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The East Asian summer monsoon (EASM) is a key component of the Asian Monsoon system affecting regional precipitation in East Asia and thus human culture, society, and development. Despite this, decadal- to centennial-scale EASM dynamics remain poorly understood over the last millennium. Here we used high-resolution benthic microfossil proxies to reconstruct EASM variability for the last 1,000 years from a brackish lake in South Korea. We compared this EASM record with Asian Monsoon proxy records from East China (i.e., continental proxy) and East Asian coasts (i.e., oceanic setting), and identified four pervasive EASM strengthening events at 1250, 1450, 1550, and 1900 CE and their deviation, likely related to land-ocean temperature gradients. Our results indicate that data from the oceanic setting under the direct influence of the Pacific High are important in understanding the EASM dynamics over the last millennium. It is also noteworthy that EASM variability shows striking similarity to changes in the Atlantic meridional overturning circulation, implying the causal relationship.

Plain Language Summary The East Asian summer monsoon (EASM) is a determining factor of regional precipitation in East Asia that subsequently affects socioeconomic activities of regional populations. However, spatiotemporal trends of decadal- to centennial-scale EASM strength over the last millennium remain poorly understood, even with precisely dated Chinese stalagmite records. Here we use microfossil proxies in a brackish lake sediment core from the East Asian coast to better understand EASM dynamics for the last 1,000 years. We identified four pervasive EASM strengthening events at 1250, 1450, 1550, and 1900 CE and found that oceanic and continental settings could partially explain spatial differences in Asian summer monsoon trends at this scale. In addition, we found that global ocean circulation was weakened during these EASM events. Our results help to better understand Asian monsoon dynamics in the past, present, and future in this rapidly changing planet.