Estimating horizontal diffusivity in the East Sea (Sea of Japan) and the northwest Pacific from satellite‐tracked drifter data

Abstract

We use data of satellite‐tracked, mixed‐layer drifters collected for the World Ocean Circulation Experiment/Tropical Ocean‐Global Atmosphere (WOCE/TOGA) Surface Velocity Program in the East Sea (Sea of Japan) and the northwest Pacific Ocean during 1991–1997 to obtain reliable estimates of geographically varying horizontal diffusivity, integral timescale and space scale. For the diffusivity we suggest calculation of both the minor principal component of the diffusivity tensor in a definition by Davis [1991] and the half growth rate of the minor principal component of the displacement variance tensor. Numerical simulation of particle motion is used to prove that the minor principal component estimates of diffusivity, in contrast to regular estimates, are insensitive to ensemble averaging over particles taken from a finite area in a shear mean flow. In the East Sea, typical values of the diffusivity, timescale, and space scale are (1.7–5.2)×107 cm2 s−1, 0.7–1.7 days, and 10–25 km, respectively. In the northwest Pacific Ocean, typical estimates of the diffusivity, timescale, and space scale in mean are a factor 2–3 larger, namely, (2.3–15.1)×107 cm2 s−1, 1.7–3.7 days, and 18–62 km, respectively. It was found that the space scale L and the first mode internal Rossby radius Ri are related as L ≈ Ri, which suggests a parameterization of lateral diffusivity k∞ of the form k∞ = u Ri, where u is the rms current velocity fluctuation.