Did monsoon govern the Asian rainy season in the early Eocene? An ensemble paleoclimate simulation perspective.

The Asian summer monsoon (ASM) is a seasonal response of the coupled land-ocean-atmospheric system, which influences more than 60% of the world’s population. Although progress has been made in understanding the ASM variability and its prediction, the timing and governing factors for the ASM initiation are still debatable as recent proxy evidence and modeling studies suggested the initiation of a wet-dry monsoonal climate from the Cretaceous period (145 million years ago, Ma) to the early Miocene or late Oligocene epoch, ∼25-22 Ma. Capitalizing on an ensemble of paleoclimate simulations for the early Eocene (56-48 Ma), we show that the Asian wet season was considerably weaker and shorter than present in the absence of an elevated heat source like the Tibetan Plateau in the early Eocene. The deficient upper tropospheric meridional temperature gradient couldn’t drive the seasonal northward migration of the precipitation band over South Asia. Additionally, the weaker cross-equatorial moisture flow was mechanically blocked by the Gangdese mountain along the southern edge of Asia, leading to significantly dry conditions in South Asia. The enhanced atmospheric greenhouse gases were inadequate to strengthen the seasonal circulation and precipitation variability to the present level. We argue that an altered wet and dry seasonality over South Asia was not necessarily qualified as the Eocene ‘monsoon’.