Reconciling South Asian Monsoon Rainfall and Wind Histories

Cenozoic evolution of South Asian Monsoon and mechanisms driving changes recorded in the geological record remain highly debated. An intensification of monsoonal rainfall recorded in sediment archives from the earliest Miocene (23-20 million years ago, Ma) is generally attributed to Himalayan uplift. However, Indian Ocean paleorecords place the onset of strong monsoons around 13 Ma, linked to strengthening of the Somali Jet that forces Arabian Sea upwelling.  In this contribution we reconcile these divergent records using Ocean-Atmosphere and ocean biogeochemistry models. Our results show that factors forcing monsoon circulation versus rainfall are decoupled and diachronous : Asian topography predominantly controlled early Miocene rainfall patterns, with limited impact on ocean-atmosphere circulation. Yet the uplift of East African and Middle Eastern topography played a pivotal role in the establishment of modern Somali Jet structure above the western Indian Ocean, while strong upwelling initiate in response to the emergence of the Arabian Peninsula. Our results emphasize a polygenetic history of the South Asian Monsoon with multiple paleogeographic controls: although elevated rainfall seasonality was likely a persistent feature since the India-Asia collision in the Paleogene, the modern-like monsoonal atmospheric circulation was only reached recently, in the late Neogene.