Mechanisms of East Asian winter monsoon response to North Atlantic freshwater forcing: Pivotal role of the Tibetan Plateau

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.