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Taking the stratospheric sudden warming (SSW) event in February 2018 as an example, the relationship between the SSW event and the intraseasonal oscillation (ISO) mode over mid-high-latitude Eurasia is investigated by daily reanalysis data. First, the 2018 SSW event and mid-high-latitude ISO are reviewed. The 2018 SSW event is a typical vortex-split event defined by the NCEP-DOE dataset, and the ISO mode features a southeastward propagation. Along with the ISO propagation, temperature anomalies developed from troposphere to stratosphere in the 2018 wintertime. It is also found that a strong ISO event occurred before the onset of SSW in this wintertime. Our analysis reveals that the correlation is significant when mid-high-latitude ISO leads the 2018 SSW event by 9–13 days. Occurrence of strong ISO 9–13 days before the SSW event is found to be conducive to its onset. The diagnosis of planetary wave activity indicates that stronger ISO leads to strengthened wavenumber-2 geopotential height anomalies; then, its in-phase superposition of climatological geopotential height makes up for the upward-propagating tropospheric planetary waves, which is in favor of the onset of the 2018 SSW event. The outcome of energy conversion equations also reveals that ISO perturbation contributes to the onset of SSW. Lastly, the contribution of the vertical component of Plumb wave activity flux propagated upward from the region of mid-high-latitude ISO is estimated at approximately 69%.

Relationship between the Intraseasonal Oscillation over Mid-High-Latitude Eurasia and the Stratospheric Sudden Warming Event in February 2018