A strong stratospheric sudden warming (SSW) event occurred in the southern hemisphere (SH) in September 2019 and significantly weakened the stratospheric polar vortex. Due to the positive zonal wind anomalies in the troposphere, the barotropic/baroclinic instability, primarily controlled by the horizontal/vertical wind shear, weakened in the mid-latitude upper troposphere from September 17 to October 15. As a result, planetary waves (PWs) deflect equatorward near the tropopause rather than vertically into the stratosphere, resulting in less perturbing of the polar vortex. After October 15, the westward zonal wind anomalies propagate downward and reach the troposphere, increasing the tropospheric barotropic/baroclinic instability. This benefits the propagation of PWs into the stratosphere, leading to the early breaking of the stratospheric polar vortex. The anomalous cooling due to enhanced upwelling in the SH mesosphere is caused by stronger stratospheric wind filtering of gravity waves (GWs), governing the early onset of polar mesospheric clouds (PMCs).