Heat transport, deep waters, and thermal gradients: Coupled simulation of an Eocene Greenhouse Climate

For the first time, a coupled general circulation model with interactive and dynamical atmospheric, oceanic, and sea-ice components, is used to simulate an Eocene (∼50 Ma) “greenhouse”climate. We introduce efficient ocean spin-up methods for coupled paleoclimate modeling. Sea surface temperatures (SSTs) and salinities evolve unconstrained, producing the first proxy data-independent estimates for these Eocene climate parameters. Tropical and extratropical model-predicted SSTs are warmer than modern values, by 3 and 5C, respectively. Salinity-driven deep water formation occurs in the North Atlantic and Tethys. The zonal average overturning circulation is weaker than modern. Eocene ocean heat transport is 0.6 PW less than modern in the Northern Hemisphere and 0.4 PW greater in the Southern Hemisphere. The model–predicted near–modern vertical and meridional Eocene temperature gradients imply that the dominant theory for maintaining low gradients—increased ocean heat transport—is incorrect or incomplete and other mechanisms should be explored.