A Typical Evolution Mode of Stratosphere–Troposphere Coupling during the Cold Air Outbreak Events in 2020/2021 Winter

Stratospheric sudden warming (SSW) events are a critical predictability source of the extended-range prediction of cold air outbreaks (CAO) because of their temporal lead information relative to the tropospheric circulation changes. Nevertheless, there is large uncertainty in the region and time of the surface air temperature in response to the SSW events, and the stratosphere–troposphere coupling process and the mechanism involved still remain unclear. Using the ERA5 reanalysis data from 1979 to 2021, this study focuses on the “displacement” type major SSW event of 2020/2021 winter and investigates the characteristics of the extratropical surface air temperature anomalies and the evolution of the coupling mode of the poleward warm air stratospheric branch (WB_ST), poleward warm air tropospheric branch (WB_TR), and equatorward cold branch (CB) of isentropic meridional mass circulation (IMMC), as well as the related wave dynamics. Results show that associated with this SSW event, the CAO events across Eurasia occurred before the subpolar westerly became easterly, while the CAO event over North America occurred after the subpolar westerly recovered. This mainly resulted from the three-stage coupling modes among the three IMMC branches during the stratospheric polar vortex oscillation, i.e., in-phase strengthening of WB_ST and WB_TR/CB, out-of-phase with a strengthened WB_ST but a weakened WB_TR/CB, and out-of-phase with a weakened WB_ST but a strengthened WB_TR/CB. The stronger CB resulted in the CAOs associated with SSW, while the stronger WB_ST predominantly contributed to the occurrence of SSW and the continuous strengthening of the negative phase of the Arctic Oscillation. The coupling mode of the three IMC branches depended on the westward tilt of waves at two critical levels, namely the tropopause level and the middle and lower tropospheric level. Anomalously strong westward tilt caused the net poleward mass transport above this level and the net equatorward transport below it and vice versa. Especially during the polar vortex recovery, the anomalously weak baroclinicity near the tropopause strengthens the WB_TR and, subsequently, the CB due to the mass continuity, which is conducive to the occurrence of CAOs. The coupling evolution mode of IMMC around the SSW in 2020/2021 winter was found to be highly consistent with that during the negative stratospheric Northern Annular Mode (NAM) events that were characterized by a lagged downward propagation of the polar stratospheric temperature anomaly to the lower troposphere. The two share common features in terms of the wave scale, i.e., the wavenumber-1 waves tend to be stronger and propagate upward in the early stage of the SSW event or the negative NAM event, and the wavenumber-2 waves are strengthened but restrained within the troposphere after the SSW.