GEOGRAPHICAL, SEASONAL AND INTERANNUAL VARIATIONS OF AIR-SEA CO2 EXCHANGE IN THE SUBTROPICAL PACIFIC SURFACE WATERS DURING 1983-1988 .1. VARIABILITIES OF OCEANIC FCO(2)

The fugacity, of CO2 (fCO(2)) in the surface seawater of the northeast (15 degrees N-45 degrees N) and southwest (25 degrees S-15 degrees S) subtropical Pacific Ocean from November 1983 to February 1988 were observed on Is cruises on the M/V Lillooet. The northeast Pacific within 15 degrees N-45 degrees N was a net CO2 sink with a mean annual Delta fCO(2) of about -5 mu atm, as the average of -19 mu atm in the boreal winter and spring and 8 mu atm in the boreal summer and autumn. Our cruises also obtained the first set of austral winter and spring fCO(2) data for the southwest subtropical Pacific and thus provided a complete seasonal cycle of surface fCO(2) for this oceanic region. The southwest Pacific within 25 degrees S-15 degrees S, with a mean annual Delta fCO(2) of about -20 mu atm, was a CO2 sink year round except during the austral summer of the El Nino whence Delta fCO(2) had positive values. The observed sea-surface fCO(2) showed large variability. Between the summer and the winter months, the seasonal amplitudes of fCO(2) (normalized to the climatological annual mean SST) ranged from 1 mu atm to 63 mu atm. The surface waters of the subtropical and the northeast Pacific also displayed annual trends in the normalized fCO(2) values. From January 1984 to January 1987, there were decreases in the seasonal means of normalized fCO(2) of 4-11 mu atm yr(-1) within 25 degrees C-15 degrees S, 15 degrees N-25 degrees N, and 25 degrees N-35 degrees N. These trends were within the uncertainty of the data and were not significant statistically. However, from summer 1985 to summer 1987, the normalized fCO(2), showed significant positive increase of 24 mu atm yr(-1) within 35 degrees N-45 degrees N. There was also an increase, though not significant statistically, of 13 mu atm yr(-1) within 25 degrees N-35 degrees N. The cause of the slightly larger increase in fCO(2), in northern subtropical gyre could be the anomalous intrusions of the Oyashio Current during the period 1984-1986 along the North Pacific Current, which was the source of the California Current bifurcating into the waters of 25 degrees N-35 degrees N and 35 degrees N-45 degrees N. These intrusions in late spring usually followed an intensified and eastward shift of the Aleutian Low which was especially persistent during 1977-1988.