Recent change in the oceanic uptake rate of anthropogenic carbon in the North Pacific subpolar region determined by using a carbon‐13 time series

Applying the δ 13 C approach to time series of observations in the North Pacific subpolar region (Station KNOT; 44°00′N, 155°00′E), we demonstrated time series of vertical distributions of oceanic anthropogenic carbon. We found that the vertical distributions of oceanic anthropogenic carbon during 1999–2006 were almost consistent with those estimated by the other carbon‐based quasi‐conservative tracer approach (ΔC*). Comparing the oceanic anthropogenic carbon contents and the water‐column inventories among 1999, 2000, and 2006, we found the recent oceanic uptake rate of anthropogenic carbon above 27.3 σ θ to be 0.86 ± 0.12 μ mol kg −1 yr −1 , which was 1.2 times higher than the expected value derived from oceanic equilibration with increasing atmospheric CO 2 . Considering the strengthened ocean stratification with a bidecadal oscillation and the recent increase in alkalinity from the Sea of Okhotsk, it was possible to explain the difference in the recent oceanic uptake rate of anthropogenic carbon between our result and the expected one.