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°N, 128°E155°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 (pCO₂^sea) 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 ± yr^-1 (n = 50). The rate of pCO₂^sea 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 pCO₂^sea a increase is suggestive of temporal variations in ΔpCO₂ distribution. After removing the long-term trend from the pCO₂^sea data, the seasonal variation of pCO₂^sea in the western North Pacific (132°E142°E) was evaluated with a linear regression between the pCO₂^sea 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 (μ 60 ±) 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 pCO₂^sea 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.