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  • Author or Editor: Vladimir M. Kamenkovich x
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Mark A. Cane, Vladimir M. Kamenkovich, and Alexander Krupitsky

Abstract

The usefulness of the concept of JEBAR, the joint effect of baroclinicity and relief, in large-scale ocean dynamics is critically analyzed. The authors address two questions. Does the JEBAR term properly characterize the joint impact of stratification and bottom topography on the ocean circulation? Do estimates of the JEBAR term from observational data allow reliable diagnostic calculations?

The authors give a negative answer to the first question. The JEBAR term need not give a true measure of the effect of bottom relief in a stratified ocean. A simple two-layer model provides examples. As to the second question, it is demonstrated that the large-scale pattern of the transport streamfunction is captured by the smoothed solution, especially with the Mellor et al. formulation of the JEBAR term. However, the calculated velocity field is very noisy and the relative errors are large.

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Alexander Krupitsky, Vladimir M. Kamenkovich, Naomi Naik, and Mark A. Cane

Abstract

A linear equivalent barotropic (EB) model is applied to study the effects of the bottom topography H and baroclinicity on the total transport and the position of the Antarctic Circumpolar Current (ACC). The model is based on the observation of Killworth that the time mean velocity field of the FRAM Model is self-similar in the vertical.

A realistic large-scale topography H̄ is constructed by filtering 5-minute resolution data with an appropriate smoothing kernel. It is shown that the asymptotic behavior of the solution of the barotropic model (a particular case of the EB model) in the limit of very small bottom friction depends on subtle details of topography and basin geometry. Given the uncertainties of the smoothing procedure the authors conclude that the barotropic model is not robust with respect to possible variations of model topography.

The authors found that the EB model with a vertical profile function similar to that of Killworth reproduces the major features of the time- and depth-averaged FRAM solution, including the position and the transport of the ACC, reasonably well. The solution is robust with respect to uncertainties in H̄. The EB model is much improved by a parameterization of the bottom friction via near-bottom velocity, which tends to shut off the flow in the shallow regions.

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Vladimir M. Kamenkovich, Yurii P. Leonov, Dmitrii A. Nechaev, Deirdre A. Byrne, and Arnold L. Gordon

Abstract

A series of numerical experiments with a two-layer primitive equation model is presented to study the dynamics of Agulhas eddies. The main goal of the paper is to examine the influence of an underwater meridional ridge (modeled after the Walvis Ridge) on an Agulhas eddy hitting it. First, the propagation of an eddy of the specified vertical structure over a flat bottom is considered, varying the initial eddy horizontal scale from 40 to 120 km. Unlike small nonlinear eddies, large nonlinear eddies (on the scale of Agulhas eddies) do not rapidly evolve into a compensated state (no motion in the lower layer). Second, the influence of a ridge on eddies of differing vertical structures having a specified intensity in the upper layer and a prescribed horizontal scale is analyzed. Significantly baroclinic eddies can cross the Walvis Ridge, but barotropic or near-barotropic ones cannot.

The evolution of eddies crossing the ridge is compared with that of initially identical eddies moving over a flat bottom and with field observations. Eddies in our model tend toward the compensated state, with a motionless lower layer, when they cross a steep ridge. This tendency appears largely independent of the initial state of the eddy. Eddies crossing the ridge, show an intensification just before the eddy center encounters the ridge, expressed as a deepening of the thermocline depth and a heightening of the sea surface elevation. This effect is large enough [O(10 cm)] that it should be noticeable in altimeter records such as the one from the Topex-Poseidon satellite. The translational speed and direction of model eddies agree with observations, even in the absence of externally prescribed large-scale currents or friction; model eddies averaged 4.6 km day−1 and moved westward.

The modeled eddies proved an effective transport for passive tracers; tracers initially located near the center of the eddy were transported with practically no losses. The influence of the ridge leads to the substantial increase of the transported tracers. Model eddies show a realistic e-folding scale for amplitude decay of 2680 km. This long scale, combined with the tracer transport, indicates that Agulhas eddies, which cross the Walvis Ridge, are capable of carrying their observed thermal and salinity anomalies far into the South Atlantic subtropical gyre.

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