Opposite terrestrial and marine South Asian summer monsoon rainfall response at Mid-Holocene

The South Asian summer monsoon (SASM) rainfall exerts a profound influence on the densely populated region and its surrounding oceans. Giving synchronous rainfall changes across the South Asian continent (SAC) and the Bay of Bengal (BoB) in the current climate, both terrestrial and marine proxies are widely used to reconstruct the past SASM variations. However, earlier studies based on proxy records and climate models have suggested an increased rainfall over SAC and a decreased rainfall over BoB at the mid-Holocene, challenging the traditional view of synchronous rainfall variation between SAC and BoB.

Using the TraCE-21ka simulation and PMIP4 models, this study delves into the underlying mechanisms of the opposite land-ocean rainfall response in MH, underscoring the crucial role of North Africa rainfall anomaly in remotely regulating the BoB rainfall. During the boreal summer, enhanced insolation amplified the land-sea thermal gradient, which further intensifies the southwest monsoon and thus leads to an increased SAC rainfall. Conversely, the reduction in BoB rainfall can be attributed to the weakness of monsoon trough circulation triggered by increased rainfall over North Africa. The North African rainfall anomaly excites Kelvin waves to its east, consequently resulting in anomalous easterlies over the northern Indian Ocean and anticyclonic circulation over the BoB. These findings not only deepen our grasp of the SASM system, but provides essential insights for interpreting the marine sedimentary records and projecting the future SASM responses.