Drivers and consequences of a stronger mid-Pliocene Atlantic Meridional Overturning Circulation

The mid-Pliocene warm period (mPWP, ~3.3 – 3 Ma) is the most recent geological period with a CO₂ concentration similar to the present day (~400 ppm). The Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) focuses on the KM5c time slice (3.205 Ma), giving insight into the climate dynamics of this period. Sea surface temperature (SST) proxies indicate amplified warming over the North Atlantic in the mPWP with respect to the pre-industrial period, which may be linked to an intensified Atlantic Meridional Overturning Circulation (AMOC). Zhang et al. (2021) reported a stronger mPWP AMOC in all the PlioMIP2 simulations but found no consistent relation to either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs. We therefore look further into the drivers and consequences of a stronger AMOC in the mPWP compared to pre-industrial simulations.

Within the PlioMIP2 ensemble, we find that all model simulations with a closed Bering Strait and Canadian Archipelago show strongly reduced freshwater transport from the Arctic Ocean into the North Atlantic. The resulting increase in sea surface salinity in the subpolar North Atlantic and Labrador Sea stimulates deepwater formation in these areas. The stronger AMOC is therefore primarily a response to the closure of the Arctic gateways. We also look at the different components of the Atlantic OHT, 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 mPWP, it is partly compensated by a reduced gyre component. Our results point towards a complex interplay between atmospheric and oceanic processes and indicate that considering these components separately allows for a better understanding of the climatic response to the AMOC strength.