Does Stratospheric Sudden Warming Occur More Frequently during ENSO Winters Than during ENSO-Neutral Winters?

Stratospheric sudden warming (SSW) events exhibit pronounced interannual variability. Based on WMO definition, it has been suggested that SSW events occur more preferably during El Niño-Southern Oscillation (ENSO) winters (both El Niño and La Niña winters) than during ENSO-neutral winters. In this study, such relationship is re-evaluated by considering six different definitions of SSW. For all definitions, SSW events are detected more frequently during El Niño winters than during ENSO-neutral winters, in consistent with a strengthened planetary-scale wave activity. However, such systematic relationship is not found during La Niña winters. While two SSW definitions, including WMO definition, show a higher SSW frequency during La Niña winters than during ENSO-neutral winters, other definitions show no difference or even lower SSW frequency. This result is insensitive to the choice of reanalysis datasets and ENSO index, indicating that the reported ENSO-SSW relationship is not robust but dependent on the details of the SSW definition. The definition-defendant ENSO-SSW relationship is discussed in terms of different background wind and different planetary-scale wave forcings during El Niño- and La Niña-winter SSW events. Implication of this finding to SSW-related downward coupling and surface climate variability is also discussed.