  We study the dynamics of two-dimensional coherent structures in planetary
atmospheres and oceans. We derive the Zakharov-Kuznetsov equation for large
scale motion from the barotropic quasigeostrophic equation in a weakly
nonlinear, long wave approximation. We consider coherent structures emerging
out of an instability caused by a narrow jet-like meanflow. We use multiple
scale analysis combined with asymptotic matching.
