Early Eocene vigorous ocean overturning and its contribution to a warm Southern Ocean

The early Eocene (similar to 55 Ma) was the warmest period of the Cenozoic and was most likely characterized by extremely high atmospheric CO2 concentrations. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 Earth system model, set up with paleogeographic reconstructions of this period from the DeepMIP project and with different levels of atmospheric CO2. When compared with proxy-based reconstructions, the simulations reasonably capture both the reconstructed amplitude and pattern of early Eocene sea surface temperature. A comparison with simulations of modern conditions allows us to explore the changes in ocean circulation and the resulting ocean meridional heat transport. At a CO2 level of 840 ppm, the early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60 degrees S), fed by deepwater formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the buildup of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with subpolar gyres, sustains the poleward advection of saline subtropical water to the convective regions in the Southern Ocean, thereby maintaining deepwater formation. This salt-advection feedback mechanism is akin to that responsible for the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3-0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7PW southward at 20 degrees S, and contributes to keeping the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO2 levels show that ocean circulation and heat transport are relatively insensitive to CO2 doubling.