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 & DEG;C) climate during the Miocene Climate Optimum (MMCO, approximately 16.9-14.7 Ma) is likely to serve as a reference for 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 with a nominal 1 & DEG; horizontal resolution ocean component model. Under a 400 ppmv CO2 concentration, the model generally simulates the MMCO temperature well with small biases at mid and low latitudes compared to the proxy data. Large biases at high latitudes show that FGOALS-g3 fails to reproduce the weak meridional gradient indicated by the proxy record. 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. Based on the Energy Balance Model decomposition, warming by the lower surface albedo reaches 1.4-2.7 & DEG;C, which is comparable to greenhouse effect warming (-2.7 & DEG;C). Accompanied by MMCO global ocean warming and land-sea distribution changes, both oceanic wind-driven and thermohaline circulations strengthen. The Antarctic Circumpolar Current in the MMCO is stronger due to the enhanced westerly wind stress and the reduced sea ice extent. 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 western North Atlantic. Combined with the supplemental water (-30 Sv), the Gulf Stream is enhanced and flows more poleward, causing the sea ice to retreat, leading to a deeper mixed layer. Consequently, the Subpolar North Atlantic salinification causes stronger convective motion and the appearance of the Atlantic Meridional Overturning Cell.