Seasonal variability of fCO2 in the Angola-Benguela region

Abstract

The data reported were collected on 28 cruises using volunteer observing ships (VOS) along the QUIMA-VOS line to gauge the fugacity of CO2 (fCO2) over surface water in the south east Atlantic Ocean, over 3 years, from July 2005 to June 2008. The VOS line crosses the region from west to east, approaching the coast along the southern latitudes, and revealing the effects of offshore filaments in the northern areas, plus direct upwelled water in the southern areas. A complex pattern of distribution appears, with areas of super-saturation and low CO2 concentrations, associated with temperature variability and ocean circulation, with the highest variability associated to the most significant upwelling cells. Climatological seasonal cycles of sea surface temperature (SST) and fCO2 for the southern Benguela region presented increased monthly anomalies and a bi-modal seasonal SST cycle, related to the proximity to coastal waters and seasonal upwelled water between September and March, reducing the maximum summer temperature. The biological drawdown of fCO2, controlled by the seasonality thanks to the upwelling activity, contributed with values ranging from 150 to over 250 μatm. From 2005 to 2008, the temperature sharply increased in the southern Benguela region, by 0.47 °C yr−1 whereas the fCO2 in seawater decreased from 2.5 μatm yr−1 in the northern Namibia cell, to 0.4 μatm yr−1 in the southern Benguela area. The latter suggests higher biological activity related to an increase in the upwelling index together with the intrusion of lower inorganic carbon content seawater in the southern area from the Agulhas bank. south of 20°S, the east Atlantic Ocean acts as a net sink of CO2, with CO2 fluxes for the year 2007 increasing from −0.56 mol m−2 yr−1 (20–24°S) to −3.24 mol m−2 yr−1 (32–34°S). In the coastal area of South Africa, the constant biologically mediated CO2 drawdown reduces the fCO2 drastically, and maintains the partial CO2 pressure below the levels to be found in the atmosphere all year round.