Effect of Ocean Gateway Changes under Greenhouse Warmth

The role of tectonic Southern Ocean gateway changes in driving Antarctic climate change at the Eocene– Oligocene boundary remains a topic of debate. One approach taken in previous idealized modeling studies of gateway effects has been to alter modern boundary conditions, whereby the Drake Passage becomes closed. Here, the authors follow this approach but vary atmospheric pCO2 over a range of values when comparing gateway configurations. They find a significantly greater sensitivity of Antarctic temperatures to Southern Ocean gateway changes when atmospheric pCO2 is high than when concentrations are low and the ambient climate is cool. In particular, the closure of the Drake Passage (DP) gap is a necessary condition for the existence of ice-free Antarctic conditions at high CO2 concentrations in this coupled climate model. The absence of the Antarctic Circumpolar Current (ACC) is particularly conducive to warm Antarctic conditions at higher CO2 concentrations, which is markedly different from previous simulations conducted under present-day CO2 conditions. The reason for this is the reduction of sea ice associated with higher CO2. Antarctic sea surface temperature and surface air temperature warming due to a closed DP gap reach values around ;58 and ;78C, respectively, for high concentrations of CO2 (above 1250 ppm). In other words, the authors find a significantly greater sensitivity of Antarctic temperatures to atmospheric CO2 concentration when the DP is closed compared to when it is open. The presence of a DP gap inhibits a return to warmer and more Eocene-like Antarctic and deep ocean conditions, even under enhanced atmospheric greenhouse gas concentrations.