Time series study of CFC concentrations in the Labrador Sea during deep and shallow convection regimes (1991–2000)

Abstract

Annual measurements of concentrations of chlorofluorocarbons (CFCs) along the Labrador Sea WOCE repeat section (AR7W) during 1991–2000 revealed increasing CFC‐12 concentrations in the surface layer (<100 m), the newly ventilated Labrador Seawater (LSW), North East Atlantic Deep Water (NEADW), and Denmark Strait Overflow Water (DSOW). In newly ventilated LSW, CFC concentrations were influenced less by CFC concentrations in the atmosphere than by convection regimes, including the deep convection years of 1991–1994 and the shallow convection years of 1995–2000. The average saturation levels for CFC‐12 and CFC‐11 in the newly ventilated LSW also showed large variations between two convection regimes, which would suggest using caution in estimating ventilation ages for the LSW away from the source. A time series of the CFC‐12 inventory in the Labrador Sea demonstrated the enhanced storage of CFC‐12 during the years of deep convection, while a lower rate of inventory increase was observed during the shallow convection period of late 1990s. This suggests that a periodic renewal of intermediate and deep waters by deep convection influences the uptake of anthropogenic gases such as CFC‐12 and presumably anthropogenic carbon dioxide.