Increased Pliocene North Atlantic Deep Water: Cause or consequence of Pliocene warming?

Raymo et al. [1996] suggested that the mid-Pliocene (similar to 3 Ma) warm period was associated with increased North Atlantic Deep Water (NADW) production. Is this circulation change a cause or consequence of Pliocene warming? We test the hypothesis that increased strength of NADW was a consequence of the warming around Antarctica affecting deep Antarctic outflow. A sensitivity experiment with an ocean general circulation model with Pliocene surface conditions changed only over the Southern Ocean (SO) indicates that warmer temperatures around Antarctica result in lower rates of sea ice formation and SO deep water outflow. The decreased abyssal density gradient in the SO directly leads to about a 20% increase in NADW outflow at 30 degrees S, a 10% increase in NADW overturning in the subpolar North Atlantic, and a 20% increase in poleward heat transport in the North Atlantic. We postulate that the largest initial Pliocene climate change was in the SO because the greater sea ice area in this region is more sensitive to inferred slightly higher CO2 levels in the mid-Pliocene.