The ocean circulation in the southern hemisphere and its climatic impacts in the Eocene

Opening of Southern Ocean gateways may have had a controlling influence on the evolution of global climate. The largest step in the history of Cenozoic climate, from an Eocene “Greenhouse” toward the modern glaciated state, is correlated with opening of a deep water connection between the Southern Indian and Southern Pacific Oceans. This study synthesizes results from previous work, assesses the possibility that ocean heat transport was much greater than modern during the globally warm Eocene, and explores the sensitivity of polar climate to large changes in ocean heat transport. The magnitudes of these sensitivities are compared with those due to greenhouse gas concentration changes. We find that ocean heat transport was unlikely to have been sufficiently greater than modern during the Eocene to explain the change in the climate. Specifically, in order to have significantly affected Antarctic climate, ocean heat transport changes would have had to change by an order of magnitude—much greater than the largest change produced in our simulations. In coupled climate model simulations, changes in CO2 concentrations(from 1120 to 560 ppm) supported by proxy records, lead to significant Antarctic responses in the absence of ocean heat transport changes. The results show that Antarctic snow accumulation is primarily controlled by changes in summer temperature, not precipitation, and that snow did not remain on Antarctica during summer months in Eocene even at low greenhouse gas concentrations. Greenhouse gas concentration changes and subsequent processes such as ice albedo and weathering feedbacks, were more likely to have been the primary cause of Antarctic glaciation than ocean heat transport changes.