Has the Drake Passage Played an Essential Role in the Cenozoic Cooling?

The Drake Passage is the seaway between South America and Antarctica. It is widely believed that the thermal isolation effects caused by the opening of the Drake Passage played an important role in the abrupt cooling that occurred at the Eocene-Oligocene boundary in the Cenozoic. These effects are also thought to be independent of the geometry of the passage. Here, the authors demonstrate that the climate impacts of the Drake Passage depend on the passage geometry by comparing the climate's sensitivity to the opening of the Drake Passage under the present and the Early Eocene land-sea configurations. These experiments show that the thermal isolation effects caused by the passage are much stronger under the present land-sea configuration. In comparison, under the Early Eocene land-sea configuration, the weak anomalies in heat transport caused by the opening of the narrow and shallow Drake Passage are not strong enough to thermally insulate Antarctica. The climate effects of the Drake Passage on the Cenozoic cooling have been overestimated in previous sensitivity studies carried out using the present land-sea configuration. Thus, it is unlikely that the opening of the Drake Passage played an essential role in the abrupt Cenozoic cooling, especially in the abrupt cooling at the Eocene-Oligocene boundary.

Effect of opening the Drake Passage on the oceanic general circulation:A box model study

The impacts of opening the Drake Passage(DP) on the oceanic general circulation are examined.When the DP is open,wind stress at mid-and high latitudes gives rise to a wind-driven gyre,which induces a meridional heat exchange between mid-and high latitudes in the Southern Ocean.After the opening of the DP,the Antarctic Circumpolar Current(ACC) forms and its associated strong temperature front blocks the heat transport from mid-latitudes to high latitudes.A simple box model is formulated,in which the effects of the wind stress(for the case of DP closed) and the thermal front(for the case of DP open) on the variability of Antarctic Bottom Water(AABW) and North Atlantic Deep Water(NADW) are explored.The sensitivity experiments demonstrate that:(1) When the DP is closed,the enhancement of the wind-driven gyre leads to the decline of AABW formation in the Southern Ocean and the increase of NADW formation in the North Atlantic.As a result,water in high latitudes of the Southern Ocean becomes warmer,so does the bottom water of global ocean.(2) When the DP is open,there is no formation of AABW until the intensity of thermal front along ACC exceeds a threshold value(it is 4.03℃ in our model).Before the formation of AABW,temperature in most of the oceans is higher than that after the formation of AABW,which usually leads to the cooling of high latitudes of the Southern Hemisphere and the bottom water in global ocean.When the strength of the thermal front is lower than the critical value,there is no AABW formation,and temperature in most of the oceans is slightly higher.These results demonstrate that during the opening of the DP,changes in wind stress and the formation of the thermal front in the Southern Ocean can substantially affect the formation of AABW and NADW,thus changing the state of meridional overturning circulation in the global ocean.