Over geological time the placement of the Earth’s continents is altered by tectonic activity such that the geometry of the oceans also changes. This can lead to changes in ocean topology, as the connectivity between basins alters, and new ocean currents and dynamics can emerge. One of the most prominent examples of this is the Antarctic Circumpolar Current (ACC), the modern ocean's strongest current and its major zonal connection between ocean basins. However, the date of inception of the ACC is debated due to uncertainty in the relative opening times of Drake Passage and the Tasman Seaway. These debated dates, some ~50Ma to ~30Ma, place the origin of the ACC within the time frame of large-scale climatic change with decreasing atmospheric carbon dioxide levels and general cooling of global climate. How the changing of the Southern Ocean’s circulation might be related to this larger climate shift is difficult to assess without a proper understanding of when the ACC began to flow and how its transport of ocean properties might have altered over time.
Using an idealised eddy-resolving numerical ocean model, we investigate whether both Drake Passage and the Tasman Seaway have to be open to allow for a substantial circumpolar current. We find that overlapping continental barriers do not impede a circumpolar transport in excess of 50 Sv, as long as a circumpolar path can be traced around the barriers. However, the presence of overlapping barriers does lead to an increased sensitivity of the current’s volume transport to changes in wind stress. This change in sensitivity is interpreted in terms of the role of pressure drops across continental barriers and submerged bathymetry in balancing the momentum input by the surface wind stress. Specifically, when the pressure drop across continents is the main balancing sink of momentum, the zonal volume transport is sensitive to changes in wind stress. Changes in zonal volume transport take place via altering the depth-independent part of the circumpolar transport rather than that arising from thermal wind shear. In such a scenario, isopycnals continue to slope steeply across the model Southern Ocean, implying a strong connection between the deep and surface oceans.