The onset of Asian Monsoons: a modelling perspective

Abstract. The Cenozoic onset and development of the Asian monsoons remain unclear and have generated much debate, as several hypotheses regarding circulation patterns at work in Asia during the Eocene have been proposed in the last decades. These include a) the existence of modern-like monsoons since the early Eocene; b) that of a weak South Asian Monsoon (SAM) and little to no East Asian Monsoon (EAM) or c) a prevalence of the Inter Tropical Convergence Zone (ITCZ) migrations, also referred to as Indonesian-Australian Monsoon (I-AM). As SAM and EAM are supposed to have been triggered or enhanced primarily by Asian paleogeographic changes, their possible inception in the very dynamic Eocene paleogeographic context remains an open question, both in the modeling and field-based communities. We investigate here Eocene Asian climate conditions using the IPSL-CM5A2 earth system model and revised paleogeographies. Our Eocene climate simulation yields atmospheric circulation patterns in Asia substantially different from modern. A large high-pressure area is simulated over the Tethys ocean, which generates intense low tropospheric winds blowing southward along the western flank of the proto Himalayan Tibetan plateau (HTP) system. This low-level wind system blocks, to latitudes lower than 10° N, the migration of humid and warm air masses coming from the Indian Ocean. This strongly contrasts with the modern SAM, during which equatorial air masses reach a latitude of 20–25° N over India and southeastern China. Another specific feature of our Eocene simulation is the widespread subsidence taking place over northern India in the mid troposphere (around 5000 m), preventing deep convective updraft that would transport water vapor up to the condensation level. Both processes lead to the onset of a broad arid region located over northern India and over the HTP. More humid regions of high seasonality in precipitations encircle this arid area, due to the prevalence of the Inter Tropical Convergence Zone (ITCZ) migrations (or Indonesian-Australian Monsoon, I-AM) rather than monsoons. Although the existence of this central arid region may partly result from the specifics of our simulation (model dependence, paleogeographic uncertainties) and has yet to be confirmed by proxy records, most of the observational evidence for Eocene monsoons are located in the highly seasonal transition zone between the arid area and the more humid surroundings. We thus suggest that a zonal arid climate prevailed over Asia before the onset of Monsoons that most likely occurred following Eocene paleogeographic changes. Our results also show that precipitation seasonality should be used with caution to infer the presence of a monsoonal circulation and that the collection of new data in this arid area is of paramount importance to allow the debate to move forward.