Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2

Atmospheric CO2 concentrations over glacial-interglacial cycles closely correspond to Antarctic temperature patterns(1). These are distinct from temperature variations in the mid to northern latitudes(2), so this suggests that the Southern Ocean is pivotal in controlling natural CO2 concentrations(3). Here we assess the sensitivity of atmospheric CO2 concentrations to glacial-interglacial changes in the ocean's meridional overturning circulation using a circulation model(4,5) for upwelling and eddy transport in the Southern Ocean coupled with a simple biogeochemical description. Under glacial conditions, a broader region of surface buoyancy loss results in upwelling farther to the north, relative to interglacials. The northern location of upwelling results in reduced CO2 outgassing and stronger carbon sequestration in the deep ocean: we calculate that the shift to this glacial-style circulation can draw down 30 to 60 ppm of atmospheric CO2. We therefore suggest that the direct effect of temperatures on Southern Ocean buoyancy forcing, and hence the residual overturning circulation, explains much of the strong correlation between Antarctic temperature variations and atmospheric CO2 concentrations over glacial-interglacial cycles.