Impact of the Tibetan Plateau and Rocky Mountains on deep ocean circulation during the Middle Miocene

Abstract. The stages of deep ocean circulation during the Miocene remain uncertain, but palaeogeographic changes likely played a role in its evolution to the Atlantic-dominated overturning state of today. Regarding orographic changes, the uplift of the Tibetan Plateau is thought to be conducive to a shift from Pacific to Atlantic deep water formation, with flattening of the Rocky Mountains having the opposite effect. Here, we perform a suite of fully coupled climate model simulations using the Community Earth System Model version 1.2 (CESM1.2) with middle Miocene palaeogeography to assess the impact of orographic and CO₂ forcing on the meridional overturning circulation (MOC). The control simulation exhibits a strong Pacific MOC (PMOC; ~20 Sv) and no Atlantic MOC (AMOC). Flattening or widening the Tibetan Plateau has no significant effect on either basin's overturning. Flattening the Rocky Mountains strengthens the PMOC by 4 Sv by altering freshwater routing into the Arctic, but does not affect AMOC. A lower CO₂ simulation (2x PI CO₂) yields no qualitative change in overturning structure, indicating that the PMOC is not solely a consequence of warm climate forcing. Our results suggest that Miocene orography influenced Pacific deep water formation but was insufficient to trigger a shift toward modern-like AMOC conditions. This implies that other tectonic and ocean gateway changes were likely necessary to enable the development of the modern Atlantic-dominated overturning regime.