Dynamic stability in the stratosphere

The general circulation of the stratosphere is described briefly in the light of recent observations, with particular emphasis on the polar vortex of winter and its evolution. It is concluded that the observed breakdowns in the polar vortex are some sort of instability phenomenon. Baroclinic instability is investigated both through a four-level finite-difference model and a two-layer analytic model. Although the evidence is not entirely conclusive, the analytic model, consisting of a tropospheric layer without shear and a stratospheric layer with constant shear, appears to be stable regardless of shear or wavelength. The finite-difference model, however, does indicate instability of the higher modes provided that the stratospheric shear is very strong. The discrepancy may lie in the fact that long internal waves are not excluded from the finite-difference model as they are from the stratospheric layer of the analytic model. Inertial and barotropic instability are also briefly discussed. Although inertial instability is dismissed as an explanation for the observed developments, barotropic instability may not so be dismissed. It is suggested that future research be directed to the problem of barotropic instability.