A Simple Method for Estimating Horizontal Diffusivity

Abstract

A common dilemma for oceanographers is the choice of horizontal diffusivity. There is no single answer as we could argue that diffusion depends precisely on those processes that cannot be sampled or modeled. Here we propose the radial offset by diffusion (ROD) method as a simple model-dependent approach for estimating these coefficients, and show its application for the southwestern South Atlantic. The method compares actual displacements of field drifters with numerical trajectory predictions. The observed–predicted differences in radial positions (radial offsets), which respond to diffusive motions not captured by the numerical model, are reproduced with a one-dimensional radial-diffusive solution through a proper selection of the diffusion coefficient. The method is tested at eight depths, from the sea surface down to 2000 m, using several drifter datasets and the Parcels software applied to the GLORYS12v1 (1/12° daily) velocity outputs. In all cases the radial offsets show Gaussian distributions that are well reproduced by the radial diffusive solution. Maximum diffusivities of 4630–4980 m2 s−1 happen in the upper 200 m of the water column and minimum values of 1080–1270 m2 s−1 occur between 1400 and 2000 m. The 15-m diffusivity is fairly constant in latitude (3850–5270 m2 s−1), but the 1000-m diffusivity decreases from 1640 to 1820 m2 s−1 north of the Polar Front to 530 m2 s−1 south of the Southern Boundary. A comparison with other diffusivity studies validates the good adequacy of the ROD method for numerical and field applications.