Impact of mid-Pliocene boundary conditions on the Atlantic Meridional Overturning Circulation (AMOC)

The mid-Piacenzian warm period (mPWP, ~3.3 – 3 Ma) is the most recent geological period with a CO₂ concentration comparable to today (~400 ppm). Evidence from proxies and global climate model simulations both indicate that the Atlantic Meridional Overturning Circulation (AMOC) was stronger during the mPWP than it is now, contributing to warmer sea surface temperatures (SSTs) in the high-latitude North Atlantic. Using the coupled atmosphere-ocean model MIROC4m, we study how increased CO₂ levels, reduced ice sheets, and altered orography and vegetation influenced the mPWP AMOC. Our findings indicate that higher CO₂ concentrations and smaller ice sheets both weaken the AMOC with respect to the pre-industrial. The stronger mPWP AMOC is therefore a consequence of the combined orography and vegetation forcing, with closed Arctic gateways accounting for approximately 80% of this forcing.  The primary mechanism driving the AMOC strengthening was a reduction in freshwater transport from the Arctic to the North Atlantic, enhanced by a decrease in surface freshwater flux into the high-latitude North Atlantic. Additionally, we find that orography and vegetation changes explain approximately 85% of above-average SST warming in the subpolar North Atlantic during the mPWP, with nearly half  of this warming attributable to the closure of the Arctic gateways.