Impact of topographic change on the East Asian monsoon in Japan and Eastern Asia during the Last Glacial Maximum

The rainfall patterns in Eastern Asia are largely a consequence of the Asian winter and summer monsoons. On timescales 10 4 –10 6 of years, the position and strength of the monsoons change as a result of variations in Northern Hemisphere summer insolation. However, locally, it is possible there is a superimposed signal due to sea level change. Due to the growth of ice sheets, primarily in northern North America and Europe, global average sea level fell about 120 m during the Last Glacial Maximum (27,000–19,500 years ago). This caused the broad shallow continental shelf regions in coastal Eastern Asia to be exposed, and also greatly limited the flow of warm Pacific water into the Sea of Japan. In this study, we perform climate modelling experiments to isolate the impact of the lower sea level on the rainfall patterns in Eastern Asia. We find that the lowered sea level locally increases the summer precipitation, impacting the southern parts of Japan as well as Eastern China. The lower sea level also causes a decrease in winter precipitation in Japan. Our oxygen isotope modelling results show that the lower sea level alone is sufficient to explain the extremely depleted δ 18 O values measured from proxies in Last Glacial Maximum sediments in the Sea of Japan. These results show that the rainfall proxy records in the coastal Eastern Asia region, especially near the East China Sea, should not be interpreted as purely due to insolation driven changes to the monsoon front, as they also contain a local signal due to sea level changes. Further impacts of sea level changes on continental proxy records are likely limited, and they are measuring a purely global climate change signal.

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