Unraveling weak and short South Asian wet season in the Early Eocene warmth

The timing and causative factors underlying the Asian summer monsoon initiation remain contentious as recent proxy data and modeling studies suggested the existence of a wet-dry monsoon-like climate from the Cretaceous period. Leveraging an ensemble of deep-time climate simulations focused on the early Eocene (DeepMIP-Eocene), we show that the early Eocene Asian wet season was notably weaker and shorter than present-day in the absence of an elevated heat source like the Tibetan Plateau. The deficient upper-tropospheric meridional temperature gradient was insufficient to propel the seasonal northward migration of the rainband over South Asia. The weaker cross-equatorial moisture flow encountered obstruction of Gangdese mountain along the southern edge of Asia, leading to significant South Asian aridity. The enhanced greenhouse effect was inadequate to augment the seasonal circulation and rainfall variability to current levels. The altered wet and dry seasonality across South Asia during the early Eocene does not meet the criteria to be classified as a monsoon, suggesting that South Asian monsoon initiation is likely contingent upon a specific level of Tibetan Plateau upliftment. The Asian wet season in the Early Eocene was notably shorter and weaker compared to the present-day Asian summer monsoon, primarily attributed to the absence of the Tibetan highland as a heat source, according to DeepMIP-Eocene multi-model ensemble paleoclimate simulations.