Decadal Variations of a Long-Term Carbon Sink in the Labrador Sea

Deep convection in the Labrador Sea ventilates the water column every winter. Depths of deep convection are influenced by the North Atlantic Oscillation (NAO). During the NAO positive phase, convection reaches over 2000m as observed in early 1990's, while during the NAO negative phase, convection becomes shallower to 500-1000m. Convection regimes in the Labrador Sea are, therefore, in the opposite phase from deep the convection in the Nordic Seas and the formation of subtropical mode water. Carbon dioxide sequestered during the winter convection in the Labrador Sea is partly transported to the south as Deep Western Boundary Current, while some spreads at the intermediate depths into the North Atlantic sub-polar gyre. Thus the Labrador Sea provides a conduit for a long-term storage of atmospheric carbon dioxide on time scales of centuries to millennium. We will present a time series study of Dissolved Inorganic Carbon (DIC), Alkalinity (Alk) and auxiliary measurements along the Labrador Sea repeat section from 1993 to 2004. Water masses in the Labrador Sea can be described by four layers from the surface to the bottom: namely, (1) new Labrador Sea Water (LSWnew) ventilated in the previous winter, (2) Labrador Sea Water produced during the exceptionally deep convection period of 1993-1994 (LSWold), (3) North East Atlantic Deep Water (NEADW) and (4) Denmark Strait Overflow Water (DSOW). All these water masses were formed in deep convection regions, LSWnew and LSWold in the Labrador Sea and NEADW and DSOW in the Nordic Seas. DIC has increased in all four water masses since 1993 with the highest rate of increase in LSWnew, corresponding to the atmospheric CO2 increase. LSWold has been isolated from the atmosphere since 1994 and accumulating DIC from the respired organic carbon. In the latter half of 1990's, during the period of shallow convection (<1000m), DIC concentrations were constantly higher in LSWold than LSWnew. However, in 2000, convection reached 1500m and DIC concentrations in LSWnew increased to the same levels as in LSWold. The inventory of DIC in the Labrador Sea was estimated to be 106 (S.D.=14) PgC from the basin wide survey of over 100 stations in 1996. The average inventory increase is 0.03 PgC/year for the period from 1993 to 2004. A sharp inventory increase in 2000 suggests the higher uptake of atmospheric CO2 during the deep convection period despite the entrainment of abundant DIC from the depth to the surface.