Explosive Growth of Asymmetric Disturbances in a Flow with Vertical Shear

The classical problem of geophysical hydrodynamics is the problem of the instability of a zonal geostrophic flow with a vertical velocity shear. At present, the instability with respect to symmetric perturbations that do not depend on the coordinate along the flow has been most thoroughly studied. For a symmetric instability to arise, the two-dimensional perturbation wave vector must lie inside a certain sector in the vertical plane of the wave numbers. In this paper, we study the instability with respect to asymmetric perturbations oriented at an angle to the flow. Fundamentally new features of the temporal dynamics of the amplitudes of such perturbations are found. The main feature is associated with the existence of a stage of exponential explosive growth of finite duration. A kinematic interpretation of this stage is given, which is related to the passage of the vertical projection of the perturbation wave vector through the sector of symmetric instability.