Climate models predict that the Northern Hemisphere (NH) will warm faster than the Southern Hemisphere (SH) in response to increasing greenhouse gases, and observations show that this trend has already begun to occur. This interhemispheric asymmetry has largely been attributed to land-ocean differences between the hemispheres and Arctic sea ice melt, while the role of ocean currents in setting this asymmetry is less well understood. This study isolates the impact of an open Southern Ocean gateway upon the interhemispheric asymmetry in transient global warming by forcing a fully coupled climate model with an increasing CO2 scenario with and without a land bridge across Drake Passage (DP). It is found that over the transient warming period, the NH-SH surface warming asymmetry is reduced in the DP closed case, by approximately 41% for sea surface temperature and approximately 6% for surface air temperature. In the DP open case, sea ice extent is far greater in the SH than in the DP closed case, whereas the sea ice response to warming in the NH is insensitive to whether or not DP is closed. These results illustrate that part of the interhemispheric asymmetry in surface warming is due to the Antarctic Circumpolar Current (ACC) thermally isolating Antarctica. The ACC limits ocean heat transport across the DP latitudes and allows a much greater coverage of sea ice in the Southern Ocean than would be the case in the absence of a circumpolar ocean.