The variations in the East Asian summer monsoon over the past 3 kyrs and the controlling factors

The mechanisms driving the variations in the centennial-scale East Asian summer monsoon (EASM) remain unclear. Here, we use the δ¹⁸O records from adult ostracode shells to reconstruct the EASM variations over the last 3 kyrs in southwestern Japan. A common variation with a 200 yr periodicity among the Asian monsoonal regions was recognized between BC 800 and BC 100. Since then, neither a correlation between the EASM variation and solar activity or a common EASM variation through EASM regions has been identified. The evidence reveals that solar activity dominantly affected the centennial-scale EASM variations throughout Asian monsoonal regions until BC 100. Furthermore, factors other than solar activity that varied and differed in specific regions controlled the EASM intensity due to decreasing summer solar insolation in the Northern Hemisphere after BC 100. These relations indicate that the dominant factor that affects the EASM variations shifts according to the solar insolation intensity.