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

Understanding the relationship between Earth's climate and the oceanic carb on cycle requires an understanding of the time-variations of CO2 in the oce an, it's exchange with the atmosphere, and the rate of uptake of anthropoge nic CO2 by the ocean. Since 1988, hydrographic and biogeochemical data have been collected at the Bermuda Atlantic Time-Series Study (BATS) site in th e Sargasso Sea, located in the North Atlantic subtropical gyre. With over a decade of oceanographic data, interannual trends of CO2 species and air-se a exchange of CO2 at BATS can be examined. Between 1988 and 1998, surface s eawater total carbon dioxide (TCO2) and salinity normalized TCO2 (nTCO(2)) increased at a rate of 2.2 +/- 6.9 and 1.6 +/- 5.8 mu molkg(-1)yr(-1), resp ectively. During the same period, the partial pressure of CO2 (pCO(2)) of s eawater increased at a rate of 1.4 +/- 10.7 mu atm yr(-1), similar to the r ate of increase in atmospheric pCO(2) ( similar to 1.3 mu atm yr(-1)). The increase in seawater TCO2 and pCO(2) 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 tre nds were examined by determining how hydrographic and biogeochemical anomal ies, or deviations from the mean state, vary over time. Significant correla tions existed between anomalies of temperature, salinity, integrated primar y production, mixed-layer depth, TCO2, salinity normalized TCO2 (nTCO(2)), and alkalinity. For example, cold temperature anomalies (up to - 0.5 degree sC) in 1992 and 1995 were associated with increased mixed-layer depth, high er rates of integrated primary production ( < similar to 100 mg C m(2) d(-1 )), and higher concentrations of nTCO(2) ( < similar to 5 mu mol kg(-1)). T he interannual anomalies of hydrography and ocean biogeochemistry were part ially linked to large-scale climate variability such as North Atlantic Osci llation (NAO) and El Nino Southern Oscillation (ENSO). Temperature, mixed-l ayer depth, primary production and TCO, anomalies were correlated with NAO variability, with cold anomalies at BATS generally coinciding with NAO nega tive states. Salinity, alkalinity and nTCO(2) anomalies were correlated wit h the Southern oscillation index (SOI), lagging ENSO events by 6-12 months. (C) 2001 Published by Elsevier Science Ltd.