Regional differences in the air–sea CO 2 flux between 3 and 14°S in the south-western tropical Atlantic

Context

The fugacity of surface-seawater CO₂ (fCO_2sw) and the sea–air CO₂ fluxes in the south-western tropical Atlantic (SWTA) were studied to increase the knowledge about the carbon cycle in this region.

Aims

This paper aims to describe the distribution of fCO_2sw in SWTA.

Methods

The fCO_2sw was measured from 2008 to 2020 by volunteer merchant ships with an onboard system that measures pCO₂ while the vessels were underway.

Key results

Higher values occurred north of 8°S than in the region south of 8°S. The north is a strong source of CO₂ for the atmosphere, with an annual mean value of 3.14 ± 0.52 mmol m⁻² day⁻¹. The south is a weaker source of CO₂, with an annual average of 0.93 ± 0.90 mmol m⁻² day⁻¹. In the months of July and August, a weak sink of CO₂ was observed, with a mean of −0.55 mmol m⁻² day⁻¹.

Conclusions and implications

The differences between these two regions are explained by the origin of the surface-water masses encountered along the ship track. The central branch of the South Equatorial Current (SEC) transports surface water, with a higher CO₂ concentration and lower salinity, north of 8°S, whereas the surface waters between 8 and 14°S come from the southern branch of the SEC. The intertropical convergence zone is another physical process influencing the region north of 8°S.