Interannual variability of the oceanic CO2 sink in the subtropical gyre of the North Atlantic Ocean over the last 2 decades

Between 1983 and 2005, continuous oceanic CO2 observations at two time series sites in the North Atlantic Ocean near Bermuda indicate that surface seawater dissolved inorganic carbon ( DIC) and pCO(2) increased annually at rates similar to that expected from oceanic equilibration with increasing CO2 in the atmosphere. In addition, seawater pH, CO32- ion concentrations, and CaCO3 saturation states have also decreased over time. There was considerable seasonal asymmetry in the oceanic CO2 sink or source rates, with wintertime air- to- sea CO2 influx greater than the summertime sea- to- air CO2 efflux. On an annual basis, the region was an oceanic sink for CO2, with a mean net annual air- sea CO2 flux rate of - 815 +/- 251 and -1295 +/- 294 mmol CO2 m(-2) yr(-1), respectively, estimated using different synoptic and data assimilation model wind speed data sets. Peak- to- peak variability of similar to 850 - 1950 mmol CO2 m(-2) yr(-1) represented an interannual variability of similar to 0.2 - 0.3 Pg C yr(-1) in the oceanic CO2 sink scaled to the subtropical gyre of North Atlantic Ocean. The long- term trend over the 1983 - 2005 period was a slight increase in the oceanic CO2 sink, associated primarily with a gradual increase in wind speed over the same period. Interannual variability of summertime ( June - September) and fall ( October - December) air- sea CO2 flux rates were correlated to the North Atlantic Oscillation ( NAO) and strongly influenced by wind events such as hurricanes. Wintertime ( January - May) air- sea CO2 flux rates were poorly correlated with the NAO and Arctic Oscillation ( AO), although gas exchange rates were similar to 11 - 40% higher during concurrent El Nino periods compared to La Nina periods.