Ocean Meridional Overturning Circulation During the Early and Middle Miocene

The Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO2 concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high-latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO2, and vegetation. The strength of the Southern Ocean-driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre-Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC.<p></p>