Stability of the stratospheric polar vortex in the winter–spring period is one of key factors of the duration and scales of stratospheric ozone depletion in a polar region. The Arctic polar vortex attains its peak intensity in winter, whereas the Antarctic vortex usually strengthens in early spring. As a result, large-scale ozone depletion occurs every year from August to November over the Antarctic, and short-term ozone loss occasionally occurs from January to March over the Arctic. In this work, we analyze the reason for the high strength and stability of the Antarctic polar vortex in the winter–spring period. A good agreement between the seasonal variations in the temperature of the subtropical lower stratosphere and in the zonal wind in the lower subpolar and polar stratosphere in the Southern Hemisphere is shown on the basis of ERA-Interim reanalysis data. The results of numerical simulations with the PlaSim-ICMMG-1.0 model show acceleration of zonal wind in the subpolar region with an increase in the temperature of the subtropical stratosphere. Thus, the winter–spring strengthening of the Antarctic polar vortex occurs due to an increase in the stratospheric equator-to-pole temperature gradient caused by the seasonal increase in the temperature of the lower subtropical stratosphere in this period.