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

Abstract

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 pCO2 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 ∼850–1950 mmol CO2 m−2 yr−1 represented an interannual variability of ∼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 ∼11–40% higher during concurrent El Niño periods compared to La Niña periods.