Sudden stratospheric warming (SSW) is an extraordinary event in winter polar stratosphere, characterized by a rapid rise of temperatures and weakening of the stratospheric polar vortex (SPV). SSW is largely induced by vertically propagating planetary waves of tropospheric origin and can, in turn, results in negative phase of Arctic oscillation in the troposphere and therefore severe surface cold surges in mid- and high-latitude regions. The longer timescale of stratospheric variability than that in troposphere makes SSW a promising predictor for surface weather and climate.

During SSW, the SPV can be either displaced from the pole or split into two pieces, i.e., displaced and split type. Different types of SPVs are revealed to be responsible for varied tropospheric impacts. Notably, ~1/3 of SSWs have ambiguous classifications across different methods because the vortex sometimes displaces and then splits within a relatively short-time period. Tropospheric influences of these mixed-type SSWs may be more complex. Their consecutive impacts and early precursors in the troposphere still remain unclear, which largely motivated our current study.

Using 42-yr ERA5 analysis data from December to March during 1979–2020, we first identified weak SPV days based on the anomalous 10-hPa geopotential, and defined an SSW event as with at least 5 consecutive weak SPV days. 29 SSW events were eventually derived with a frequency of ~0.7 event per year. These SSW events were then classified into different types using the k-means clustering method. 11 out of 29 SSW events were found to be mixed-type with a routine transformation from displacement to split. The mixed-type events are featured by longer mean duration (22 days) and stronger negative North Atlantic oscillation downward into the troposphere than both displaced and split type events, which is conducive to the prominent cold surges over high-latitude Asia and mid-latitude North America.

Tropospheric precursors in different types of SSW events were further elucidated by planetary wave decomposition. Before the onset of SSW, the mid-level low (high) pressure anomaly over Alaska associated with North America warming (cooling) can effectively enhance the upward propagation of wave 1 (2), which contributes to the following formation of displaced (split) SPV. Notably, once the vortex is displaced, the mid-level Ural anomaly becomes a major discriminator for subsequent morphology of SPV. The Ural anomalous high can significantly enhance the upward propagation of wave 2, which helps convert displaced vortex into split one and therefore facilitates the formation of mixed-type SSW events.