Spatial and Temporal Variability of Diapycnal Mixing in the Indian Ocean. Issue 7 (12th July 2021)

• Record Type: Journal Article
• Authors: Katsumata, K.; Talley, L. D.; Capuano, T. A.; Whalen, C. B.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-12
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC017257
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The 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 m 2 s −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. Plain Language Summary: The ocean transports heat and freshwater in north‐south directions by the meridional overturning circulation, by which an inflow at some density returns at another density. The circulation requires mixing to convert the inflow into the outflow. We estimated this mixing using data collected from 10 sections that criss‐crossed the Indian Ocean. The mixing varies spatiotemporally, but in a section along 95°E, persistent spatial patterns were found during three occupations in 1995, 2007, and 2016. The fluxes across the density surfaces associated with the mixing of temperature and salinity vary primarily with the strength of turbulence; the impacts of the fluxes associated with temperature and salinity on the density of the seawater are comparable at high latitudes. The strength of turbulence depends on the roughness of bottom topography. The smooth bottom and greater depth of the eastern basins result in weaker mixing than the western basins. Turbulence also depends on surface winds. We used these relationships to estimate mixing in the entire Indian Ocean. Mixing was found insufficient to account for the mixing needed to explain the observed meridional overturning circulation. This deficit might be explained by consideration of unobserved high‐mixing events, enhanced mixing near the bottom, and mixing by deep equatorial jets. Key Points: In meridional section, turbulence variability is larger spatially than temporally; thus basin‐scale turbulence "geography" is repeatable Correlations of diapycnal diffusivity with bottom topography roughness & surface winds were used to map the turbulence in the Indian Ocean Additional mixing such as unobserved events, near‐bottom turbulence, & equatorial mixing is necessary to close the buoyancy budget