The anomalous stratospheric state favoring the occurrence of sudden stratospheric warmings (SSW) is usually referred to as vortex preconditioning. This study investigates the role of vortex preconditioning in triggering strong and weak SSWs by using ERA5 reanalysis data. Strong and weak SSWs are distinguished by the amplitude of zonal-wind deceleration of sudden stratospheric deceleration events. The robust stratospheric anomalies before strong SSWs last longer compared to weak SSWs, accompanied by stronger amplification of stratospheric wave activity near the warming. The stratospheric anomalies before weak SSWs have two significant enhancement stages, with two processes of stratospheric wave amplification. Robust stratospheric anomalies generally appear before tropospheric wave events (TWEs) followed by the strong and weak SSWs, which are absent before TWEs without SSWs. Stratospheric meridional potential vorticity gradient events (SPVEs) are defined to represent the anomalous stratospheric state during vortex preconditioning. The SPVEs can effectively modulate the stratospheric upward wave activity. No strong lower-tropospheric wave forcing is seen for the composites of both strong and weak SSWs preceded by SPVEs. These SSWs account for about 59% of the total SSWs. Furthermore, about 23% of strong SSWs and 36% of weak SSWs are only preceded by SPVEs without TWEs, indicating the major role of vortex preconditioning in triggering these SSWs. The SPVEs can be caused by wave breaking in the surf zone or the enhanced polar vortex, while the SPVEs preceded by clear wave breaking may be more favorable to the occurrence of SSWs.

SSW,Vortex preconditioning