Dynamical Aspects of Stratospheric Vacillations in a Highly Truncated Model

Abstract

Dynamics of stratospheric vacillations, which were first obtained by Holton and Mass in a highly truncated spectral model, are investigated with an EP flux diagnosis based on the transformed Eulerian-mean equations. The vacillations, which are purely periodic solutions under constant external conditions, can be divided into a dynamically active period and an inactive period. Dynamics controlling the variations are different between the two periods. During the active period the mean zonal winds vary vigorously like a stratospheric sudden warming. The variation is mainly due to the wave driving, although a large part of it is canceled by the Coriolis torque of the "residual" circulation. The transience term is important in all of the layers to determine the wave driving, while the dissipation term has a comparable magnitude in the upper layers. The wave transience is directly associated with vertical propagations of the wave-activity pulse, which is generated not at the bottom boundary, but inthe interior. On the other hand, during the inactive period, easterly winds and negative values of the meridional gradient of the zonal mean potential vorticity appear in the lower layers within 10-30 km. These layers prevent the wave from propagating upward. The mean zonal winds above the layers are accelerated gradually by the Coriolis torque of the "residual" circulation due to the diabatic heating.