Estimation of Horizontal Turbulent Diffusivity from Deep Argo Float Displacements

Abstract

Abstract We use an analog method, based on displacements of Argo floats at their parking depth (nominally located around 1000 dbar) from the ANDRO dataset, to compute continuous, likely trajectories and estimate the Lagrangian dispersion. From this, we find that the horizontal diffusivity coefficient has a median value around 500 m2 s−1 but is highly variable in space, reaching values from 100 m2 s−1 in the gyre interior to 40 000 m2 s−1 in a few specific locations (in the Zapiola Gyre and in the Agulhas Current retroflection). Our analysis suggests that the closure for diffusivity is proportional to eddy kinetic energy (or square of turbulent velocity) rather than (absolute) turbulent velocity. It is associated with a typical turbulent time scale of 4–5.5 days, which is noticeably quite constant over the entire globe, especially away from coherent intense currents. The diffusion is anisotropic in coherent intense currents and around the equator, with a primary direction of diffusion consistent with the primary direction of horizontal velocity variance. These observationally based horizontal diffusivity estimations, and the suggested eddy kinetic energy closure, can be used for constraining, testing, and validating eddy turbulence parameterization.