Variations and mechanisms of the Asian summer monsoon revealed by stalagmite δ¹⁸O records

The Asian summer monsoon is an important part of the global climate system and a hot issue of the earth system science. The variation of the Asian paleo-monsoon revealed by δ¹⁸O data from the Asian stalagmites deepens our understanding of the mechanisms of its spatiotemporal evolution. However, the main controlling factors of the stalagmite δ¹⁸O records in the East Asian and Indian summer monsoon regions in different time scales remain controversial. Aiming at this problem, we reviewed the high-resolution stalagmite δ¹⁸O records in the East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) regions and revealed that they are in response to the variation in the Northern Hemisphere summer insolation driven by the precession on orbital scale. The specific mechanism involves the variation of zonal sea-land thermal contrast, seasonal humidity changes and cycles in low-level convergence, and the variation of summer length. These mechanisms jointly lead to increased summer rainfall with depleted δ¹⁸O in the EASM and ISM regions during high Northern Hemisphere summer insolation. On millennial scale, the stalagmite δ¹⁸O records in EASM and ISM regions are in response to the abrupt North Atlantic climate changes consistently, by mainly the weakening of water vapor fractionation in the Indian Ocean due to southward migration of the intertropical convergence zone (mainly influenced the ISM and EASM regions) and seasonal variation of water vapor sources and rainfall amount regulated by the westerly (mainly influenced the EASM region). Finally, at the centennial or shorter scale, the stalagmite δ¹⁸O records in both EASM and ISM regions are influenced by large-scale atmospheric circulation associated with EI Niño-Southern Oscillation. In the future, the development of more high-resolution stalagmite δ¹⁸O and other rainfall indicators records in key areas would facilitate our understanding of the relationship between atmospheric circulation and rainfall changes in the Asian summer monsoon region.