The breakdown of the polar vortex in the stratosphere is considered as a barotropic instability process of an irregularly shaped vortex.
Numerical tests are carried out based on the observed height patterns of 50 mb level in the cases of 15 November 1957, 15 January 1958, and 20 January 1958 as initial value problems for artificially introduced small disturbances.
Kinetic energy of disturbances remains constant for the former two, but in the case of 20 January 1958, just a few days before the occurrence of breakdown, the kinetic energy increases. The patterns of disturbance thus computed were similar to the observed height changes for the latter two cases.