Abstract:

The densest waters of the global ocean form on Antarctica’s continental shelves, after which they tumble into the abyss and slowly spread around the globe. In steady state, the rate of Antarctic Bottom Water (AABW) formation in the Southern Ocean must be balanced by the rate of a water mass transformation by turbulent mixing in the interior ocean. In the first part of the talk, I will describe how, over the past decade, our understanding of this mixing-driven upwelling has been turned upside down, with most of the upwelling transport now thought to occur near the sea floor as turbulent buoyancy fluxes are forced to converge by the no-flux boundary condition. In the second part of the talk, I will show that climate models project formation of AABW to decrease significantly over the coming decades, primarily due to freshwater forcing from a melting cryosphere. Under strong enough formation, new AABW formation ceases completely and the standing stock of AABW that fills the bottom 10% of the global ocean becomes unventilated by the large-scale circulation, adjusting only very slowly due to turbulent diffusion. Finally, I will hint at the potential impacts of these circulation changes on the carbon cycle by evaluating the ocean’s projected uptake, storage, and outgassing of chlorofluorocarbons over the 21st century.