Decadal Trend of Dissolved Inorganic Carbon in the Subarctic Western North Pacific Ocean

The dissolved inorganic carbon (DIC) and related chemical species have been measured from 1992 to 2007 at Stations KNOT (44°N, 155°E) and K2 (47°N, 160°E) in the subarctic western North Pacific. DIC corrected for the contribution of the biological activity significantly increased at rates of 0.8-1.6 μmol kg-1 y-1 in the temperature minimum (~100m), which is the remnant of the mixed layer from the previous winter, and intermediate waters (120-380m). This rate decreased with increasing density. To estimate the CO2 uptake rate in this region, we calculated the water column inventory of corrected DIC increase. The water column inventory of CO2 increase was estimated to be 0.42 ± 0.09 mol m-2 y-1, which is almost the same as that previously reported in the subarctic western North Pacific (0.66 ± 0.22 mol m-2 y-1; 1973-1993 [Ono et al., 2000]) and in the global ocean (0.51 ± 0.09 mol m-2 y-1; 1990s [Bindoff et al., 2007]). The corrected DIC increase in the temperature minimum and upper intermediate waters (100-200m, 1.3-1.5 μmol kg-1 y-1) was controlled not only by that expected from oceanic equilibration with increasing anthropogenic CO2 in the atmosphere (0.7 μmol kg-1 y-1), but by the decrease in the difference between atmospheric and oceanic CO2 in the winter mixed layer (0.6-0.8 μmol kg-1 y-1). This decrease, in turn, is caused by the accumulation of CO2 due to the reduction of CO2 emission in winter. If the atmospheric and oceanic CO2 in winter continue to increase at the same rates as during 1992-2007, the calculated CO2 in the temperature minimum will reach that of the atmosphere by 2030- 2040. This would result in the western North Pacific acting as a sink for atmospheric CO2 throughout the year and could accelerate the decrease of pH in surface waters.