Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models

A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m23, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv [ 106 m3 s21). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO2.