Isolated vortex evolution in 2 and 4 mode models

Abstract

The evolution of an isolated nonlinear baroclinic vortex on a β-plane is examined in 2 and 4 vertical model quasi-geostrophic models. The results show large differences, particularly in the vortex trajectory. In the 2 mode model the vortex propagates first towards the east, then towards the south, and later towards the southwest. In the 4 mode model the vortex propagates towards the east-southeast throughout the integration period. The eastward propagation in both models is due to modongenesis in the barotropic (i.e. first) mode. Although the energy of the third and fourth modes is only about 1% of the energy of the barotropic mode, they have a significant influence on the evolution of the initial vortex, since their potential vorticity is about 30–50% of the vorticity of the barotropic mode. The simulated vortex evolution is examined for the North Atlantic, the North Pacific and the eastern Mediterranean according to their characteristic stratification. The initial strength of the nonlinearity (i.e. the ratio between the nonlinear advection and linear wave propagation), and the initial ratio between its radius and Rossby radius of deformation are assumed equal for all three regions. An east-southeast propagation is observed, but the eastward (nondimensional) propagation rate in the North Atlantic is significantly slower than in the others. It is suggested that the modon with the baroclinic rider, as develops in the 4 mode ocean, is a good model for the Gulf Stream rings and for the eastern Mediterranean anticyclonic eddies. This is attributed to its being nondispersive for a long period of time, and to its speed of propagation being mainly determined by the mean current advection.