Is the Indian Ocean MOC driven by internal wave breaking?

Abstract

This dissertation investigates the energetics of the Indian Ocean Meridional Overturning Circulation (MOC) using hydrographic data (Part I), and the interaction between a broad band internal wave field and a mean flow using idealized numerical simulations (Part II). The main objective of this work is to quantify how much energy is needed to drive the Indian Ocean MOC and to compare this with the energy available in the internal wave field. The
turbulent dissipation needed to sustain the MOC is estimated by assuming a 'mixing efficiency' of 0.2 and an advective-diffusive balance in neutral density layers. The advective transport of mass into this box-model is based on
published estimates of the flow field at 32ºS and the Indonesian Through-flow. A comparison of the large scale dissipation rates with estimates of the input of energy by the tides and the wind shows that most published overturning solutions require more energy than is likely to be available. This result suggests that energy budgets may be useful as constraints in inverse models. Estimates of turbulent dissipation due to internal wave breaking are inferred from in-situ observations of shear and strain using a fine scale parameterization. The isoneutral mean of the inferred internal wave dissipation rates is about one order of magnitude smaller than dissipation rates inferred from the large scale flow fields. This result appears robust when considering potential sampling biases in the internal wave observations and leads to the main conclusion of this work: the Indian Ocean MOC cannot primarily be driven by internal wave breaking. A preliminary investigation into other processes capable of dissipating energy in the ocean interior shows that the MOC may be closed by hydraulic turbulence in the numerous Fracture Zones in the Indian Ocean.

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Identifiers

ORCID for Alberto C. Naveira Garabato: orcid.org/0000-0001-6071-605X

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