NCEP reanalyses are used to isolate systematic variations in the stratosphere that operated coherently over the last 4 decades with the 11-year variation of UV irradiance. Only a small systematic variation is visible at low frequency, which would reflect a simple linear response that drifts with solar flux. However, a systematic variation manifests itself prominently at high frequency, which involves changes between neighboring years. The systematic variation of interannual variability reflects a more complex, nonlinear response to the 11-year variation of UV irradiance. It is analogous to a similar variation found earlier in the QBO of equatorial wind.
Interannual variability undergoes a frequency modulation that systematically alters its phase during winter, when the polar-night vortex is sensitive to equatorial wind. The modulation appears prominently in the wintertime tendency of temperature. It has the same structure as anomalous temperature associated with changes in wave forcing of the residual mean circulation. Each reflects anomalous downwelling over the Arctic, compensated at lower latitude by anomalous upwelling. The resemblance between these anomalous structures suggests that the 11-year modulation of interannual variability enters through systematic changes of the residual circulation.
Accompanying the modulation of zonal-mean structure is an amplification and decay of wavenumber 1 at high latitude. It reflects a poleward advance and retreat of the critical region, or surf zone of planetary waves, where their absorption forces residual motion. The modulation of wave structure parallels the modulation of equatorial wind. Both imply a reversal of anomalous downwelling between solar min and solar max, one broadly consistent with the observed reversal of anomalous temperature.
More generally, anomalous residual motion couples the troposphere to the stratosphere, through transfers of mass across the tropopause. Air is rejected to the troposphere at high latitude, through anomalous downwelling. It must be compensated at low latitude by the absorption of air from the troposphere, through anomalous upwelling. Interannual changes in the polar stratosphere are, in fact, shown to be accompanied by coherent changes inside the tropical troposphere. There, signatures of anomalous warming and upper-tropospheric divergence reflect an intensification of organized convection and the Hadley circulation, one that accompanies the intensification of the residual mean circulation.