Distribution and ventilation of water masses in the Labrador Sea inferred from CFCs and carbon tetrachloride

An extensive study of halocarbon tracers, chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl4), based on over 100 stations in the Labrador Sea, demonstrated water mass structure and ventilation processes. Regional convection and isopycnal mixing of four water masses including newly ventilated Labrador Sea Water (LSWshallow), Labrador Sea Water produced in the winter of 1993–1994 (LSWdeep), North East Atlantic Deep Water (NEADW) and Denmark Strait Overflow Water (DSOW) from the surface to the bottom of the Labrador Sea were resolved from tracer distributions. Saturation levels varied spatially with different tracers and with convection regimes during the production of water masses. Average saturation levels in the LSWshallow for the entire Labrador Sea were 77% for CFC-12, 73% for CFC-11 and 92% for CCl4, respectively. Saturation levels during the time of LSW formation estimated from the area of deep convection were over 84% for CFC-12, 77% for CFC-11 and 99% for CCl4. Regional convection and subsequent horizontal mixing resulted in further reduction in saturation levels to 76%, 70% and 88%, respectively, when LSWshallow exited out to the south at the mouth of the Labrador Sea. LSWdeep was produced during the exceptionally deep convection winter of 1993–1994 and contained lower saturation levels than the Labrador Sea Water produced during weak convection regimes (LSWshallow). CFC-12, CFC-11 and CCl4 saturation levels in the LSWdeep were 66%, 61% and 76%, respectively, indicating that convection regime influenced tracer saturation levels. Estimates of ventilation ages depend strongly on the saturation levels. We calculated ventilation ages using CFC-12 with a range of saturation levels and with the saturation level calibrated tracer ratio, CFC-12/CCl4. The best estimates of ventilation ages for the NEADW and DSOW, obtained using the CFC-12/CCl4 ratio, were 11–13 and 5–8 years, respectively.