The spatial–temporal evolution of the Asian summer monsoon during the late Miocene and potential CO2 forcing: A data–model comparison

The long-term evolution of the Asian summer monsoon and its drivers are important for understanding future Asian summer monsoon variations. Due to inconsistencies among proxy data and/or different interpretations, controversies remain regarding the long-term evolution and drivers of the East and South Asian summer monsoons (EASM and SASM) during the late Miocene. There are two main viewpoints involving an overall strengthening or weakening trend, which is typically attributed to uplift of the Himalaya–Tibetan Plateau or global cooling. A detailed synthesis of paleoenvironmental reconstructions is required to understand the evolution of the EASM and SASM during the late Miocene. Here, we compiled paleoenvironmental reconstructions for the EASM and SASM during the late Miocene and used numerical simulations to investigate their evolution and potential drivers. The synthesis of the paleoenvironmental reconstructions indicates that the late Miocene climate: (1) underwent an overall drying trend in northern China, but a wetting trend in the South China Sea and surrounding areas; and (2) became progressively drier on the northern Indian subcontinent, but not on the southern Indian subcontinent. The modeling results indicate that: (1) EASM circulation overall weakened, whereas SASM circulation weakened (strengthened) in the northern (southern) part of South Asian monsoon domain; (2) summer precipitation decreased (increased) in the northern (southern) part of East and South Asian monsoon domains, which is roughly agreement with the paleoclimate records. Our results suggest that a decline in atmospheric CO2 may have been a key driver of the evolution of the EASM and SASM during the late Miocene.