On the Stability of Discrete $$N+1$$ Vortices in a Two-Layer Rotating Fluid: The Cases $$N=4,5,6$$

A two-layer quasigeostrophic model is considered in the $$f$$ -plane approximation. The stability of a discrete axisymmetric vortex structure is analyzed for the case where the structure consists of a central vortex of arbitrary effective intensity $$\Gamma$$ and $$N$$ ( $$N=4,5$$ and $$6$$ ) identical peripheral vortices. The identical vortices, each having a unit effective intensity, are uniformly distributed over a circle of radius $$R$$ in the lower layer. The central vortex lies either in the same or in another layer. The problem has three parameters $$(R,\Gamma,\alpha)$$ , where $$\alpha$$ is the difference between layer nondimensional thicknesses. The cases $$N=2,3$$ were investigated by us earlier. The theory of stability of steady-state motions of dynamical systems with a continuous symmetry group $$\mathcal{G}$$ is applied. The two definitions of stability used in the study are Routh stability and $$\mathcal{G}$$ -stability. The Routh stability is the stability of a one-parameter orbit of a steady-state rotation of a vortex structure, and the $$\mathcal{G}$$ -stability is the stability of a three-parameter invariant set $$O_{\mathcal{G}}$$ , formed by the orbits of a continuous family of steady-state rotations of a two-layer vortex structure. The problem of Routh stability is reduced to the problem of stability of a family of equilibria of a Hamiltonian system. The quadratic part of the Hamiltonian and the eigenvalues of the linearization matrix are studied analytically. The results of theoretical analysis are sustained by numerical calculations of vortex trajectories.