Long‐term trend of the partial pressure of carbon dioxide (pCO2) in surface waters of the western North Pacific, 1984–1993

ABSTRACT The ocean is an important sink for anthropogenic CO2 emissions, but there are only a few measurements which confirm the oceanic CO2 uptake. Since 1981, partial pressure of CO2 (pCO2) in the western North Pacific (35°N−3°N, 128°E−155°E) and the overlying air have been measured periodically to clarify the seasonal and long‐term trends of the oceanic carbonate system. The partial pressure of CO2 in surface seawater (pCO2sea) observed every boreal winter during the period from 1984 to 1993 give a growth rate of 1.8 ± 0.6 µatm yr−1 (n = 27) north of 15°N and 0.5 ± 0.7 µatm yr−1 (n = 23) south of 14°N with an average of 1.2 ± 0.9 µatm yr−1 (n = 50). The rate of pCO2sea a increase north of 15°N is equal to that of atmospheric CO2 (1.8 µatm yr−1) during the same period but that south of 14°N is lower. The difference in rate of pCO2sea a increase is suggestive of temporal variations in ΔpCO2 distribution. After removing the long‐term trend from the pCO2sea data, the seasonal variation of pCO2sea in the western North Pacific (132°E−142°E) was evaluated with a linear regression between the pCO2sea and sea surface temperature (SST). Generally, a thermodynamic process (temperature effect) plays a predominant role in determining the seasonal variations of pCO2sea. South of 14°N, however, a clear interannual variability is significant relative to the seasonal changes if an El Niño event is accompanied by enhanced vertical mixing. The annual air‐sea CO2 flux showed a large influx of CO2 into the ocean north of 27°N (Kuroshio Counter Current) because of a large negative ΔpCO2 (− 60 µatm) and strong wind during the winter season. Toward the south, the annual average air‐sea CO2 flux increased by 9 mmol m−2 day−1 from – 8 mmol m−2 day−1 at 31°N to 1 mmol M−2 day−1 at 5°N. South of 10°N, the ocean acts as a source for atmospheric CO2 (0.2‐0.7 mmol m−2 day−1), but this is a considerably weaker source as compared with those of the central and eastern equatorial Pacific. The observed increase of pCO2sea and the estimated air/sea CO2 flux suggest the importance of carbon transport from the mixed layer to the intermediate/deep water in the area of Subtropical Mode Water formation, south of the Kuroshio and east of Japan.