Dynamics associated with a stronger mid-Pliocene Atlantic Meridional Overturning Circulation in PlioMIP2

Sea surface temperature (SST) proxies of the mid-Pliocene warm period (3.264-3.025 Ma) indicate amplified warming over the North Atlantic with respect to the pre-industrial period, which may be linked to an intensified Atlantic Meridional Overturning Circulation (AMOC) in the mid-Pliocene. Earlier results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that all models in the PlioMIP2 ensemble simulate a stronger AMOC in the mid-Pliocene than in the pre-industrial. However, no consistent relationship has been found between the stronger mid-Pliocene AMOC and either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs.

In this study, we look further into the drivers and consequences of a stronger AMOC in mid-Pliocene compared to pre-industrial simulations in PlioMIP2. We find that all model simulations with a closed Bering Strait and Canadian Archipelago show reduced freshwater transport from the Arctic Ocean into the North Atlantic. The resulting increase in salinity in the subpolar North Atlantic and Labrador Sea drives the stronger AMOC in the mid-Pliocene. To investigate the ensemble’s variable response of the total Atlantic OHT to the stronger AMOC, we separate the Atlantic OHT into two components associated with either the overturning circulation or the wind-driven gyre circulation. While the ensemble mean of the overturning component is increased significantly in magnitude in the mid-Pliocene, it is partly compensated by a reduction of the gyre component in the northern subtropical gyre region. This reduction originates from a zonal asymmetry in mid-Pliocene warming in the subtropical North Atlantic, rather than through changes in atmospheric forcing of the gyre. Our results indicate that these components should be considered separately to gain a more complete understanding of the Atlantic OHT response to a stronger mid-Pliocene AMOC. In addition, we show that the AMOC exerts a stronger influence on North Atlantic SSTs in the mid-Pliocene than in the pre-industrial, providing a possible explanation for the improved agreement of the PlioMIP2 ensemble mean SSTs with reconstructions in the North Atlantic.