We have investigated the seasonal and inter-annual variations of the difference in partial pressure of CO
2 between surface seawater (pCO
2sea) and overlying air (pCO
2air) and the air-sea CO
2 flux in the mid-latitudes of the western North Pacific (WNP; 25-40°N, 140-170°E) and eastern North Pacific (ENP; 25-40°N, 120-150°W) from 1999 to 2006 using the latest voluntary observation ship data. In the WNP and ENP, the area-averaged ΔpCO
2 (pCO
2air-pCO
2sea) was at its minimum in late summer (-4.6 to 6.7 µatm in the WNP and -32.5 to -20.5 µatm in the ENP) and at its maximum in late winter (51.0 to 59.8 µatm in the WNP and 35.1 to 46.2 µatm in the ENP). The WNP acts as a moderate sink for atmospheric CO
2 (4.1 to 5.5 mmol m
-2d
-1), while the ENP acts as a weak sink (1.1 to 1.9 mmol m
-2d
-1). Because ΔpCO
2 is mainly controlled by pCO
2sea, we have evaluated the effect of the factors controlling pCO
2sea: sea surface temperature (SST), salinity (SSS), dissolved inorganic carbon (TCO
2), and total alkalinity (A
T). In the WNP, not only SST but also TCO
2 plays an important role in the seasonal pCO
2sea variation, while the SST could only explain most of the pCO
2sea variation in the ENP. From 1999 to 2006, pCO
2sea increased at a significantly lower rate (0.53 ± 0.11 µatm yr
-1) than pCO
2air (1.81 ± 0.01 µatm yr
-1) in the WNP, and at a slightly lower rate in the ENP (1.32 ± 0.16 µatm yr
-1). The air-sea CO
2 flux increased at a rate of 0.19 ± 0.05 mmol m
-2d
-1 yr
-1 in the WNP and 0.09 ± 0.03 mmol m
-2d
-1 yr
-1 in the ENP, suggesting that the WNP is a stronger sink for atmospheric CO
2.
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