Investigation of the dynamics of the Agulhas Current System in a numerical model

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Meteorology and Physical Oceanography

First Committee Member

Douglas B. Boudra, Committee Chair

Second Committee Member

Donald B. Olson, Committee Member


A one-layer model demonstration shows that both inertia and internal friction may account for a partial retroflection where a linear, weakly viscous system has none. When stratification is introduced and baroclinicity increased, the stretching term exerts an increasing influence. With 40 km resolution, terms included so that the numerical model conserves potential vorticity become important as well. When grid resolution is halved, the importance of the extra conserving terms diminishes and the stretching term exerts an even greater influence. The importance of a substantial viscous stress curl along the coast of Africa, as provided by the no-slip condition, is illustrated through comparison with a slippery Africa experiment. Finally, an experiment with a more realistic South African coastal geometry, giving a more realistic order of importance to the planetary vorticity advection in the separating Agulhas is described.Experiments in which rings form exhibit considerably larger values of mean to eddy kinetic energy transfer than those in which no rings form. In a low Rossby number experiment with a rectangular Africa, rings are formed almost continuously, and basin mode resonance plays a significant role in ring formation. Whether a form of instability (barotropic or baroclinic) plays an important role as well is unclear. In two high Rossby number experiments, one with rectangular and the other with triangular African geometry, basin mode resonance is not a factor, and, it appears that ring formation is associated with release of mixed barotropic-baroclinic instability.It is found that, as the ring rounds the tip of Africa, there is only a small influence of the large scale flows on the ring propagation. On the other hand, the presence of the African continent provides an additional westward movement in addition to the one due to $\beta$. As soon as the ring drifts into the South Atlantic subtropical gyre, advection by the large scale flows dominates the ring motion. Furthermore, the model rings have a coherent structure all the way to the bottom and the barotropic component of the flow enhances the westward propagation due to $\beta$ through vertical coupling. (Abstract shortened with permission of author.)


Physical Oceanography

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