm, indicating an important role for the Atlantic Meridional Overturning Circulation (AMOC) on millennial to centennial timescales. We suggest solar insolation and the AMOC as the dominant factors on orbital timescale and millennial to centennial timescales, respectively. Both factors modulate moisture variations in the ASM margin by influencing the interplay between the ASM and the Westerlies.