Chlorofluorocarbon‐derived formation rates of the deep and bottom waters of the Weddell Sea

Abstract

The spreading and mixing of deep and bottom waters from the Weddell Sea (originating with potential temperature <0°C) are investigated through analysis of new and historical CFC measurements. We investigate the component that enters the southern Indian Ocean with a simple one‐dimensional advection‐diffusion model of the deep boundary current. This model does not require a condition of mixing solely with tracer‐free water to be imposed. As boundary conditions, we derive time histories for the CFC concentrations of newly formed Weddell Sea deep and bottom waters. A range of best fit velocities is obtained by determining which model velocities produce model tracer concentrations closest to measured concentrations. From this, we derive a formation rate of 3.2±1.5 Sv for the waters originating with temperatures lower than 0°C that subsequently enter the southern Indian Ocean. This compares to an upper limit for these waters of 5.7 Sv derived from the CFC‐11/CFC‐12 ratio. Whilst the former value is dependent on the highly idealized concepts of the model used, the latter is a more robust upper limit. Using the CFC‐11/CFC‐12 ratio measured at Vema Channel during the World Ocean Circulation Experiment cruise A23, an upper limit of 0.9 Sv is derived for the formation rate of waters colder than 0°C that enter the South Atlantic. Overall, an upper limit of 6.6 Sv is derived for the rate of formation of the deep and bottom waters of the Weddell Sea. A better representation is obtained by combining the upper limit to the South Atlantic component with the model‐derived southern Indian Ocean estimate; we thus estimate the formation rate of the deep and bottom waters in the Weddell Sea to be 3.7±1.6 Sv. Whilst the range obtained is large and the assumptions implicit in the calculations are potentially significant, we believe this more realistically represents the true formation rate than estimates derived using an imposed condition of tracer‐free mixing. The range can be narrowed in future as more data become available.