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

The South Asian summer monsoon (SASM) significantly intensified during the Middle Miocene (17–12 Ma), but the driver to this change remains an open question. The uplift of the Himalaya (HM) and Iranian Plateau (IP), and global CO2 variation are prominent factors among suggested drivers. Particularly the impact of high CO2 on the Miocene SASM is little studied despite a large 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 the two factors are comparable in the core SASM region while in the western region, the topographic forcing is still the dominant driver. A thermodynamical process is proposed to link the uplift of the IP and enhanced SASM through latent heating release. Compared with reconstructions, the response of SASM to the IP uplift is in good agreement with observed precipitation and wind while the effects of the HM uplift and CO2 variation are inadequate to interpret the proxies.