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The East Asian Monsoon (EAM) impacts storms, freshwater availability, wind energy production, coal consumption, and subsequent air quality for billions of people across Asia. Despite its importance, the EAM's long-term behavior is poorly understood. Here we present an annually resolved record of EAM variance from 1584 to 1950 based on radiocarbon content in a coral from the coast of Vietnam. The coral record reveals previously undocumented centennial scale changes in EAM variance during both the summer and winter seasons, with an overall decline from 1600 to the present. Such long-term variations in monsoon variance appear to reflect independent seasonal mechanisms that are a combination of changes in continental temperature, the strength of the Siberian High, and El NifioSouthern Oscillation behavior. We conclude that the EAM is an important conduit for propagating climate signals from the tropics to higher latitudes.

Plain Language Summary The monsoon systems across Asia are critical to the many Asian communities for which they supply freshwater and control winter severity. Even small changes in the amount of summer precipitation or strength of winter storms can have devastating effects. Our instrumental records of the monsoons are limited, and therefore so is our understanding of their behavior. Here, we use relative amounts of radiocarbon to reconstruct both the summer and winter East Asian Monsoon from 1584 to 1950. We find that there is significant variability in the monsoon system that appears to be influenced by both tropical and high-latitude climate systems, including a long-term decline in monsoon variability and an inverse relationship between the winter monsoon and the Siberian High at multidecadal frequencies. Both the summer and winter monsoon records indicate that the monsoon system may be a path of communication between the tropics and higher latitudes.