Evolution of Rings in Numerical Models and Observations

Observed properties of rings are compared to rings produced in a two-gyre wind driven circulation model and in a model of the Agulhas retroflection. Their temporal evolution is discussed in terms of structure and translation rate. In both observations and numerical model results, propagation speeds 2 to 5 times faster than of an equivalent isolated eddy (which is of the order of the long Rossby wave speed) were observed. The decay rate of model rings with a lateral viscosity of 330m²s⁻¹ is found to be faster than in observations. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom and it seems likely that this may also be the case in real oceanic rings. In the specific case of the model Agulhas ring, the factors influencing its motion and evolution are isolated in a series of subsidiary experiments. 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 β. As soon as the ring drifts into the South Atlantic tropical gyre, advection by the large scale flows dominates the ring motion.