A simple differential diffusion model to account for the discrepancy between 223Ra‐ and 224Ra‐based eddy diffusivities

Abstract

A series of 223Ra (t1/2 = 11.4 d) and 224Ra (t1/2 = 3.66 d) measurements made in the Mid‐Atlantic Bight yield eddy diffusivity (K) estimates of 1.2 ± 0.3 × 102 m2 s−1 and 1.4 ± 0.2 × 102 m2 s−1, respectively. These results fall in line with previous studies from multiple locations throughout the ocean, in which 224Ra‐based eddy diffusivities invariably exceed those determined using 223Ra. Such a pattern conflicts with the Fickian model for eddy diffusivity, in which K is constant. Moreover, this trend runs counter to the length scale‐dependent view of eddy diffusion, which suggests that K values estimated using 223Ra should exceed those of 224Ra, because the length scale of the former is greater than that of the latter. A finite mixing‐length model based on the concept of differential diffusion is used to provide an explanation for this discrepancy.