Environmental Science and Engineering Seminar

The bottom third of the global ocean is filled by dense Antarctic Bottom Water (AABW). As its name suggests, its origins can be traced to the very cold and dense waters formed through ice-ocean interactions on the Antarctic shelf; as these sourced Dense Shelf Waters (DSW) tumble down the Antarctic continental slope, they entrain overlying Circumpolar Deep Waters and produce Antarctic Bottom Water. In a steady state, the rate of formation of Antarctic Bottom Water in the Southern Ocean must be balanced by the rate of destruction to the north. Theory and modeling suggests this destruction is due to a combination of small-scale diapycnal mixing and geothermal heating; however, exactly where and how this occurs remains somewhat mysterious.

In this talk, I will apply Water Mass Transformation theory to GFDL's eddy-permitting coupled climate model simulations to describe the full life-cycle of simulated AABW. I will begin by discussing the steady preindustrial context in which AABW formation, export, and destruction are well-balanced. I will show that the modeled transformation of DSW into AABW is largely due to spurious numerical mixing (a model error) but that the destruction of AABW is largely due to parameterized mixing (mostly induced by locally-breaking internal tides). I will also briefly describe how ideas from discrete graph flow theory may help disentangle the complicated spatial structure of AABW export pathways. Finally, I will show that anthropogenic forcing results in a significant reduction (and lightening) of DSW and AABW formation, collapsing the lower cell of the ocean overturning circulation and resulting in a 20% contraction of AABW by 2100; linear extrapolation suggests AABW would vanish by 2350 or so.