Interannual variability of oceanic CO2 and biogeochemical properties in the Western North Atlantic subtropical gyre

Understanding the relationship between Earthʹs climate and the oceanic carbon cycle requires an understanding of the time-variations of CO2 in the ocean, itʹs exchange with the atmosphere, and the rate of uptake of anthropogenic CO2 by the ocean. Since 1988, hydrographic and biogeochemical data have been collected at the Bermuda Atlantic Time-Series Study (BATS) site in the Sargasso Sea, located in the North Atlantic subtropical gyre. With over a decade of oceanographic data, interannual trends of CO2 species and air–sea exchange of CO2 at BATS can be examined. Between 1988 and 1998, surface seawater total carbon dioxide (TCO2) and salinity normalized TCO2 (nTCO2) increased at a rate of 2.2±6.9 and 1.6±5.8 μmol kg−1 yr−1, respectively. During the same period, the partial pressure of CO2 (pCO2) of seawater increased at a rate of 1.4±10.7 μatm yr−1, similar to the rate of increase in atmospheric pCO2 (∼1.3 μatm yr−1). The increase in seawater TCO2 and pCO2 can be attributed to a combination of uptake of anthropogenic CO2 from the atmosphere and interannual changes in hydrographic properties of the subtropical gyre. Underlying interannual trends were examined by determining how hydrographic and biogeochemical anomalies, or deviations from the mean state, vary over time. Significant correlations existed between anomalies of temperature, salinity, integrated primary production, mixed-layer depth, TCO2, salinity normalized TCO2 (nTCO2), and alkalinity. For example, cold temperature anomalies (up to −0.5°C) in 1992 and 1995 were associated with increased mixed-layer depth, higher rates of integrated primary production (<∼100 mg C m2 d−1), and higher concentrations of nTCO2 (<∼5 μmol kg−1). The interannual anomalies of hydrography and ocean biogeochemistry were partially linked to large-scale climate variability such as North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO). Temperature, mixed-layer depth, primary production and TCO2 anomalies were correlated with NAO variability, with cold anomalies at BATS generally coinciding with NAO negative states. Salinity, alkalinity and nTCO2 anomalies were correlated with the Southern oscillation index (SOI), lagging ENSO events by 6–12 months.