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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 ~6-85 × 107 cm2 s−1 for diffusivity, ~2-7 days for integral time scale, and ~33-223 km for length scale. Large diffusivities (>20 × 107 cm2 s−1) occur in the central-eastern equatorial IO and the eastern African coast. Small diffusivities (~6-8 × 107 cm2 s−1) appear in the subtropical gyre of the southern IO and the southeastern Arabian Sea. The equatorial IO has the largest zonal diffusivity (~85 × 107 cm2 s−1), corresponding to the largest time scale (~7 days) and length scale (~223 km), while the eastern coast of Somalia has the largest meridional diffusivity (~31 × 107 cm2 s−1). The minor component of the Lagrangian length scale is approximately equal to the first baroclinic Rossby radius ( R1) at midlatitudes ( R1 ~ 30-50 km), while the major component equals R1 in the equatorial region ( R1 > 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. A significant result is that the drifter-derived diffusivities asymptote to constant values in relatively short time lags (~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. [ABSTRACT FROM AUTHOR]

Using the 1985-2013 record of near-surface currents from satellite-tracked drifters, the pseudo-Eulerian statistics of the near-surface circulation in the Indian Ocean (IO) are analyzed. It is found that the distributions of the current velocities and mean kinetic energy (MKE) in the IO are extremely inhomogeneous in space and nonstationary in time. The most energetic regions with climatologic mean velocity over 50 cm s−1 and MKE over 500 cm2 s−2 are found off the eastern coast of Somalia (with maxima of over 100 cm s−1 and 1500 cm2 s−2) and the equatorial IO, associated with the strong, annually reversing Somalia Current and the twice-a-year eastward equatorial jets. High eddy kinetic energy (EKE) is found in regions of the equatorial IO, western boundary currents, and Agulhas Return Current, with a maximum of over 3000 cm2 s−2 off the eastern coast of Somalia. The lowest EKE (<500 cm2 s−2) occurs in the south subtropical gyre between 30° and 40°S and the central-eastern Arabian Sea. Annual and semiannual variability is a significant fraction of the total EKE off the eastern coast of Somalia and in the central-eastern equatorial IO. In general, both the MKE and EKE estimated in the present study are qualitatively in agreement with, but quantitatively larger than, estimates from previous studies. These pseudo-Eulerian MKE and EKE fields, based on the most extensive drifter dataset to date, are the most precise in situ estimates to date and can be used to validate satellite and numerical results. [ABSTRACT FROM AUTHOR]