Abstract
The seawater fugacity of CO 2 ( f CO 2 ) has been monitored hourly at an instrumented mooring at 6°S, 10°W since 2006. The mooring is located in the South Equatorial Current and is affected by the equatorial Atlantic cold tongue. This site is characterized by large seasonal sea surface temperature variations (>4°C). The f CO 2 is measured by a spectrophotometric sensor deployed at about 1.5 meters deep. Measurements of seawater f CO 2 , sea surface temperature (SST) and sea surface salinity (SSS) are used to calculate total dissolved inorganic carbon (TCO 2 ) and pH. Total alkalinity (TA) is calculated using an empirical relationship with SSS determined for this region. Satellite chlorophyll-a concentrations at 6°S, 10°W are low (<0.2 mg m -3 ) but some peaks over 0.8 mg m -3 are sometimes detected in August. Nevertheless, the site is a permanent source of CO 2 to the atmosphere, averaging 4.7 ± 2.4 mmol m -2 d -1 over 2006-2021. Despite the weakening of the wind, the CO 2 flux increases significantly by 0.20 ± 0.05 mmol m -2 d -1 yr -1 . This suggests that the source of CO 2 is increasing in this region. This is explained by seawater f CO 2 increasing faster than the atmospheric increase during 2006-2021. Most of the seawater f CO 2 increase is driven by the increase of TCO 2 , followed by SST. The f CO 2 increase leads to a pH decrease of -0.0030 ± 0.0004 yr -1 . The SST anomalies (SSTA) at 6°S, 10°W are correlated to the Tropical Southern Atlantic (TSA) index and to the Atlantic 3 region (ATL3) index with a correlation coefficient higher than 0.75. The strong positive phase of both ATL3 and TSA, observed towards the end of the time-series, is likely contributing to the strong increase of seawater f CO 2 .