Variability in Southern Ocean air-sea carbon dioxide fluxes estimated from biogeochemical profiling floats

Ship-based observational estimates, along with the majority of coupled models, find that the Southern Ocean takes up a significant amount of carbon dioxide (CO₂) in the contemporary climate system, typically around 0.9 PgC y^-1. The strength of the Southern Ocean CO₂ sink has, however, been called into question by a new estimate calculated with measurements from autonomous biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project. In particular, air-sea CO₂ fluxes computed from float observations show significant wintertime outgassing in the high-latitude zone around Antarctica where upwelling brings old, carbon-rich deep waters to the surface ocean. This strong outgassing signal contributes to a much smaller Southern Ocean carbon uptake estimated from the floats for the period from May 2014 to May 2017, approximately 0.1 PgC y^-1. Here we extend these biogeochemical float-based flux estimates to include data through May 2018. Using the resulting updated data set, the interannual and spatial variability in Southern Ocean air-sea CO₂ flux is investigated, with a focus on characterizing the drivers of variability. We conclude by discussing the implications of these findings for predicting changes in ocean carbon uptake.