Synchronous or asynchronous Holocene Indian and East Asian summer monsoon evolution: A synthesis on Holocene Asian summer monsoon simulations, records and modern monsoon indices

Holocene climate records obtained from the Asian summer monsoon domain suggest a regionally-delineated response to changing summer monsoon. The interaction between the East Asian summer monsoon (EASM) and the Indian summer monsoon (ISM), two subsystems of the Asian summer monsoon, has been considered as a factor that explains those inconsistent Holocene climate records. However, this assumption is not valid when the relationship between the two subsystems is not clear. This paper presents a literature review regarding climate simulation of the Asian summer monsoon for testing the long-term relationship. The absolutely-dated Holocene speleothem records in the EASM domain and the ISM domain were compared to verify the simulation results. In addition, a unified monsoon index, which has a unified solid dynamic basis and is appropriate for different monsoon regions, was used in order to identify the modern relationship between the two subsystems. The speleothem records show more synchronous than asynchronous on the Holocene millennial-scale monsoon evolution, furthermore the two subsystems respond to the Younger Dryas (YD) and 8.2 ka events in a similar way. However, these monsoon simulations roughly suggest that the two subsystems respond to Holocene climate change in different ways. While the simulations were mostly performed in a certain period of the Holocene, the speleothem records provided a relatively continuous Asian summer monsoon history. Therefore, time scales could affect the comparison between simulations and speleothem records. Then, we further discussed the interaction between the Asian monsoon subsystems according to simulations and modern monsoon indices. Overall, the relationship between the two subsystems is more complicated than synchronous or asynchronous, which is a dynamic relationship and related to the atmosphere–land–ocean–vegetation interaction. In addition, the relationship can vary over different time scales, and the links between time scales should be paid more attention to. Besides, the interaction between the westerly winds and the Asian summer monsoon in the mid-latitudes of East Asia will profoundly affect those areas in response to Holocene climate change. It is recommended that further research should be emphasized in dynamic mechanisms between the Asian summer monsoon subsystems and between the Asian monsoon and the westerly winds.