LONG-TERM TREND OF THE PARTIAL-PRESSURE OF CARBON-DIOXIDE (PCO(2)) INSURFACE WATERS OF THE WESTERN NORTH PACIFIC, 1984-1993

The ocean is an important sink for anthropogenic CO2 emissions, but th ere are only a few measurements which confirm the oceanic CO2 uptake. Since 1981, partial pressure of CO2 (pCO(2)) in the western North Paci fic (35 degrees N-3 degrees N, 128 degrees E-155 degrees E) and the ov erlying air have been measured periodically to clarify the seasonal an d long-term trends of the oceanic carbonate system. The partial pressu re of CO2 in surface seawater (pCO(2)(sea)) observed every boreal wint er during the period from 1984 to 1993 give a growth rate of 1.8 +/- 0 .6 mu atm yr(-1) (n = 27) north of 15 degrees N and 0.5 +/- 0.7 mu atm yr(-1) (n = 23) south of 14 degrees N with an average of 1.2 +/- 0.9 mu atm yr(-1) (n = 50). The rate of pCO(2)(sea) increase north of 15 d egrees N is equal to that of atmospheric CO2 (1.8 mu atm yr(-1)) durin g the same period but that south of 14 degrees N is lower. The differe nce in rate of pCO(2)(sea), increase is suggestive of temporal variati ons in Delta pCO(2) distribution. After removing the long-term trend f rom the pCO(2)(sea) data, the seasonal variation of pCO(2)(sea) in the western North Pacific (132 degrees E-142 degrees E) was evaluated wit h a linear regression between the pCO(2)(sea) and sea surface temperat ure (SST). Generally, a thermodynamic process (temperature effect) pla ys a predominant role in determining the seasonal variations of pCO(2) (sea). South of 14 degrees N, however, a clear interannual variability is significant relative to the seasonal changes if an El Nino event i s accompanied by enhanced vertical mixing. The annual air-sea CO2 flux showed a large influx of CO2 into the ocean north of 27 degrees N (Ku roshio Counter Current) because of a large negative Delta pCO(2) (-60 mu atm) and strong wind during the winter season. Toward the south, th e annual average air-sea CO2 nux increased by 9 mmol m(-2) day(-1) fro m -8 mmol m(-2) day(-1) at 31 degrees N to 1 mmol m(-2) day(-1) at 50 N. South of 10 degrees N, the ocean acts as a source for atmospheric C O2 (0.2-0.7 mmol m(-2) day(-1)), but this is a considerably weaker sou rce as compared with those of the central and eastern equatorial Pacif ic. The observed increase of pCO(2)(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 f ormation, south of the Kuroshio and east of Japan.