The Structure of an East Australian Current Anticyclonic Eddy

John C. Andrews Australian Defence Scientific Service, Department of Defence, Weapons Research Establishment, Salisbury, South Australia

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Paul Scully-Power Royal Australian Navy Research Laboratory, Garden Island, New South Wales, Australia

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Abstract

An intense, anticyclonic, warm core winter eddy off the east coast of Australia was surveyed with an airborne radiation thermometer, expendable bathythermographs and a continuously recording surface thermosalinograph in September 1974. The eddy had a diameter of 250 km, a dynamic relief of 0.7 dyn-m and a mixed layer depth extending to over 300 m in the core. A strong current ring was present halfway from the center to the edge of the eddy with surface speeds ranging from 0.6 to 1.78 m s−1. The dynamics of the eddy are related to previous knowledge of mesoscales in the East Australian Current region. The simplicity of the eddy structure defines a dominant and lone horizontal wavenumber whose inverse is the Rossby radius of deformation predicted by complex baroclinic instability theory. This simplicity allows a very simple eddy model to be proposed with first vertical baroclinic mode structure with a vertical depth scale of 430 m. The interior deep mixed layer was completely enclosed by a shell of isothermal water; this double layering indicates that large-scale entrainment of surface water may be an important feature of eddy generation off East Australia.

Abstract

An intense, anticyclonic, warm core winter eddy off the east coast of Australia was surveyed with an airborne radiation thermometer, expendable bathythermographs and a continuously recording surface thermosalinograph in September 1974. The eddy had a diameter of 250 km, a dynamic relief of 0.7 dyn-m and a mixed layer depth extending to over 300 m in the core. A strong current ring was present halfway from the center to the edge of the eddy with surface speeds ranging from 0.6 to 1.78 m s−1. The dynamics of the eddy are related to previous knowledge of mesoscales in the East Australian Current region. The simplicity of the eddy structure defines a dominant and lone horizontal wavenumber whose inverse is the Rossby radius of deformation predicted by complex baroclinic instability theory. This simplicity allows a very simple eddy model to be proposed with first vertical baroclinic mode structure with a vertical depth scale of 430 m. The interior deep mixed layer was completely enclosed by a shell of isothermal water; this double layering indicates that large-scale entrainment of surface water may be an important feature of eddy generation off East Australia.

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