Zonal flow generation by the stretching of Rossby modons

The numerical study is presented of the zonal flow generation in shallow rotating fluids and in magnetized plasmas, in a strongly nonlinear regime described by the Charney-Hasegawa-Mima equation. It is demonstrated that coherent vortices, often regarded as the building blocks of the strong turbulence, are unstable in the presence of inhomogeneities. While the monopolar vortices, both cyclones and anticyclones, are rapidly dispersed by a finite Rossby velocity, the dipolar vortices (or modons) undergo a qualitative modification by the action of the scalar nonlinearity arising from the β effect. The westward propagating modons rapidly topple, disintegrating into two monopoles that propagate independently and rapidly disperse. Conversely, for the eastward propagating modons, the β-effect produces the change of the direction of the propagation, followed by the stretching in the east-west direction. On a long time scale, such modons expand to a length equal to the size of the computational box, and essentially an one-dimensional zonal flow is created, whose transverse (north-south) scale is determined by the initial size of the modon.