Air–sea CO₂ flux in the Pacific Ocean for the period 1990–2009

Abstract. Air–sea CO₂ fluxes over the Pacific Ocean are known to be characterized by coherent large-scale structures that reflect not only ocean subduction and upwelling patterns, but also the combined effects of wind-driven gas exchange and biology. On the largest scales, a large net CO₂ influx into the extratropics is associated with a robust seasonal cycle, and a large net CO₂ efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air–sea CO₂ flux from a variety of products, drawing upon a variety of approaches in three sub-basins of the Pacific Ocean, i.e., the North Pacific extratropics (18–66° N), the tropical Pacific (18° S–18° N), and the South Pacific extratropics (44.5–18° S). These approaches include those based on the measurements of CO₂ partial pressure in surface seawater (pCO₂sw), inversions of ocean-interior CO₂ data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO₂sw data, and inversions of atmospheric CO₂ measurements. Long-term means, interannual variations and mean seasonal variations of the regionally integrated fluxes were compared in each of the sub-basins over the last two decades, spanning the period from 1990 through 2009. A simple average of the long-term mean fluxes obtained with surface water pCO₂ diagnostics and those obtained with ocean-interior CO₂ inversions are −0.47 ± 0.13 Pg C yr⁻¹ in the North Pacific extratropics, +0.44 ± 0.14 Pg C yr⁻¹ in the tropical Pacific, and −0.37 ± 0.08 Pg C yr⁻¹ in the South Pacific extratropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO₂ taken up over the North and South Pacific extratropics is released back to the atmosphere from the tropical Pacific. These estimates of the regional fluxes are also supported by the estimates from OBGCMs after adding the riverine CO₂ flux, i.e., −0.49 ± 0.02 Pg C yr⁻¹ in the North Pacific extratropics, +0.41 ± 0.05 Pg C yr⁻¹ in the tropical Pacific, and −0.39 ± 0.11 Pg C yr⁻¹ in the South Pacific extratropics. The estimates from the atmospheric CO₂ inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO₂sw data and pCO₂sw diagnostics. In the South Pacific extratropics, where CO₂ variations in the surface and ocean interior are severely undersampled, the difference in the air–sea CO₂ flux estimates between the diagnostic models and ocean-interior CO₂ inversions is larger (0.18 Pg C yr⁻¹). The range of estimates from forward OBGCMs is also large (−0.19 to −0.72 Pg C yr⁻¹). Regarding interannual variability of air–sea CO₂ fluxes, positive and negative anomalies are evident in the tropical Pacific during the cold and warm events of the El Niño–Southern Oscillation in the estimates from pCO₂sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation Index, but the peak-to-peak amplitudes tend to be higher in OBGCMs (0.40 ± 0.09 Pg C yr⁻¹) than in the diagnostic models (0.27 ± 0.07 Pg C yr⁻¹).