Mapping the decadal evolution of Antarctic Bottom Water ventilation from CFCs

Geoffrey Gebbie, Woods Hole Oceanographic Inst., Physical Oceanography, Woods Hole, MA, United States and Sarah G Purkey, Scripps Institution of Oceanography, University of California San Diego, San Diego, CA, United States
Abyssal waters in the Southern Ocean have undergone a warming and freshening over the past three decades that appears to be inconsistent with a steady advective-diffusive model. One hypothesis is that changes in Antarctic Bottom Water formation and ventilation rates can explain the temperature and salinity observations. Here we develop a new method to utilize the transient tracers, CFC-11 and CFC-12, as observed in hydrographic surveys to overcome the limitations of sparse observations. We produce objectively-gridded maps of CFCs evolution by modeling the oceanic subsurface response, accounting for the data uncertainty, and applying a new Green's function "time-correction" technique. Annually-resolved maps from 1940 to 2017 illustrate that CFCs are now approaching the equator in bottom waters for the first time. In addition, non-steady Antarctic Bottom Water ventilation can be inferred from the spatial distribution of CFCs, giving promise that transient tracers contain clues that will ultimately explain abyssal warming and freshening.