Previous studies show that the variability associated with the winter-mean North Atlantic Oscillation (NAO) tends to be largely modulated according to the solar cycle so that the anomalous zonal wind signal extends to the upper stratosphere when solar activity is high, but the vertical extension is limited when solar activity is low. To examine the physical mechanism of the solar cycle modulation of the winter-mean NAO, momentum and wave-energy budget analyses have been performed using the Japanese Reanalysis (JRA-55) dataset from 1958 to 2021. Momentum analysis shows that the enhancement of zonal wind in High Solar (HS) years is due to direct enhancement of wave forcing. Wave-energy analysis shows that baroclinic energy conversion from zonal-mean flow in the region from the surface to the middle stratosphere and barotropic energy conversion to the zonal-mean flow around the middle stratosphere tend to be enhanced in HS winters. Both enhancements are related to the enhancement of the zonal wind. Our analysis suggests that the climatological basic states tend to become more unstable during HS winters.