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Eddy–Mean Flow Interaction in Zonal Oceanic Jet Flow along Zonal Ridge Topography

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  • 1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon
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Abstract

Motivated by satellite altimeter observations of enhanced sea level variability near steep topographic slopes in the Southern Ocean, effects of topography on the spatial distribution of mesoscale eddies and on eddy–mean flow interaction are investigated using a two-layer, wind-forced, quasigeostrophic channel model. The principal topography, a zonal ridge with a zonal modulation of ridge height and width, is an idealized version of a segment of the Southeast Indian Ridge along the path of the Antarctic Circumpolar Current. Geosat altimeter observations in this region suggest that spatial variations of eddy energy are related to alongstream modulations of ridge morphology.

The time-mean flow and distribution of time-dependent eddies in the model are sensitive to relatively subtle alongstream variations of topography. Topographic steering leads to alongstream variations of time-mean baroclinic and barotropic shear and to alongstream variations in the meridional position of the jet relative to the crest of the zonal ridge. Linear stability analysis demonstrates that zonal variations of flow stability are strongly coupled to the topography. Unstable mode growth rates are largest where topographic steering forces the jet into regions of reduced ambient potential vorticity gradient. Growth rates are lower where topography steers the jet into regions of higher ambient potential vorticity gradient. As a result, the largest eddy energies occur downstream of zonal modulations of ridge height or width. Unlike flows over flat-bottom topography, the zonal distribution of unstable mode growth rate is negatively correlated with velocity shear. Analysis of area-averaged mean-to-eddy energy conversions shows that zonal modulations of topography modify the regime of flow instability. Baroclinic instability and recycling of eddy energy in the upper part of the water column occur in cases with zonally uniform topography. Mixed baroclinic–barotropic instability and strong downward transfers of eddy energy occur in cases with zonal modulations of topography. Local vorticity analyses demonstrate that alongstream variations of topography produce strong zonal modulations of flow dynamics. Zonal variations of topography shift the region of eddy-influenced dynamics within the model domain and modify the relative contributions of the mean and eddy field to the time-mean vorticity balance. When interpreted in the context of Southern Ocean dynamics, these results suggest that eddy-active regions near steep topographic slopes may contribute disproportionately to the dynamics of the Antarctic Circumpolar Current.

* Current affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York.

Corresponding author address: Dr. Donna L. Witter, Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964.

Email: dwitter@ldeo.columbia.edu

Abstract

Motivated by satellite altimeter observations of enhanced sea level variability near steep topographic slopes in the Southern Ocean, effects of topography on the spatial distribution of mesoscale eddies and on eddy–mean flow interaction are investigated using a two-layer, wind-forced, quasigeostrophic channel model. The principal topography, a zonal ridge with a zonal modulation of ridge height and width, is an idealized version of a segment of the Southeast Indian Ridge along the path of the Antarctic Circumpolar Current. Geosat altimeter observations in this region suggest that spatial variations of eddy energy are related to alongstream modulations of ridge morphology.

The time-mean flow and distribution of time-dependent eddies in the model are sensitive to relatively subtle alongstream variations of topography. Topographic steering leads to alongstream variations of time-mean baroclinic and barotropic shear and to alongstream variations in the meridional position of the jet relative to the crest of the zonal ridge. Linear stability analysis demonstrates that zonal variations of flow stability are strongly coupled to the topography. Unstable mode growth rates are largest where topographic steering forces the jet into regions of reduced ambient potential vorticity gradient. Growth rates are lower where topography steers the jet into regions of higher ambient potential vorticity gradient. As a result, the largest eddy energies occur downstream of zonal modulations of ridge height or width. Unlike flows over flat-bottom topography, the zonal distribution of unstable mode growth rate is negatively correlated with velocity shear. Analysis of area-averaged mean-to-eddy energy conversions shows that zonal modulations of topography modify the regime of flow instability. Baroclinic instability and recycling of eddy energy in the upper part of the water column occur in cases with zonally uniform topography. Mixed baroclinic–barotropic instability and strong downward transfers of eddy energy occur in cases with zonal modulations of topography. Local vorticity analyses demonstrate that alongstream variations of topography produce strong zonal modulations of flow dynamics. Zonal variations of topography shift the region of eddy-influenced dynamics within the model domain and modify the relative contributions of the mean and eddy field to the time-mean vorticity balance. When interpreted in the context of Southern Ocean dynamics, these results suggest that eddy-active regions near steep topographic slopes may contribute disproportionately to the dynamics of the Antarctic Circumpolar Current.

* Current affiliation: Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York.

Corresponding author address: Dr. Donna L. Witter, Lamont-Doherty Earth Observatory of Columbia University, Rt. 9W, Palisades, NY 10964.

Email: dwitter@ldeo.columbia.edu

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