New insight into pattern divergence of the Indian summer monsoon during the Holocene

As one of the paramount components of Asian monsoon and global atmospheric circulation system, the ISM (Indian summer monsoon) plays a vitally important role in the natural environment, ecological balance, and cultural development. Moreover, understanding the ISM could provide valuable insights into global hydrological, atmospheric, and carbon cycles. In recent decades, many ISM records based on different archives have been established within the Holocene, however, issues still remain: the TP (Tibetan Plateau) and the Indian subcontinent are often analyzed separately, despite the both are strongly influenced by the ISM, impeding comprehensive understanding of the monsoon and its object relationship with the East Asian summer monsoon (EASM). Here, covering the both regions, 65 published paleoclimate records within the Holocene have been collected, and conclusions are drawn as follow: (1) There are two evolution patterns of the ISM during the Holocene. The first pattern with the Holocene Optimum (HO) at ~10–6 ka BP, mainly distributing in the Arabian Sea and its surrounding areas, the Indian subcontinent, western central TP, northern TP, and southern China. The second pattern with the HO at ~8–3 ka BP, mainly distributing in the Bay of Bengal and its surrounding areas and southern TP. (2) The delay between the optimum of the second pattern and insolation maximum may be caused by the variation of high-latitude ice cover, although the underlying mechanism concerning the spatial distribution of the two patterns is still unclear. (3) The ISM and the EASM evolve synchronously, and share similar pattern divergence. Arguments over this issue mainly owe to the selected records originating from different patterns. This study makes up the gap in the pattern divergence of monsoon evolution, deepening the understanding of the ISM, even entire Asian monsoon system, and therefore is of great significance for future climate prediction.