South Asian summer monsoon enhanced by the uplift of the Iranian Plateau in Middle Miocene

Abstract. The South Asian summer monsoon (SASM) significantly intensified during the Middle Miocene (17–12 Ma), but the driver of this change remains an open question. The uplift of the Himalaya (HM) and the Iranian Plateau (IP) and global CO2 variation are prominent factors among suggested drivers. Particularly, the impact of high CO2 levels on the Miocene SASM has been little studied, despite the wide range of reconstructed CO2 values around this period. Here we investigate their effects on the SASM using the fully coupled Ocean–Atmosphere Global Climate Model, CESM1.2, through a series of 12 sensitivity experiments. Our simulations show that the IP uplift plays a dominant role in the intensification of the SASM, mainly in the region around northwestern India. The effect of the HM uplift is confined to the range of the HM and its vicinity, producing orographic precipitation change. The topography forcing overall out-competes CO2 variation in driving the intensification of the SASM. In the case of extremely strong CO2 variation, the effects of these two factors are comparable in the core SASM region, while in the western region, the topographic forcing is still the dominant driver. We propose a thermodynamical process linking the uplift of the IP and the enhanced SASM through the release of latent heat. When compared with reconstructions, the simulated response of SASM to the IP uplift is in good agreement with observed precipitation and wind field, while the effects of the HM uplift and CO2 variation are inadequate to interpret the proxies.