Understanding the causes and consequences of the northward extension of the tropical monsoon in Asia in the Eocene.

Previous geological studies have revealed that a zonal climate pattern with an arid zonal band prevailed in the mid-latitude of East Asia in the Paleogene. On the south of the arid band, a tropical monsoon climate controlled South China, Indochina, and India. In the Eocene, the boundary between the arid and the tropical monsoon climate largely moved northward. However, the reason behind this change remains unclear. Here, with the fast version of the Norwegian Earth System Model (NorESM1-F), we conduct experiments under different Eocene paleogeography conditions to analyze the evolution of the tropical Asian monsoon from the early Eocene to the late Eocene. Our results demonstrate that the northward movement of Indian plate causes a reorganization in low-level westerlies in summer. After the full merging of the India-Asia continents, modern-like low-level tropical circulations come into being and lead to the wetting in southern China. • The East Asian arid band moved northward during the Eocene. • Indian plate's northward movement reorganized low-level southwesterlies in the Eocene. • Modern-like low-level tropical circulations formed in the late Eocene. • Wetting in southern China does not indicate a monsoon climate in the late Eocene. [ABSTRACT FROM AUTHOR]