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Abstract. Air-sea CO 2 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 CO 2 influx into the extratropics is associated with a robust seasonal cycle, and a large net CO 2 efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air-sea CO 2 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 • 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 CO 2 partial pressure in surface seawater (pCO 2 sw), inversions of ocean-interior CO 2 data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO 2 sw data, and inversions of atmospheric CO 2 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 pCO 2 diagnostics and those obtained with ocean-interior CO 2 inversions are −0.47 ± 0.13 Pg C yr 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 CO 2 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 CO 2 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 CO 2 inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO 2 sw data and pCO 2 sw diagnostics. In the South Pacific extratropics, where CO 2 variations in the surface and ocean interior are severely undersampled, the difference in the air-sea CO 2 flux estimates between the diagnostic models and ocean-interior CO 2 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 CO 2 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 pCO 2 sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation In...

[1] Ocean acidification resulting from increases in present and future atmospheric CO 2 levels could seriously affect diverse coastal and oceanic ecosystems. In this work, we determine that a significant trend in ocean acidification is superposed on the large seasonal and interannual variabilities of acidity in surface waters off the south coast of Honshu, Japan, based on our repeated observations of partial pressure of CO 2 ( pCO 2 ), total inorganic carbon (TCO 2 ), and pH. Multiple regression analysis of TCO 2 as a function of temperature, salinity, and timing of observations shows that TCO 2 increased at a rate of +1.23 ± 0.40 mmol kg −1 yr −1 for the period 1994-2008, while no long-term change has been determined for total alkalinity calculated from TCO 2 and pCO 2 in seawater. These results indicate that pH and the aragonite saturation state (W arag ) are decreasing at a rate of −0.020 ± 0.007 decade −1 and −0.12 ± 0.05 decade −1 , respectively. If future atmospheric CO 2 levels keep increasing as predicted by the Intergovernmental Panel on Climate Change emission scenario A1FI, which postulates intensive fossil fuel use associated with very rapid economic growth, a further reduction of −0.8 to −1.0 in W arag is likely in the next 50 years. Such a rapid reduction of W arag could have negative impacts on a variety of calcareous organisms.

A B S T R A C TWe estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3 • N to 33 • N along the repeat hydrographic line at 137 • E in the western North Pacific Ocean. Estimates were based on the observational records of oceanic CO 2 partial pressure and related surface properties over the last two decades. The computed pH time series both for 25 yr in winter (late January-early February) and for 21 yr in summer (June-July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer. The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr −1 (average, 0.0018 ± 0.0002 yr −1 ) in winter and from 0.0008 to 0.0019 yr −1 (average, 0.0013 ± 0.0005 yr −1 ) in summer. The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25 • N) in summer. We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.