Indo‐Pacific sector dominates Southern Ocean carbon outgassing

The Southern Ocean modulates the climate system by exchanging heat and carbon dioxide (CO2) between the atmosphere and deep ocean. While this region plays an outsized role in the global oceanic anthropogenic carbon uptake, CO2 is also released to the atmosphere across large swaths of the Antarctic Circumpolar Current (ACC). Southern Ocean outgassing has long been attributed to remineralized carbon from upwelled deep water, but the precise mechanisms by which this water reaches the surface are not well constrained from observations. Using data from a novel array of autonomous biogeochemical profiling floats, we examine Southern Ocean air–sea CO2 fluxes and the pathways that transfer carbon from the ocean interior into the mixed layer where air–sea exchange occurs. These float-based flux estimates of unprecedented spatial resolution indicate that carbon outgassing occurs predominantly in the Indo-Pacific sector of the ACC due to variations in the mean surface ocean partial pressure of CO2 (pCO2). We show that this zonal asymmetry in surface pCO2, and consequently air–sea carbon fluxes, stems primarily from regional variability in the mixed-layer entrainment of upwelled carbon-rich deep water. These results suggest that a sustained circumpolar observing system is crucial to monitor future changes in oceanic carbon release and uptake.

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