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) produ ction. Is this circulation change a cause or consequence of Pliocene warmin g? We test the hypothesis that increased strength of NADW was a consequence of the warming around Antarctica affecting deep Antarctic outflow. A sensi tivity experiment with an ocean general circulation model with Pliocene sur face conditions changed only over the Southern Ocean (SO) indicates that wa rmer temperatures around Antarctica result in lower rates of sea ice format ion and SO deep water outflow. The decreased abyssal density gradient in th e 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.