Numerical simulation of the formation of tripolar vortices by the method of contour dynamics

It is found that tripolar structures can appear either in the presence of three or more lines of discontinuities in the vorticity field if the latter is compensated, or in the presence of two lines if the total circulation is not equal to zero. It is shown by the linear analysis of stability that the boundaries of the core and shell in these cases have different sensitivities to small perturbations. Numerical simulation by the method of contour dynamics makes it possible to distinguish zones with different types of motions in the domain of instability: rotational, in which tripoles are formed, and translational, in which the vortex breaks down into dipolar structures. When the generalized vorticity equation that includes the entrainment effects is used, the mechanism of cyclonic-anticyclonic asymmetry, which governs the transformation of an axisymmetric vortex, is realized. In this case, there exists a domain where tripolar or paired dipolar configurations may develop depending on the sign of the vorticity of the core.