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Dynamic and Double-Diffusive Instabilities in a Weak Pycnocline. Part I: Observations of Heat Flux and Diffusivity in the Vicinity of Maud Rise, Weddell Sea

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  • 1 Department of Oceanography, Naval Postgraduate School, Monterey, California
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Abstract

An expedition to study the stability of the weakly stratified water column in the eastern Weddell Sea was undertaken in the austral winter of 2005. A regional CTD survey around Maud Rise delineated water mass boundaries associated with flow around the seamount and identified areas most susceptible to overturning. A downstream region of the seamount Taylor column was found least stable, with a potential density difference across the pycnocline less than 0.018 kg m−3. Intensive water column measurements, including 1300 profiles of temperature, conductivity, and fast-response microconductivity, were made during a series of 13 drift stations to investigate vertical turbulent transports and the evolution of water column stability. The dependence of pycnocline turbulent diffusivity kT on Froude number Fr (turbulence generated by internal wave shear) and density ratio Rρ (turbulence generated by diffusive layering and possibly diapycnal cabbeling) is investigated. The Fr alone cannot explain completely the observed kT variability. Instead, there is also a strong dependence on Rρ. Turbulent diffusivity is an order of magnitude larger in the weakly stratified Taylor cap over Maud Rise (where Rρ approaches one) than in the surrounding water column that is unaffected by flow around Maud Rise. In terms of water column stability, diffusive heat flux across the pycnocline inhibits winter ice growth and densification of the surface layer. The observed Rρ dependence of kT thus provides a strong negative feedback on the winter evolution of the Maud Rise area water column toward overturning instability.

Corresponding author address: William Shaw, Department of Oceanography, Naval Postgraduate School, 833 Dyer Road, OC Dept., Rm. 328 Spanagel Hall, Monterey, CA 93943. E-mail: wjshaw@nps.edu

Abstract

An expedition to study the stability of the weakly stratified water column in the eastern Weddell Sea was undertaken in the austral winter of 2005. A regional CTD survey around Maud Rise delineated water mass boundaries associated with flow around the seamount and identified areas most susceptible to overturning. A downstream region of the seamount Taylor column was found least stable, with a potential density difference across the pycnocline less than 0.018 kg m−3. Intensive water column measurements, including 1300 profiles of temperature, conductivity, and fast-response microconductivity, were made during a series of 13 drift stations to investigate vertical turbulent transports and the evolution of water column stability. The dependence of pycnocline turbulent diffusivity kT on Froude number Fr (turbulence generated by internal wave shear) and density ratio Rρ (turbulence generated by diffusive layering and possibly diapycnal cabbeling) is investigated. The Fr alone cannot explain completely the observed kT variability. Instead, there is also a strong dependence on Rρ. Turbulent diffusivity is an order of magnitude larger in the weakly stratified Taylor cap over Maud Rise (where Rρ approaches one) than in the surrounding water column that is unaffected by flow around Maud Rise. In terms of water column stability, diffusive heat flux across the pycnocline inhibits winter ice growth and densification of the surface layer. The observed Rρ dependence of kT thus provides a strong negative feedback on the winter evolution of the Maud Rise area water column toward overturning instability.

Corresponding author address: William Shaw, Department of Oceanography, Naval Postgraduate School, 833 Dyer Road, OC Dept., Rm. 328 Spanagel Hall, Monterey, CA 93943. E-mail: wjshaw@nps.edu
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