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

Abstract

n 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 R? (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 R?. Turbulent diffusivity is an order of magnitude larger in the weakly stratified Taylor cap over Maud Rise (where R? 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 R? dependence of kT thus provides a strong negative feedback on the winter evolution of the Maud Rise area water column toward overturning instability.