Spatial and Temporal Variability of Diapycnal Mixing in the Indian Ocean

Abstract

AbstractThe rate of turbulent kinetic energy dissipation and diapycnal diffusivity are estimated along 10 hydrographic sections across the Indian Ocean from a depth of 500 m to the seabed. Six sections were occupied twice. On the meridional section, which is nominally along 95°E, spatial patterns were observed to persist throughout the three occupations. Since the variability in diffusivity exceeds the variability in the vertical gradients of temperature and salinity, we conclude that the diffusive diapycnal fluxes vary mostly with diffusivity. In high latitudes, diapycnal diffusions of both temperature and salinity contribute almost equally to density diffusion, particularly across isopycnals just above the salinity maximum, while mainly temperature contributes in other latitudes. The known zonal difference in turbulence is reproduced. Diffusivity from the seabed to 4,000 m above the seabed has an exponential profile with a mode value of 4 × 10−4 m2s−1 at 1,000 m above the seabed and is positively correlated with topographic roughness as reported previously. It is found that the diffusivity also correlates with wind power injected through the surface at near‐inertial frequencies 10–80 days before the observations. These correlations were used to interpolate the observation‐based turbulence quantities to the entire Indian Ocean. Although the dissipation averaged along selected neutral‐density surfaces is less than the dissipation needed to explain the meridional overturning circulation evaluated across 32°S latitude, this may be explained by effects not captured by the ship‐based observations and parameterization. These effects likely include unobserved high‐mixing events, near bottom processes (e.g., hydraulic jumps), and deep equatorial jets.