Reexamination on the type classification of Stratospheric Sudden Warming

Abstract

During stratospheric sudden warmings (SSW), stratospheric polar vortex is characterized by displaced off the pole toward low-latitude (displacement-type) or split into two vortexes (split-type) by undergoing the distortion and the division due to wave-mean interaction. We find that even though SSW events are identified as split-type, the evolutions can be distinctively different depending on the dominant wave disturbance prior to SSW in the middle stratosphere. Thereby, we classify the split-type SSW into displacement-split type (DS type) and split- split type (SS type). The frequencies of the both split types in the reanalysis data are almost similar and show distinct dynamic features during prewarming phase. In the SS type, the characteristics of the known split type are more clearly seen. The DS type is rather similar to the displacement-type characteristics prior to the SSW. In the WACCM simulation with climatological surface boundary conditions, however, the number of split-type SSW is much less than the observed as previous studies reported and we found that this largely due to the inability of WACCM to simulate SS type. Most of the split-type SSW are simulated as DS type, and the incidence rate of SS type to DS type is very low to 0.21 level.