MOTION AND EVOLUTION OF OCEANIC RINGS IN A NUMERICAL-MODEL AND IN OBSERVATIONS

Observed properties of oceanic rings are compared to rings produced in a two-gyre wind-driven numerical ocean model and in a model of the South Atlantic/Indian Ocean. Their temporal evolution is discussed in terms of structure and translation rate. They exhibit substantial similarity in terms of thermocline depth, ring size, swirl velocities, and translations speeds. In both observations and numerical model results, the propagation speeds are 2 to 5 times faster than that of an equivalent isolated eddy (which is of the order of the long Rossby wave speed). This is attributed to advection by the mean flows. Furthermore, it is observed that the model rings have a coherent structure all the way to the bottom. There is strong evidence that this is also the case in real oceanic rings. One major difference between observed and modeled rings is in their decay rate. The temporal decay of the rings in the models and observations is therefore discussed in relation to the decay mechanisms at work and the frictional parameterization of the model.