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 CO 2 observations at two time series sites in the North Atlantic Ocean near Bermuda indicate that surface seawater dissolved inorganic carbon (DIC) and p CO 2 increased annually at rates similar to that expected from oceanic equilibration with increasing CO 2 in the atmosphere. In addition, seawater p H, CO 3 2− ion concentrations, and CaCO 3 saturation states have also decreased over time. There was considerable seasonal asymmetry in the oceanic CO 2 sink or source rates, with wintertime air‐to‐sea CO 2 influx greater than the summertime sea‐to‐air CO 2 efflux. On an annual basis, the region was an oceanic sink for CO 2 , with a mean net annual air‐sea CO 2 flux rate of −815 ± 251 and −1295 ± 294 mmol CO 2 m −2 yr −1 , respectively, estimated using different synoptic and data assimilation model wind speed data sets. Peak‐to‐peak variability of ∼850–1950 mmol CO 2 m −2 yr −1 represented an interannual variability of ∼0.2–0.3 Pg C yr −1 in the oceanic CO 2 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 CO 2 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 CO 2 flux rates were correlated to the North Atlantic Oscillation (NAO) and strongly influenced by wind events such as hurricanes. Wintertime (January–May) air‐sea CO 2 flux rates were poorly correlated with the NAO and Arctic Oscillation (AO), although gas exchange rates were ∼11–40% higher during concurrent El Niño periods compared to La Niña periods.