Characteristics of the Near-Surface Currents in the Indian Ocean as Deduced from Satellite-Tracked Surface Drifters. Part II: Lagrangian Statistics

Lagrangian statistics of the surface circulation in the Indian Ocean (IO) are investigated using drifter observations during 1985-2013. The methodology isolates the influence of low-frequency variations and horizontal shear of mean flow. The estimated Lagrangian statistics are spatially inhomogeneous and anisotropic over the IO basin, with values of similar to 6-85 x 10(7) cm(2) s(-1) for diffusivity, similar to 2-7 days for integral time scale, and similar to 33-223km for length scale. Large diffusivities (>20 x 10(7) cm(2) s(-1)) occur in the central-eastern equatorial IO and the eastern African coast. Small diffusivities (similar to 6-8310 7 cm(2) s(-1)) appear in the subtropical gyre of the southern IO and the southeastern Arabian Sea. The equatorial IO has the largest zonal diffusivity (similar to 85 x 10(7) cm(2) s(-1)), corresponding to the largest time scale (7 days) and length scale (similar to 223 km), while the eastern coast of Somalia has the largest meridional diffusivity (similar to 31 3 10 7 cm(2) s(-1)). The minor component of the Lagrangian length scale is approximately equal to the first baroclinic Rossby radius (R-1) at midlatitudes (R-1 similar to 30-50 km), while the major component equals R-1 in the equatorial region (R-1 > 80 km). The periods of the energetic eddy-containing bands in the IO in Lagrangian spectra range from several days to a couple of months, where anticyclones dominate. Asignificant result is that the drifter-derived diffusivities asymptote to constant values in relatively short time lags (similar to 10 days) for some subregions of the IO if they are correctly calculated. This is an important contribution to the ongoing debate regarding drifter-based diffusivity estimates with relatively short Lagrangian velocity time series versus tracer-based estimates.