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The Interaction between the Bottom Mixed layer and Mesoscale Motions of the Ocean: A Numerical Study

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  • 1 Institute of Oceanographic Sciences, Wormely, Godalming, Survey England
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Abstract

The interaction between a bottom mixed layer and a mesoscale eddy field is studied using a numerical model of a two-layer quasi-geostrophic fluid above a mixed layer. The height of the mixed layer is assumed to be restricted by stratification. A shear exists between the advection velocities within the mixed layer and the geostrophic flow above. The linear stability of such a system has been investigated by Richards. The present paper investigates nonlinear effects. The development of the instability discovered by Richards caused by the presence of the mixed layer, is found to be limited after 20 days by the overturning of the wave on the mixed layer-interior interface. For some nonlinear flows, however, there is a strong interaction between the mixed layer and the internal motions of the fluid. The differential advection of the structures on the mixed layer-interior interface can produce a negative topographic drag and transfer energy from the eddying motions into a mean flow. This enhances the β-effect and can drive zonal flows. Allowing mixing to take place between the mixed layer and interior fluid reduces this effect

The horizontal structure of the mixed layer is studied for various velocity and length scales of the mesoscale motions. Horizontal advection is found to play a dominant role in the dynamics of the bottom mixed layer making the use of one-dimensional models for the layer inappropriate unless an allowance for advection is made. A comparison of the model with observations made by Armi and D'Assaro of the horizontal structure of the bottom mixed layer suggests that some of the features they observe way be accounted for by forcing due to mesoscale eddies.

Abstract

The interaction between a bottom mixed layer and a mesoscale eddy field is studied using a numerical model of a two-layer quasi-geostrophic fluid above a mixed layer. The height of the mixed layer is assumed to be restricted by stratification. A shear exists between the advection velocities within the mixed layer and the geostrophic flow above. The linear stability of such a system has been investigated by Richards. The present paper investigates nonlinear effects. The development of the instability discovered by Richards caused by the presence of the mixed layer, is found to be limited after 20 days by the overturning of the wave on the mixed layer-interior interface. For some nonlinear flows, however, there is a strong interaction between the mixed layer and the internal motions of the fluid. The differential advection of the structures on the mixed layer-interior interface can produce a negative topographic drag and transfer energy from the eddying motions into a mean flow. This enhances the β-effect and can drive zonal flows. Allowing mixing to take place between the mixed layer and interior fluid reduces this effect

The horizontal structure of the mixed layer is studied for various velocity and length scales of the mesoscale motions. Horizontal advection is found to play a dominant role in the dynamics of the bottom mixed layer making the use of one-dimensional models for the layer inappropriate unless an allowance for advection is made. A comparison of the model with observations made by Armi and D'Assaro of the horizontal structure of the bottom mixed layer suggests that some of the features they observe way be accounted for by forcing due to mesoscale eddies.

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