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The subject of localized topographic circulation has received much attention in the past, in particular Taylor columns have been extensively studied and several observations indicate their existence in the real ocean. In contrast, we study here the closed circulation created above a large scale seamount, in the absence of a mean flow, by the interaction of the eddy field with the topography. This study is based on the mean state theory of Dewar (1998) which relies in part on the downgradient diffusion of potential vorticity (PV) by eddies. Using a multilayered eddy resolving quasigeostrophic model, the eddy parameterization is directly confirmed through computation of eddy PV fluxes. The critic raised by Cummins (2000) about a spurious source of angular momentum above the anomalous topography due to the parameterization is seen to depend on the geometry of the basin. In the present case, the rectangular domain can maintain a pressure gradient torque which closes the angular momentum balance. The time variability of the circulation is characterized and interpreted in the light of the mean state theory result that bottom friction and eddy diffusivity control the circulation. A very low frequency mode internal to the topographic circulation is unraveled which is related to eddy diffusivity and PV homogenization . Interactions with higher frequency basin modes are also seen to provoke the sheddings of the circulation away from its topographic anchor. The combination of these two modes leads to different regimes. In the unstable regime, the circulation is destabilized by perturbations and shed away. In the stable regime, the circulation is stronger and remains above the topography anomaly. An anticyclonic wave with azimuthal mode number one is also observed in the numerical simulation, that is similar to observations. The observation in the Argentine Basin of a strong anticyclone above the Zapiola Drift is the main motivation for the present study. Results from the present study are compared to observations of the Zapiola Anticyclone, as well as its numerical simulation with the primitive equations model SPEM.
Diffusion, Time Variability, Potential Vorticity, Quasigeostrophy, Eddies, Topographic Circulation
Date of Defense
June 16, 2005.
Submitted Note
A Dissertation Submitted to the Department of Oceanography in Partial FulfiLlment of the Requirements for the Degree of Doctor of Philosophy.
Bibliography Note
Includes bibliographical references.
Publisher
Florida State University
Identifier
FSU_migr_etd-1412
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