Evidence of the Solar Cycle in the General Circulation of the Stratosphere

Abstract

NCEP reanalyses are used to isolate systematic variations in the stratosphere that operated coherently over the last four decades with the 11-yr 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, Fs. However, a systematic variation manifests itself prominently at high frequency, which involves changes between neighboring years. Corresponding to interannual variability, the systematic variation at high frequency reflects a more complex, nonlinear response to the 11-yr variation of UV irradiance. It is analogous to a similar variation found earlier in the quasi-biennial oscillation (QBO) of equatorial wind, uEQ. Interannual variability undergoes a frequency modulation that systematically alters its phase during winter, when planetary waves couple the polar and equatorial stratosphere. The polar-night vortex is then sensitive to equatorial wind, which itself varies systematically with Fs. Monte Carlo simulations indicate that the systematic variation of wintertime phase is highly significant. The systematic variation appears prominently in the wintertime tendency of temperature, which is coupled directly to the residual mean circulation. In fact, the anomalous wintertime tendency operating coherently with Fs has the same basic structure as that operating coherently with anomalous forcing of the residual circulation. Each reflects anomalous downwelling over the Arctic that is compensated at lower latitude by anomalous upwelling. The resemblance of these anomalous structures suggests that the systematic variation at high frequency enters through changes of the residual circulation. Accompanying the variation of zonal-mean structure is a systematic amplification and decay of wavenumber 1 at high latitude. It represents a poleward advance and retreat of the critical region, or surf zone, where planetary waves experience strong absorption that forces residual motion. This variation of wave structure, along with the anomalous residual motion it forces, parallels the systematic variation of equatorial wind. Wintertime-mean uEQ suggests a reversal of anomalous downwelling between solar min and solar max, one broadly consistent with the observed reversal of anomalous temperature.