Is the deep Indian Ocean MOC sustained by breaking internal waves?
The Indian Ocean hosts a vigorous basin-scale overturning that constitutes one of the major deep upwelling branches of the global meridional overturning circulation (MOC). The extent to which the deep Indian Ocean MOC is sustained by breaking internal waves is assessed by quantifying and comparing the energetics of the overturning and those of the regional internal wave field. A range of published inverse estimates of the circulation across 32°S is used to assess the basin average buoyancy fluxes. The turbulent dissipation needed to sustain the MOC ranges between 0.17 ± 0.04 and 1.19 ± 0.17 TW, which is consistent with the estimated 0.35−0.26+1.04 TW dissipated by breaking internal waves, as inferred from observed fine structure. Both estimates of turbulent dissipation are consistent with the total energy input into the regional internal wave field (0.21−0.05+0.08 TW) based on published estimates of energy conversion from winds, tides and geostrophic bottom flows. However, a discrepancy arises when comparing the energetics at different density levels. At mid-ocean density levels (∼1000–3000 m) the dissipation of internal wave energy is found to be significantly smaller (factor 5–10) than the dissipation needed to sustain inverse estimates of the MOC. The uncertainty related to undersampling of internal wave breaking hot spots was analyzed and found to be small, which suggests that mixing processes other than wave breaking due to weak wave-wave interactions, may be significant in the deep Indian Ocean.