Drifter-derived estimates of lateral eddy diffusivity in the World Ocean with emphasis on the Indian Ocean and problems of parameterisation

Abstract

The Global Drifter Program data set, collected in the period 1979–2011, is divided in drogue-on and drogue-off subsets which are then used to develop 5°×5° bin estimates of lateral eddy diffusivity in the World Ocean's mixed and surface layers, respectively, by means of a modification of the Davis (1987. Journal of Marine Research, 430 (45), 635–666.; 1991. Deep Sea Research. 38, 432 (1), S531–S571.) approach. The analysis is geographically focused on the Indian Ocean, the region where such calculations have been lacking. As the Indian Ocean circulation is strongly determined by monsoonal winds, an effort was made to calculate the diffusivity relative to monsoon-driven mean currents. The drifter-derived estimates of the asymptotic lateral diffusivity at large time lags K∞, were found to be below 1×104m2s−1 almost everywhere in the World Ocean, while the maximum diffusivity Kmax, equivalent to integrating the Lagrangian velocity autocorrelation to the first zero crossing, exceeds K∞ in strong ocean currents by a factor of 2–4. At mid latitudes (20°≤|ϕ|≤50°), the estimates of K∞ in the mixed and surface layers are shown to follow the frozen field regime parameterisation K∞~EKE×Lbc, where EKE is the eddy kinetic energy and Lbc is the baroclinic Rossby deformation radius. In contrast, at low latitudes (|ϕ|<29°) in the surface layer, the fixed float regime parameterisation Kmax,K∞=EKE×T0, where T0=1.9 days and 1.3 days is the typical value of the Lagrangian integral timescale in the cases of Kmax and K∞, respectively, is preferable.