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

Abstract

Air-sea fluxes of CO₂ in surface seawater of the northeast (15°N-45°N) and southwest (25°S-15°S) subtropical Pacific Ocean from January 1985 to February 1988 were estimated. Initially, the net CO₂ flux unadjusted for the temperature deviation at the sea surface microlayer was estimated as a multiplicative product of the CO₂ transfer velocity, the solubility of CO₂ in seawater, and the difference in CO₂ fugacities between surface seawater and air. The fugacities of CO₂ in surface seawater and in the overlying air were given in Part I of this report. We used the Liss-Merlivat formulation to estimate the CO₂ transfer velocity from wind speed and seawater temperature. The monthly mean CO₂ fluxes were also adjusted for the temperature deviation ΔT between the mixed layer and the surface skin. The mean ΔT was estimated from the climatological means of solar radiation and heat fluxes across the boundary layer. Estimated ΔT for 1985-1988 averaged about 0.3 °C. The overall effect of the depressing skin temperature was an increased strength of the CO₂ sink, as expressed by a factor φ to estimate the surface temperature deviation on the net CO₂ flux for a given month and oceanic zone. The relative shifts between unadjusted and adjusted fluxes on a seasonal basis for each 10° latitudinal bands in the subtropical Pacific could vary between + 56% and - 71%. During the study period, the adjustments increased the net CO₂ fluxes within 25°Sμ15°S by 5% in austral winter and 56% in austral summer. Within 15°N-35°N, CO₂ effluxes reduced in the summer by 39% but increased in the winter by 15%. In northern waters within 35°Nμ45°N, the summer CO₂ effluxes were reduced by about 14%, whereas the winter CO₂ influxes showed an increased of about 7%. Thus, it is important to include the skin temperature effect in deriving the correct magnitude of the air-sea CO₂ flux. In the boreal winter/spring, the northeast Pacific within 15°Nμ35°N was a CO₂ sink with the adjusted net CO₂ flux averaged about - 1.7 mmol CO₂ m^-2 d^-1 during the 1986/1987 El Niño and about - 2.2 mmol m^-2 d^-1 before and after the El Niño. In the boreal summer/autumn, the CO₂ fluxes averaged, respectively, about 0.4 mmol m^-2 d^-1 and 0.6 mmol m^-2 d^-1 during the El Niño and the non-El Niño periods. Within 35°N-45°N, the average CO₂ flux was about 1.4 mmol m^-2 d^-1 in summer; about - 5.2 mmol m^-2 d^-1 in the winter/spring. The southwest subtropical Pacific within 25°5-15°S was basically a CO₂ sink year round with net CO₂ flux averaged at about - 2.0 mmol m^-2 d^-1 in austral winter/spring and - 0.7 mmol m^-2 d^-1 in austral summer/autumn.