Abstract
Internal variations of the troposphere–stratosphere coupled system with intraseasonal and interannual timescales are investigated in a parameter sweep experiment with a simple global circulation model under a periodic annual forcing. In order to examine the role of forced planetary waves in the variations, the amplitude of a sinusoidal surface topography is chosen as an experimental parameter; 100-yr integrations are performed for each of 10 topographic amplitudes from 0 to 3000 m.
The extratropical stratospheric circulation depends on the topographic amplitude in its mean seasonal march and intraseasonal and interannual variations. In the run without the topography, the stratospheric circulation is basically driven thermally and hardly shows interannual variation in any seasons. In the runs in which the topography is included, on the other hand, the stratospheric circulation is dynamically active and shows large interannual variation in different seasons, that is, spring for small topographic amplitudes (around 500 m) and winter for large amplitudes (around 1000 m). The mean seasonal march and interannual variation in these runs of small and large amplitudes resemble those in the Southern and Northern Hemispheres, respectively.
In this study, the annual forcing is introduced only in the stratosphere while the tropospheric condition is kept constant in time, in order to investigate downward influence from the stratosphere to the troposphere in the seasonal march. The annual response of the atmosphere can significantly penetrate into the troposphere, depending on the topographic amplitude. The downward penetration is significant for the amplitudes of planetary and synoptic-scale waves, while it is negligible for zonal mean quantities.
The empirical orthogonal function and lag correlation analyses show that a sequence of variability associated with stratospheric sudden warmings (SSWs) in the run of the topographic amplitude of 1000 m is characterized by poleward and downward propagation of anomalies of the zonal mean zonal wind and the planetary wave amplitude. Preconditioning for and the aftereffect of SSWs extend through both the stratosphere and troposphere. One month before SSWs, the polar night jet and the tropospheric jet shift poleward while planetary waves amplify in the troposphere and stratosphere. The anomalies of the zonal wind and wave amplitude further propagate poleward and downward for several months after SSWs.
Corresponding author address: Dr. Masakazu Taguchi, Department of Atmospheric Sciences, Box 351640, University of Washington, Seattle, Washington, 98195-1640. Email: taguchi@atmos.washington.edu
