Distinct Roles of Atlantic Multidecadal Variability and External Forcings on East Asian Summer Precipitation

Abstract Both the Atlantic multidecadal variability (AMV) and external forcings are recognized as key drivers of interdecadal variations in East Asian summer precipitation. However, considerable debate persists regarding the specific contributions of the AMV and external forcings to these variations. To address this issue, we analyzed both the large ensemble historical all-forcing twentieth-century experiments and two types of North Atlantic pacemaker experiments—one with time-evolving external forcings and one with fixed 1920 forcings. Our analysis demonstrates that AMV is a dominant driver of interdecadal variations in East Asian summer precipitation. In particular, a negative AMV-like North Atlantic cooling weakened the East Asian summer monsoon, leading to the late 1960s shift through two atmospheric pathways: a midlatitude Rossby wave train and an equatorial Kelvin wave. The external forcings have contributed to a drying trend along the Yellow and Huaihe River valleys since the 1960s, when the anthropogenic forcings have been strong. The shift observed in the late 1990s, characterized by a northward migration of the precipitation belt, was driven by both the positive AMV phase and the external forcings. Significance Statement East Asian summer precipitation has exhibited pronounced interdecadal variability since the twentieth century, significantly affecting local ecosystems, water resources, and industries. This variability is influenced by multidecadal oceanic variability, such as the Atlantic multidecadal variability (AMV), and external forcings, though distinguishing their impacts has been challenging. By analyzing multitype large ensemble and pacemaker simulations, we demonstrate that both AMV and external forcings regulate interdecadal shifts in East Asian summer precipitation. The negative AMV phase led to the late 1960s shift, while the late 1990s shift was influenced by both the positive AMV phase and external forcings. Our findings contribute to the long-standing debate regarding the roles of oceanic variability and external forcings in the interdecadal variation of East Asian summer precipitation.