Seasonal Tropospheric Distribution and Air-Sea Fluxes of Atmospheric Potential Oxygen From Global Airborne Observations

Seasonal change of atmospheric potential oxygen (APO similar to O-2 + CO2) is a tracer for air-sea O-2 flux with little sensitivity to the terrestrial exchange of O-2 and CO2. In this study, we present the tropospheric distribution and inventory of APO in each hemisphere with seasonal resolution, using O-2 and CO2 measurements from discrete airborne campaigns between 2009 and 2018. The airborne data are represented on a mass-weighted isentropic coordinate (M-theta e) as an alternative to latitude, which reduces the noise from synoptic variability in the APO cycles. We find a larger seasonal amplitude of APO inventory in the Southern Hemisphere relative to the Northern Hemisphere, and a larger amplitude in high latitudes (low M-theta e) relative to low latitudes (high M-theta e) within each hemisphere. With a box model, we invert the seasonal changes in APO inventory to yield estimates of air-sea flux cycles at the hemispheric scale. We found a larger seasonal net outgassing of APO in the Southern Hemisphere (518 +/- 52.6 Tmol) than in the Northern Hemisphere (342 +/- 52.1 Tmol). Differences in APO phasing and amplitude between the hemispheres suggest distinct physical and biogeochemical mechanisms driving the air-sea O-2 fluxes, such as fall outgassing of photosynthetic O-2 in the Northern Hemisphere, possibly associated with the formation of the seasonal subsurface shallow oxygen maximum. We compare our estimates with four model- and observation-based products, identifying key limitations in these products or in the tools used to create them.

Automated summary

In this study, seasonal changes in atmospheric potential oxygen (APO) were analyzed to understand air-sea oxygen fluxes. The researchers observed larger seasonal amplitude of APO in the Southern Hemisphere compared to the Northern Hemisphere. Through a box model, they estimated air-sea flux cycles at the hemispheric scale. These findings contribute to understanding the mechanisms of air-sea oxygen fluxes and global climate change.