Effect of the Drake Passage Throughflow on Global Climate

The role of the Southern Ocean in global climate is examined using three simulations with a coupled model employing geometries different only at the location of Drake Passage (DP). The results of three main experiments are examined: 1) a simulation with DP closed, 2) an experiment with DP at a shallow (690 m) depth, and 3) a realistic DP experiment. The climate with DP closed is characterized by warmer Southern Hemisphere surface air temperature (SAT), little Antarctic ice, and no North Atlantic Deep Water (NADW) overturn. On opening the DP to a shallow depth of 690 m there is an increase in Antarctic sea ice and a cooling of the Southern Hemisphere but still no North Atlantic overturn. On fully opening the DP, the climate is mostly similar in the Southern Hemisphere to DP at 690 m, but the model now simulates NADW formation and a warming in the Northern Hemisphere. This suggests the North Atlantic thermohaline circulation depends not only on the existence of a DP throughflow, but also on the depth of the sills in the Southern Ocean. The closed DP experiment exhibits a large amount of deep-water formation [57 Sv (Sv [ 106 m3 s21)] in the Southern Hemisphere; this reduces to 39 Sv for the shallow DP case and 14 Sv when DP is at 2316 m, its modern-day depth. NADW formation is shut down in both DP closed and shallow experiments, which accounts for the warming in the Northern Hemisphere observed when the DP is opened. SAT differences between the DP open and closed climate are seasonal. The largest SAT changes occur during winter in areas of large sea ice change. However, summer conditions are still significantly warmer when DP is closed (regionally up to 48C). Summer SAT is the most important factor determining whether an Antarctic ice sheet can build up. Therefore our study does not exclude the possibility that changes in ocean gateways may have contributed to the glaciation of Antarctica. Overall, these experimental results support paleoclimatic evidence of rapid cooling of the Southern Ocean region soon after the isolation of Antarctica.