The origin of the modern-like East Asian Monsoon: insights from new data synthesis and climate modelling

It is debated as to whether the modern-like East Asian monsoon formed during the late Oligocene–early Miocene or the Eocene. To resolve this dispute requires a comprehensive and updated synthesis of the available geological records and a reliable modelling study. Here, we investigate Cenozoic climate patterns over East Asia by compiling geological records and conducting climate modelling for key geological periods based on our improved paleogeographies. Geological records suggest that a zonal (semi-)arid climate pattern was dominant over tectonic timescales during most of the Paleogene in large areas of East Asia, with marked fluctuations between dry and wet conditions over orbital timescales, and a modern-like monsoon-dominated climate pattern has formed since the late Oligocene–early Miocene (ca. 28–22 Ma). Our simulations show that a zonal dry belt extended across East Asia during the late Eocene, and a monsoon-dominated pattern had already formed over East Asia by the early Miocene. In addition, our simulations further indicate a strong sensitivity of East Asian rainfall to orbital forcing, which can explain the seemingly unstable character (i.e., wet–dry fluctuations) of the dry belt across East Asia during the Eocene. Furthermore, our results suggest that paleogeographic changes, particularly uplift of the Tibetan Plateau to moderate–high elevations and its paleolatitude approaching present-day location during the late Oligocene–early Miocene, rather than atmospheric CO₂ levels, played a crucial role in the establishment of the modern-like East Asian monsoon.