Impact of Mid-Pliocene Boundary Conditions on the Atlantic Meridional Overturning Circulation (AMOC)

We use the coupled atmosphere-ocean model MIROC4m to investigate the effect of mid-Pliocene boundary conditions on the Atlantic Meridional Overturning Circulation (AMOC), studying the impact of increased CO₂, reduced ice sheets and altered orography and vegetation. We find that a higher CO₂ concentration and smaller ice sheets both weaken the AMOC with respect to the pre-industrial. The stronger mid-Pliocene AMOC is therefore a consequence of mid-Pliocene orography and vegetation, where the closed Arctic gateways are responsible for approximately 80% of the AMOC strengthening. The main mechanism for mid-Pliocene AMOC strengthening is reduced transport of freshwater from the Arctic into the North Atlantic, enhanced by a decrease of surface freshwater flux into the high-latitude North Atlantic.