Sources of air-sea CO2 flux variability

To make predictions on the behavior of carbon pools under a changing climate, it is important to understand the interannual variability of air-sea and air-land vegetation carbon fluxes. One issue of recent debate is the role played by the North Atlantic Ocean. Predictions of ocean biosphere models imbedded in ocean circulation models suggest that North Atlantic variability is small, while time-series data from one ocean station and a previous inversion study of atmospheric data seem to indicate the opposite.

Using a novel inverse method, Manuel Gloor and colleagues obtained flux estimates that agreed significantly better with ocean process model predictions than the prior inversion study. In particular, the comparison showed that for 1983-1998, both inversion results and biogeochemical models place the primary source of global CO2 air-sea flux variability in the Pacific Ocean. Both methods also indicate that the Southern Ocean is the second-largest source of air-sea CO2 flux variability, while the variability throughout the Atlantic, including the North Atlantic, is diagnosed and predicted to be small. The study therefore also suggests that extrapolation of partial pressure difference from one station to entire ocean regions is likely not permissible.

 


Constraining ocean circulation with atmospheric oxygen

In two parallel studies, Science group researchers presented and analyzed spatial gradients in atmospheric oxygen concentrations that reflect ocean carbon fluxes and atmospheric circulation. The results demonstrate the existence of an atmospheric signature, derived from ocean-atmosphere exchange of O2 and CO2, predicted for years by modeling studies.

These modeling studies had predicted that concentrations of an air-sea gas exchange indicator called atmospheric potential oxygen, or APO (approximately O2+1.1CO2), would exhibit a positive bulge across the equator. Until recently, O2/N2 latitudinal cross-sections across the tropics did not exist. It was thus unclear if the prediction of a large APO signal merely reflected deficiencies in ocean models, perhaps due to unrealistic upwelling rates in the tropics.

In a new analysis by Manuel Gloor and colleagues, recent O2/N2 data of Y. Tohjima (NIES, Japan) were compared with predictions of atmospheric APO based on ocean interior data. Excellent agreement between the new data and the model result confirms the APO bulge predicted in simulations, and demonstrates that equatorial upwelling predicted by the ocean models is realistic.

In the parallel study, Michael Bender and colleagues present new O2/N2 and CO2 data from equatorial samples collected on the NOAA research vessel Ka’imimoana. They determine meridional gradients from pole to pole by comparing these data with results from the global sampling programs at Princeton and Scripps (lab of R. F. Keeling). The results again show good agreement with predictions based on ocean carbon cycle models and atmospheric circulation models..