Unraveling the mechanisms and implications of a stronger mid-Pliocene AMOC in PlioMIP2

The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period in which the atmospheric CO 2 concentration was approximately equal to the concentration we measure today (ca. 400 ppm). Sea surface temperature (SST) proxies indicate above-average warming over the North Atlantic in the mid-Pliocene with respect to the pre-industrial period, which may be linked to an intensiﬁed Atlantic Meridional Overturning Circulation (AMOC). Earlier results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the ensemble simulates a stronger AMOC in the mid- Pliocene than in the pre-industrial. However, no consistent relationship between the stronger mid-Pliocene AMOC and either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs has been found. 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 10 Labrador Sea drives the stronger AMOC in the mid-Pliocene. To investigate the dynamics behind 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 indicates that the lack of relationship between the total OHT and AMOC is due to 15 changes in OHT by the subtropical gyre. The overturning and gyre components should therefore be considered separately to gain a more complete understanding of the 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. we can see that the MMM freshwater transport increase of 0.021 Sv (+11%) across the GSR in the mid-Pliocene is relatively small and compensates a third of the decrease in freshwater transport into the Labrador Sea. In addition, the box plots indicate that the decrease in freshwater transport into the Labrador Sea is a more consistent feature among the models than the increase of freshwater transport 345 across the GSR. These results are in line with the decrease of the MMM total freshwater transport from meridional temperature gradient at the surface would induce a weakening in the subtropical gyre circulation and thus a decrease in of 505 the Atlantic OHT gyre component. While this seems to be a plausible mechanism for explaining the opposite behavior of the overturning and gyre OHT component, our results do not show that this mechanism plays role in the response of the PlioMIP2 ensemble as there are no substantial changes in the gyre circulation itself. It appears that changes in the ocean temperature and salinity ﬁelds are responsible for altering the OHT and freshwater transport by the subtropical gyre. We observe that between 20 � N and 40 � N, the eastern part of the Atlantic basin becomes relatively warmer and saltier than the western in the mid- 510 Pliocene. This results in the gyre circulation transporting relatively more heat and salt southwards The relatively high originate from the even warmer and saltier mid-Pliocene subpolar North related to AMOC Arctic gateway closure, respectively. The warm and salty water in the subpolar North Atlantic is transported eastwards by the northern branch of the subtropical gyre and subsequently transported southwards, resulting in the zonal asymmetry we It is possible 515 that the weakened subpolar gyre circulation plays a role in increased eastward transport of the warm and