Baroclinic instability and nonlinear oscillations in the truncated SQG model.

Baroclinic instability of a flow with a constant vertical shear in a zonal channel has been studied. The maximally truncated SQG model was used to describe the dynamics of flow disturbances. A dynamic system describing nonlinear interactions between unstable counter‐propagating Rossby waves with one zonal wave number and a neutral mode independent of zonal coordinate has been formulated within the framework of this model. Both analytical and numerical solutions of the system show that exponentially increasing disturbances at the linear stage of instability development give way to nonlinear oscillations (vacillations). The occurrence of such oscillations is caused by the law of conservation of surface potential energy. According to numerical estimates, the period of oscillations is of the order of a month, which is in agreement with measurement data obtained for the winter atmosphere. [ABSTRACT FROM AUTHOR]