Erosion of a Surface Vortex by a Seamount on the β Plane

Steven Herbette Joint Research Center, Institute of Environment and Sustainability, Ispra, Italy

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Yves Morel EPSHOM/CMO, Brest, France

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Michel Arhan LPO, UMR 6523 CNRS/IFREMER/UBO, Plouzané, France

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Abstract

This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole influences the near-bottom signature of the original vortex with a strong temporal variability. This reflects on the manner in which the surface eddy feels an underlying seamount. Sensitivity experiments show that the eddy erosion rate after encountering a seamount can no longer be related to basic parameters such as the minimum eddy–seamount distance, as it was on the f plane. The additional vorticity poles influencing the eddy on the β plane make the result of the eddy–seamount encounter very sensitive to small variations of the initial conditions, and impossible to predict.

* Current affiliation: LOCEAN, UMR 7659 CNRS/IRD/UPMC, Paris, France

Corresponding author address: Dr. Steven Herbette, LOCEAN, UMR 7659 CNRS/IRD/UPMC, 4, Place Jussieu, 75252 Paris, France. Email: steven.herbette@lodyc.jussieu.fr

Abstract

This paper investigates the behavior of a surface-intensified anticyclone encountering a seamount on the β plane in a stratified ocean. The eddy may be strongly eroded, and sometimes subdivided, provided that it gets close enough to the seamount. In case of subdivision, the detached part has a vertical structure different from that of the initial eddy, and a subsurface vortex may result. The basic erosion mechanism previously observed with f-plane experiments is still active on the β plane. Deep fluid motions induced by the initial vortex across the isobaths generate topographic vortices whose upper parts exert a shear/strain on the initial eddy, causing its filamentation. On the β plane, this process is further complicated by the presence of additional eddies created by fluid motion across the planetary vorticity gradient. Experiments without any topography show that these eddies by themselves can erode the initial vortex. In particular, a deep positive potential vorticity pole influences the near-bottom signature of the original vortex with a strong temporal variability. This reflects on the manner in which the surface eddy feels an underlying seamount. Sensitivity experiments show that the eddy erosion rate after encountering a seamount can no longer be related to basic parameters such as the minimum eddy–seamount distance, as it was on the f plane. The additional vorticity poles influencing the eddy on the β plane make the result of the eddy–seamount encounter very sensitive to small variations of the initial conditions, and impossible to predict.

* Current affiliation: LOCEAN, UMR 7659 CNRS/IRD/UPMC, Paris, France

Corresponding author address: Dr. Steven Herbette, LOCEAN, UMR 7659 CNRS/IRD/UPMC, 4, Place Jussieu, 75252 Paris, France. Email: steven.herbette@lodyc.jussieu.fr

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