Seasonal and interannual variability of oceanic carbon dioxide species at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site

The seasonal and interannual dynamics of the oceanic carbon cycle and the strength of air-sea exchange of carbon dioxide are poorly known in the North Atlantic subtropical gyre. Between October 1988 and December 1993, a time series of oceanic measurements of total carbon dioxide (TCO2), alkalinity (TA) and calculated pCO2 was obtained at the Bermuda Atlantic Time-series Study (BATS) site (31°50′N, 64°10′W) in the Sargasso Sea. These measurements constitute the most extensive set of CO2 species data collected in the oligotrophic North Atlantic. Seasonal changes in surface and water-column CO2 species were ∼40–50 μmol kg−1 in TCO2, ∼20 μmol kg−1 in TA, and ∼90–100 μatm in calculated pCO2. These large changes were driven principally by deep convective winter mixing, temperature forcing and biological activity. TA was well correlated with salinity (with the exception of a 15–25 μmol kg−1 drawdown of TA on one cruise resulting from open-ocean calcification). TCO2 and pCO2 were well correlated with seasonal temperature changes (8–9°C). Other underlying processes, such as biological production, advection, gas exchange of CO2 and vertical entrainment, were important modulators of the carbon cycle, and their importance varied seasonally. Each spring-to-summer, despite the absence of measurable nutrients in the euphotic zone, a 35–40 μmol kg−1 decrease in TCO2 was attributed primarily to the biological uptake of TCO2 (evaporation/precipitation balance, gas exchange, and advection were also important). An increase in TCO2 during the fall months was associated primarily with entrainment of higher TCO2 subsurface waters. These seasonal patterns require a reassessment of the modelling of the carbon cycle using nutrient tracers and Redfield stoichiometries. Overall, the region is a weak sink (0.22–0.83 mol C m−2 year−1) for atmospheric CO2. Upper ocean TCO2 levels increased between 1988 and 1993, at a rate of ∼ 1.7 μmol kg−1 year−1. This increase appears to be in response to the uptake of atmospheric CO2 through gas exchange or natural variability of the subtropical gyre.