Mechanisms of eddy dissipation in the Southern Ocean

Abstract

The Southern Ocean is a region of fast currents, energetic eddies, large amplitude internal waves and strong turbulence. It is also a place where substantial quantities of heat and carbon are exchanged between the ocean and atmosphere. Our ability to predict global climatic changes relies, in part, on understanding the physical processes occurring there.

This thesis adds to the growing body of knowledge about Southern Ocean dynamics by using in-situ observations from profiling floats to study a lee wave generated in the Drake Passage by the flow of the Antarctic Circumpolar Current over topography. It is the first unambiguous example of such a wave in the Southern Ocean and is found to be associated with large fluxes of energy and momentum, as well as elevated turbulent dissipation. A key finding is that the energy flux is two orders of magnitude larger than the depth integrated dissipation, indicating that the majority of the energy may not be dissipated locally.

The wave observation was made possible by developing a method for measuring vertical velo- city from profiling floats. The essence of the method is to model the expected steady motion of the float and subtract this from the observed motion to retrieve the vertical velocity. It is easily applicable to many similar floats and has the potential to provide a global picture of vertical flows in the ocean.

The interaction of eddies and internal waves away from boundaries is investigated for the first time in the Southern Ocean using data from a mooring array in the Scotia Sea. Theoretical arguments are made to treat the interaction as a viscous coupling between internal wave stress and eddy strain. The results imply that eddy dissipation by interaction with the internal wave field is an important energy sink and comparable in magnitude to lee wave generation and bottom boundary layer viscous processes.

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Identifiers

ORCID for Jesse Cusack: orcid.org/0000-0003-2065-4397

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

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