Determining the Strength of the Deep Western Boundary Current Using the Chlorofluoromethane Ratio

Abstract

The dilution of a passive tracer during deep water formation and the subsequent advection/mixing in a deep boundary current is modeled with application to chlorofluoromethanes (CFMs) in the North Atlantic. Two different types of boundary currents are considered: a uniform flow and a simple shear flow. In each case the core of the flow mixes with surrounding water, which continually accumulates CFMs. In an extreme case the coupled systems predicts that the CFM ratio in the current is unaltered from the ratio of its source water (save for a time lag). More realistic cases however suggests that the ratio is not a conserved quantity, but is substantially altered in both the overflow basin and boundary current. Matching the model results to CFM data collected near the Grand Banks gives a predicted (average) core speed of 5–10 cm s−1 for the Deep Western Boundary Current, and provides a constraint on the transport and diffusivity of the flow as well.