Simulation of the climate and ocean circulations in the Middle Miocene Climate Optimum by a coupled model FGOALS-g3

The warmer-than-present (5-10 ℃) climate during the Miocene Climate Optimum (MMCO, approximately 16.9-14.7 Ma) is likely to serve as a reference for the future pessimistic warming scenarios. Forced with MMCO boundary conditions, the warming and ocean circulation changes are simulated by the fully-coupled climate model FGOALS-g3. Under 400 ppmv CO₂ concentration, the model generally simulates the MMCO temperature well with the small biases in the mid and low latitudes compared to proxy data. Large biases in the high latitudes show that FGOALS-g3 fails to reproduce the weak meridional gradient indicated by proxy. MMCO surface albedo decreases significantly owing to changes in worldwide forest cover in the boundary condition and the amount of sea ice melt due to the warming climate compared with the PI run. Accompanied by global ocean warming and land-sea distribution changes in MMCO, both oceanic wind-driven and thermohaline circulations strengthen. The intensified MMCO Atlantic Meridional Overturning Cell (AMOC) relative to PI is likely linked to the altered ocean-gateway configuration, particularly at low and middle latitudes. When the MMCO Panama Seaway and Tethys Seaway open, waters from the Pacific and the Indian Ocean converge and mix in the west of the North Atlantic. Joined by this water supplement (~30 Sv) from other ocean basins, the Gulf Stream enhances and flows more poleward, more heat and salinity are carried to the Subpolar North Atlantic. Consequently, the sea ice retreats and the Atlantic Meridional Overturning Cell enhances in the North Atlantic in the MMCO.