Orbital Effect on North Atlantic Freshwater Forced Millennial-Scale East Asian Summer Monsoon Variability During the Holocene

Understanding millennial-scale East Asian summer monsoon (EASM) variability is critical for climate projection in East Asia in the context of slowing Atlantic Meridional Overturning Circulation (AMOC) caused by global warming. This study performs two groups of sensitivity experiments at 8.2 and 4.2 ka BP, both forced by the same North Atlantic freshwater hosing and corresponding orbital parameters using the Community Earth System Model, to explore the orbital effect on millennial-scale EASM variability. Our simulations reveal that EASM variability differs significantly between these two cold events. During the 8.2 ka BP (4.2 ka BP) (0 BP = 1950 AD) cold event, North Atlantic freshwater forcing weakens the EASM, generating a meridional tripole (dipole) pattern of EASM precipitation anomalies. Furthermore, the same freshwater discharge causes a greater decline in the EASM, contrasting to a slight weaker AMOC slowdown, at 4.2 ka BP than at 8.2 ka BP. This suggests orbital parameters significantly influence the EASM response to North Atlantic cold events. The East Asian subtropical westerly jet, whose meridional position and intensity are controlled by summer insolation on an orbital timescale, plays a key role in this orbital effect. The westerly jet's meridional position determines the spatial pattern of EASM precipitation anomalies, while its intensity affects the amplitude of the EASM's response to freshwater forcing. Because of a larger EASM decline due to freshwater hosing with lower summer insolation, we anticipate a profound declining EASM precipitation in the future with low summer insolation if the AMOC continues to weaken.