Short-term changes in the partial pressure of CO₂ in eastern tropical Atlantic surface seawater and in atmospheric CO₂ mole fraction

Abstract

Continuous monitoring of oceanic and atmospheric CO₂ during periods of 8 to 12 days in two particular locations of the tropical Atlantic was carried out during June-August 1986. At the first location (convergence zone; about 5°N, 20°W), the ocean surface was generally slightly undersaturated with respect to the atmosphere, whereas at the second one (Guinea Dome; 12°N, 22° W), the ocean surface was an important source zone of CO₂ for the atmosphere. The results of PCO₂ in surface seawater, after correction of the temperature effect, show a decrease between morning and evening related to photosynthetic activity. Over a 3-day period, the CO₂ concentration in the air above the ocean may vary up to ±0.6 ppm d^-1 at the same location, whereas the wind direction hardly changes. The net CO₂ flux changes considerably during a short period (8 to 12 days) in the same place, as does the gas transfer coefficient across the sea surface through the variation of the wind speed: the variability is about 80—90%. The net CO₂ flux calculated from mean data is lower (about 30%) than the net CO₂ flux calculated from data taken over short time intervals. In the Guinea Dome area in summer, the net CO₂ flux can be as high (1.8 mmol m^-2 d^-1) as in the equatorial area.