A Numerical Study of the Interaction between a Deep Cold Jet and the Bottom Boundary Layer of the Ocean

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  • 1 Department of Oceanography, Florida State University, Tallahassee, Florida
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Abstract

A two-dimensional (x-z) primitive equation model is used to study the interaction between a deep cold jet on a sloping bottom and the bottom boundary layer (BBL) of the deep ocean. Two closure schemes are used: a standard second order turbulence closure (SOTC) scheme (the level 2 1/2 model of Mellor and Yamada), and a new eddy viscosity closure scheme (K-model). The latter is a computationally simple model that produces very similar eddy viscosity and velocity fields as the more complicated SOTC-model while saving about 20% of the computational time.

The results of the numerical simulations compare favorably to observations from the base of the North Atlantic continental rise where the cold jet known as the Cold Filament (CF) is found. The interaction between the CF and the BBL is found to be dominated by cross-isotherm Ekman flow, resulting in an asymmetry effect with different dynamics at each one of the fronts associated with the CF. Some of the unusual characteristics of this region are explained with the aid of the numerical experiments. These are: velocity profiles significantly different from those obtained by classical Ekman dynamics, unstable BBLs and detachment of bottom layers. Spatial variations in the characteristics of the BBL which are often neglected in deep-ocean studies are found to be significant in this region.

Abstract

A two-dimensional (x-z) primitive equation model is used to study the interaction between a deep cold jet on a sloping bottom and the bottom boundary layer (BBL) of the deep ocean. Two closure schemes are used: a standard second order turbulence closure (SOTC) scheme (the level 2 1/2 model of Mellor and Yamada), and a new eddy viscosity closure scheme (K-model). The latter is a computationally simple model that produces very similar eddy viscosity and velocity fields as the more complicated SOTC-model while saving about 20% of the computational time.

The results of the numerical simulations compare favorably to observations from the base of the North Atlantic continental rise where the cold jet known as the Cold Filament (CF) is found. The interaction between the CF and the BBL is found to be dominated by cross-isotherm Ekman flow, resulting in an asymmetry effect with different dynamics at each one of the fronts associated with the CF. Some of the unusual characteristics of this region are explained with the aid of the numerical experiments. These are: velocity profiles significantly different from those obtained by classical Ekman dynamics, unstable BBLs and detachment of bottom layers. Spatial variations in the characteristics of the BBL which are often neglected in deep-ocean studies are found to be significant in this region.

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