Geographical, seasonal and interannual variations of air-sea CO₂ exchange in the subtropical Pacific surface waters during 1983–1988

Abstract

The fugacity of CO₂ (fCO₂) in the surface seawater of the northeast (15°Nμ45°N) and southwest (25°S-15°S) subtropical Pacific Ocean from November 1983 to February 1988 were observed on 18 cruises on the M/V Lillooet. The northeast Pacific within 15°N-45°N was a net CO₂ sink with a mean annual ΔfCO₂ of about - 5 ±, as the average of - 19 ± in the boreal winter and spring and 8 ± in the boreal summer and autumn. Our cruises also obtained the first set of austral winter and spring fCO₂ data for the south-west subtropical Pacific and thus provided a complete seasonal cycle of surface fCO₂ for this oceanic region. The southwest Pacific within 25°Sμ15°S, with a mean annual ΔfCO₂ of about - 20 ±, was a CO₂ sink year round except during the austral summer of the El Niño whence ΔfCO₂ had positive values. The observed sea-surface fCO₂ showed large variability. Between the summer and the winter months, the seasonal amplitudes of fCO₂ (normalized to the climatological annual mean SST) ranged from l ± to 63 ±. The surface waters of the subtropical and the northeast Pacific also displayed annual trends in the normalized fCO₂ values. From January 1984 to January 1987, there were decreases in the seasonal means of normalized fCO₂ of 4-11 ± yr^-1 within 25°S-15°S, 15°N-25°N, and 25°N-35°N. These trends were within the uncertainty of the data and were not significant statistically. However, from summer 1985 to summer 1987, the normalized fCO₂ showed significant positive increase of 24 ± yr^-1 within 35°N-45°N. There was also an increase, though not significant statistically, of 13 ± yr^-1 within 25°N-35°N. The cause of the slightly larger increase in fCO₂ 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°N-35°N and 35°N-45°N. These intrusions in late spring usually followed an intensified and eastward shift of the Aleutian Low which was especially persistent during 1977-1988.