Characteristics of the near-surface currents in the Indian Ocean as deduced from satellite-tracked surface drifters. Part II: Lagrangian statistics

Abstract

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 ~33-223km 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. Asignificant 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.