Latitudinal and Seasonal Variations in the Atmospheric CO2 and O2: Results From Shipboard Sampling in the West and North Pacific Ocean

Atmospheric potential oxygen (APO), a tracer defined as a combination of O₂ and CO₂ concentration (Stephens et al., 1998), is useful to study air-sea gas exchange related to the physical and biological oceanic processes. This is because air-sea O₂ and CO₂ exchanges mainly cause the variation in APO but terrestrial biotic exchanges do not affect it. For example, coupled ocean-atmosphere models predicted equatorial elevation of the annual mean APO. However, this distinct distribution has not been well validated because of the lack of the regular APO observation from the equatorial regions. To investigate the latitudinal distribution of APO, we have collected air samples on board regular service cargo ships between Japan and the United States and between Japan and Australia (or New Zealand) since December 2001. The flask samples were sent back to laboratory in the National Institute for Environmental Studies, and the O₂/N₂ ratios and CO₂ concentrations were determined. We binned the observed data by 10-degree latitude bands, and calculated the average seasonal cycle and deseasonalized trend for each bin. The peak-to-peak amplitude of the average seasonal cycles increased toward high latitude in both hemisphere and its minimum is placed to the about 10-degree north of the equator. The meridional distribution of annual mean APO, showing decreasing gradient toward high latitude in both hemispheres, agrees well with model simulation by Gruber et al., (2001).