Observed APO Seasonal Cycle in the Pacific: Estimation of Autumn O2 Oceanic Emissions

Abstract

In this work, we investigated the seasonal cycle of atmospheric potential oxygen (APO), a unique tracer of air-sea gas exchanges of molecular oxygen (O₂) and carbon dioxide (CO₂), expressed as APO = O₂ + 1.1 × CO₂. APO data were obtained from flask air samples collected since the late 1990s at three Japanese ground stations and on commercial cargo ships sailing between Japan and Australia/New Zealand, North America, and Southeast Asia. We also analyzed the APO spatial distribution and seasonal cycles with simulations from an atmospheric transport model using climatological oceanic O₂ fluxes from an empirical product that relate O₂ flux to ocean heat as input. Model simulations reproduced the observed APO seasonal cycles generally well, but with larger amplitudes and earlier occurrence of seasonal minima and maxima than in the observations. Moreover, the observed seasonal cycles exhibited larger APO enhancements than the simulations in autumn and early winter, especially in the North Pacific at 20°N–60°N. These enhancements remained when refining the comparison by adjusting the simulated APO peak-to-peak amplitudes and seasonal phases to the observations. This suggests additional O₂ emissions in the North Pacific, not well expressed in the air-sea O₂ fluxes used as input for our model simulations. The average autumn enhancement at 40°N–60°N was approximately twice that measured at 20°N–40°N. Confirming previous studies, our results indicate two distinct mechanisms possibly contributing to the additional oceanic O₂ emissions: outgassing from a subsurface shallow oxygen maximum at 20°N–40°N and autumn phytoplankton bloom at 40°N–60°N.