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

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 extra-tropics is associated with a robust seasonal cycle, and a large net CO₂ efflux from the tropics is associated with substantial inter-annual 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 extra-tropics (18° N-66° N), the tropical Pacific (18° S-18° N), and the South Pacific extra-tropics (44.5° S-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, inter-annual 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 ± 0.13PgCyr^-1 in the North Pacific extra-tropics, +0.44 ± 0 .14PgCyr^-1 in the tropical Pacific, and -0.37 ± 0.08PgCyr^-1 in the South Pacific extra-tropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO₂ taken up over the North and South Pacific extra-tropics 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 ± 0.02PgCyr^-1 in the North Pacific extratropics, +0.41 ± 0.05PgCyr^-1 in the tropical Pacific, and -0.39 ± 0.11 PgCyr^-1 in the South Pacific extra-tropics. 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 extra-tropics, where CO₂ variations in the surface and ocean interior are severely under-sampled, the difference in the air-sea CO₂ flux estimates between the diagnostic models and ocean interior CO₂ inversions is larger (0.18 PgCyr^-1). The range of estimates from forward OBGCMs is also large (-0.19 to -0.72 PgCyr^-1). Regarding inter-annual 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 Nino 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 PgCyr^-1) than in the diagnostic models (0.27 ± 0.07PgCyr^-1). [ABSTRACT FROM AUTHOR]