Non-stationary evolution of centennial to decadal East Asian summer monsoon variability modulated by trans-oceanic interactions over the last millennium

Centennial to decadal variability of the East Asian summer monsoon (EASM) and its mechanisms during the preindustrial era remain unclear, hindering predictions of future changes in a warm world. Here, we investigate the EASM variability over the last millennium by analyzing high-resolution and well-replicated δ 18 O records from three annually laminated stalagmites in eastern China. Segmented correlation analysis reveals a four-stage evolution in the spatial pattern of EASM rainfall, synchronized with Northern Hemisphere temperature changes. This evolution, analogous to modern observations, reflects meridional displacements of the monsoon frontal rainfall belt, which is dynamically linked to overall monsoon intensity and modulated by the interplay between Pacific and North Atlantic oceanic variability. Furthermore, decomposition and spectral analysis identify the first principal component (PC1 div ) of EASM variations, dominated by centennial to decadal variability, with a significant non-stationary temporal evolution. During both natural and anthropogenic warm periods, PC1 div is characterized by a 20- to 40-year periodicity, linked to the Pacific Decadal Oscillation (PDO). However, this PDO-EASM coupling weakened during the Little Ice Age (LIA), when PC1 div instead displayed a prominent ∼50-year periodicity linked to Atlantic Multidecadal Variability. Model simulations from the CESM Last Millennium Ensemble reveal that the ∼50-year North Atlantic variability during the LIA was predominantly driven by volcanic forcing, which enhanced the Atlantic-EASM teleconnection via atmospheric Rossby wave trains. Our findings highlight the critical role of the Pacific-Atlantic ocean variability in regulating the non-stationary evolution of EASM variability on centennial to decadal timescales.