Stratospheric Response to the 11-year Solar Cycle: Breaking Planetary Waves, Internal Reflection and Resonance

Breaking planetary waves (BPWs) play a crucial role in the seasonal development of the stratospheric polar vortex. Nonlinear reflection from the region of BPWs and resonant growth of reflected waves can alter the net wave forcing of the stratosphere. In this study, reanalysis data sets are used to evaluate an 11-year solar-cycle modulation of BPWs, nonlinear reflection and possible resonant growth of the reflected waves during Northern winter. It is shown that BPWs are enhanced in the middle and upper stratosphere during high solar activity winters. Enhanced dissipation of planetary-scale Rossby waves with zonal wavenumber 1 occurs in the polar upper stratosphere and the stratospheric surf zone at 35-45°N, 7-20 hPa. These wave-1 anomalies are accompanied by enhanced downward or poleward reflection of planetary waves with zonal wavenumbers 2 and 3 (wave-2/3) from the regions with enhanced BPWs. Consistent with the enhanced BPWs, significantly enhanced poleward focusing of transient planetary waves is detected in late winter. Resonance growth of the reflected wave-2/3 also contributes to enhanced late winter wave-driving. These wave anomalies are further discussed in terms of changes in phase-speed spectra of transient planetary wave fluxes. It is also shown that nonlinear critical layer absorption/reflection plays a key role in inducing the stratospheric changes. Thus, the solar-induced changes involving preconditioning BPWs and ‘internal-mode’ resonance differ distinctly from the commonly invoked mechanism regarding a poleward and downward propagation of zonal-mean wind anomalies. These results allows us to better explain the observed reversal of the solar-cycle wind anomalies between early winter when the polar vortex is found to be stronger during high solar activity years, and late winter when the polar vortex is weaker.