Northward extension of the East Asian summer monsoon during the mid-Holocene

Previous studies based on multiple paleoclimate archives suggested that during the mid-Holocene (MH, 6000 years before present day), the East Asian summer monsoon (EASM) had a prominent intensification and northward extension. However, current climate model simulations with orbital forcing alone present an underestimation of the magnitude of changes in the EASM. In the current work, we show that considering a vegetated and dust-reduced Sahara in the MH can significantly strengthen the EASM intensity and expand its northernmost boundary northward compared to the results with orbital forcing alone. The vegetation change over the Sahara is the dominant factor for the variation in the EASM, while the dust reduction plays a smaller role. The vegetated Sahara causes a westward shift of the Walker circulation, accompanied with enhancement of the western Pacific subtropical high (WPSH), which then results in a strengthened EASM. On one hand, the change in the Walker circulation induces decreased rainfall over the western equatorial Pacific, intensifying the WPSH through the Gill-Matsuno response. On the other hand, the shift in the Walker circulation is associated with a stronger local Hadley circulation, reinforcing the WPSH. Finally, our results show that the westward expansion of the WPSH is mainly caused by the local strengthening of the Hadley circulation.